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The main objective of this thesis was the design and synthesis of perylene bisimide dyes with sufficient water-solubility for the construction of self-assembled architectures in aqueous solutions. Beside these tasks another goal of this project was the control over the self-assembly process in terms of aggregate size and helicity, respectively. Within this thesis an appropriate synthesis for spermine-functionalized perylene bisimide dyes was developed and conducted successfully. The characterization of these building blocks and their course of self-assembly were investigated by NMR, UV/Vis and fluorescence spectroscopy as well as by atomic force and transmission electron microscopy. For the better understanding of the experimental results theoretical calculations were performed.
This PhD thesis introduced several concepts for the construction of new supramolecular assem-blies in polar solvents. Although the building blocks differ in their binding mode and association strength they follow the same principle: one main driving force for the self-assembly in polar solutions in combination with one texturing force. The main self-assembly process is based on the mutual interaction of hydrogen-bond enforced ion pairs which deliver the association energy needed for stable, supramolecular structures even in polar solvents. The texturing force itself is represented by the linkers between the zwitterionic building blocks or parts of them. The different length and functionalization of the linkers have a tremendous influence on the mode of self-assembly leading to cyclic dimers, vesicles, layers or solid spheres. Hence, this principle is suitable for the construction of programmable monomers. Since the derivatisation of the main binding motive is rather simple it offers a great number of new and undoubtedly fascinating structures with potential applications in material and biomimetic science.
Die absoluten Konfigurationen von mehr als 20 neuartigen Naturstoffen und Syntheseprodukten mit unterschiedlichen Chiralitätselementen (stereogene Zentren, chirale Achsen und chirale Ebenen) wurden durch Vergleich ihrer experimentellen CD-Spektren mit den quantenchemisch berechneten der jeweils möglichen Stereoisomere aufgeklärt. Zur Simulation des molekularen CD kamen dabei semiempirische Verfahren (CNDO/S und OM2) und die zeitabhängige Dichtefunktionaltheorie (TDDFT) zum Einsatz.
In the first part of the work three polycarbazoles poly[N-((4-dimesitylboryl)-3,5-dimethylphenyl)-carbazole]-2,7-diyl P1, poly[N-((4-dimesitylboryl)-3,5-dimethylphenyl)-carbazole]-3,6-diyl P2 and poly[N-(4-(diphenylmethylene)-phenyl)- carbazole]-2,7-diyl P3 were synthesized by Yamamoto coupling reaction and their spectroscopic and electrochemical properties were investigated. Absorption and fluorescence characteristics of P1 and P3 were found to be similar to other 2,7-linked polycarbazoles, whereas P2 shows a CT absorption band arising from a shift of electron density from the nitrogen of the carbazole donor to the triarylborane acceptor. This causes a negative solvatochromic absorption and a positive solvatochromic fluorescence behaviour and is responsible for the significantly enlarged fluorescence quantum efficiency in solution and solid state compared to other 3,6-linked polycarbazoles. Thus the spectroscopic properties are governed by the connection pattern: the 2,7-linked polycarbazoles are not affected by the acceptor substituent due to the rigid poly-para-phenylene-like backbone structure, whereas the 3,6-linked polycarbazole P2 is dominated by the properties of the monomer unit due to its more flexible (less conjugated) structure. The oxidative processes of P1-P3 have been investigated in detail by cyclic voltammetry, which are similar to known 2,7- and 3,6-polycarbazoles. The reversible reduction found for P1 and P2, respectively, is attributed to the reduction of the triarylborane moiety. No reduction process referring to the carbazole moiety was observed. Due to its better solubility compared to P1 and P3 only P2 was used as active layer in an OLED device (ITO/P2/Al). The electroluminescence spectrum revealed CIE coordinates of (0.17, 0.21). In the second part of the work the low band gap polyradical poly{[((2,3,4,5,6-pentachlorophenyl)-bis(2,3,5,6-tetrachlorophenyl)methyl radical)-4,4’-diyl]-alt-4,4’-bis(vinylphenyl)-4-(2-ethylhexyloxy)phenylamin} P4 was synthesized by Horner-Emmons reaction. It shows an IV-CT band in the NIR, which arises from an ET from the triarylamine donor to the PCTM radical acceptor. This transition is confined to one monomer unit as deduced from comparison with the monomer spectra. HOMO and LUMO of P4 determined by cyclic voltammetry are at -5.5 and -4.5 eV, respectively. The smaller electrochemical band gap (1.0 eV) compared to the optical band gap (1.2 eV) is probably caused by ion pairing effects in the electrochemical experiments and indicates a low exciton binding energy. Femtosecond-pump-probe transient absorption spectroscopy revealed the spectral features of the oxidized triarylamine donor and the reduced PCTM acceptor similar to the spectra obtained separately for positive and negative potentials by spectroelectrochemistry. Thus the ET event causing the IV-CT absorption band could unambiguously be identified. The decay of the IV-CT state was found to be biexponential. The fast solvent dependent decay component is ascribed to the direct decay from the IV-CT state to the ground state, whereas the slow solvent independent decay component is tentatively attributed to an equilibrium formation of the IV-CT state and a completely charge separated state formed by charge migration along the polymer backbone. Well balanced ambipolar charge transport with hole and electron mobilities of ca. 3 × 10-5 cm2 V-1 s-1 was found in OFET devices (BG/TC structure) comprising an additional insulating organic PPcB layer. Polymer/polymer BHJ solar cell devices with the structure glass/ITO/PEDOT:PSS/(P3HT/P4)/Ca/Al yielded a power conversion efficiency of 3.1 × 10-3 %, VOC = 0.38 V, JSC = 2.8 × 10-2 mA cm-2 and FF = 0.29 for the 1:4 (P3HT/P4) blend ratio. The improper solid state morphology of P4 that causes the unsatisfying performance of OFET and solar cell devices renders P4 less suitable for these applications, whereas the hypothesis of charge migration in the excited state is worth to be investigated in more detail.
Within this thesis the interactions between novel corannulene derivatives in solution as well as in the solid state by changing the imide residue of a literature known extended corannulene dicarboximide were investigated, in order to obtain a better understanding of the packing and possible charge transport in potential applications. Accordingly, the goal of the work was to synthesize and investigate an electron-poor corannulene bis(dicarboximide) based on previously published work but with higher solubility and less steric encumbrance in imide position to enable self-assembly in solution.
To obtain further insights into the conformational stability, structure and chiroptical properties of heavily twisted PBIs another aim of this thesis was the design, synthesis, and optoelectronic investigation of various fourfold directly arylated PBIs by substitution in bay position with smaller hydrocarbons with different steric demand, i.e., benzene, naphthalene and pyrene, which should be separable by chiral high performance liquid chromatography (HPLC).
As of yet, no concise study concerning the optical and electronic properties of differently core-substituted PBIs in the neutral as well as the mono- and dianionic state in solution is available, which also elucidates the origin of the different optical transitions observed in the absorption and emission spectra. Thus, in this thesis, the investigation of five PBI derivatives with different frontier energetic levels to produce a reference work of reduced PBIs was tackled.
A series of perylene bisimide (PBI) dyes bearing various aryl substituents in 1,6,7,12 bay positions has been synthesized by Suzuki cross-coupling reaction. These molecules exhibit an exceptionally large and conformationally fixed twist angle of the PBI π-core due to the high steric congestion imparted by the aryl substituents in bay positions. Single crystal X-ray analyses of phenyl-, naphthyl- and pyrenyl-functionalized PBIs reveal interlocked π-π-stacking motifs, leading to conformational chirality and the possibility for the isolation of enantiopure atropoisomers by semipreparative HPLC. The interlocked arrangement endows these molecules with substantial racemization barriers of about 120 kJ mol\(^{−1}\) for the tetraphenyl- and tetra-2-naphthyl-substituted derivatives, which is among the highest racemization barriers for axially chiral PBIs. Variable temperature NMR studies reveal the presence of a multitude of up to fourteen conformational isomers in solution that are interconverted via smaller activation barriers of about 65 kJ mol\(^{−1}\). The redox and optical properties of these core-twisted PBIs have been characterized by cyclic voltammetry, UV/Vis/NIR and fluorescence spectroscopy and their respective atropo-enantiomers were further characterized by circular dichroism (CD) and circular polarized luminescence (CPL) spectroscopy.
The self-assembly of a bowl-shaped naphthalimide-annulated corannulene of high solubility has been studied in a variety of solvents by NMR and UV/Vis spectroscopy. Evaluation by the anti-cooperative K\(_2\)-K model revealed the formation of supramolecular dimers of outstanding thermodynamic stability. Further structural proof for the almost exclusive formation of dimers over extended aggregates is demonstrated by atomic force microscopy (AFM) and diffusion ordered spectroscopy (DOSY) measurements as well as by theoretical calculations. Thus, herein we present the first report of a supramolecular dimer of an annulated corannulene derivative in solution and discuss its extraordinarily high thermodynamic stability with association constants up to > 10\(^6\)M\(^-\) \(^1\) in methylcyclohexane, which is comparable to the association constants given for planar phthalocyanine and perylene bisimide dyes.
The presented work in the field of supramolecular chemistry describes the synthesis and detailed investigation of (bi)pyridine-based oligo(phenylene ethynylene) (OPE) amphiphiles, decorated with terminal glycol chains. The metal-ligating property of these molecules could be exploited to coordinate to Pd(II) and Pt(II) metal ions, respectively, resulting in the creation of novel metallosupramolecular π-amphiphiles of square-planar geometry.
The focus of the presented studies is on the self-assembly behaviour of the OPE ligands and their corresponding metal complexes in polar and aqueous environment. In this way, the underlying aggregation mechanism (isodesmic or cooperative) is revealed and the influence of various factors on the self-assembly process in supramolecular systems is elucidated. In this regard, the effect of the molecular design of the ligand, the coordination to a metal centre as well as the surrounding medium, the pH value and temperature is investigated.
Self-organization and self-sorting processes are responsible for the regulation and control of the vast majority of biological processes that eventually sustain life on our planet. Attempts to unveil the complexity of these systems have been devoted to the investigation of the binding processes between artificial molecules, complexes or aggregates within multicomponent mixtures, which has facilitated the emergence of the field of self-sorting in the last decade. Since, artificial systems involving discrete supramolecular structures, extended supramolecular aggregates or gel-phase materials in organic solvents or—to a lesser extent—in water have been investigated. In this review, we have collected diverse strategies employed in recent years to construct extended supramolecular aggregates in water upon self-sorting of small synthetic molecules. We have made particular emphasis on co-assembly processes in binary mixtures leading to supramolecular structures of remarkable complexity and the influence of different external variables such as solvent and concentration to direct recognition or discrimination processes between these species. The comprehension of such recognition phenomena will be crucial for the organization and evolution of complex matter.
No abstract available
Large-scale multireference configuration interaction (MRD-CI) calculations in a flexible atomic orbital (AO) basis are employed to study the reaction of C\(_2\)H\(_4\) with CH\(_2\) in its firSt triplet and singlet state. The minimum energy path (MEP) of both reactions is calculated, and different mechanisms are discussed. To examine the possible participation of the singlet state in the overall reaction starting from the triplet channel and terminating in the singlet-state c-C\(_3\)H\(_6\), various cuts through both hypersurfaces are calculated. lt is found that favorable interconversion from the trip1et to the singlet surface can only occur at !arge separations of the two fragments of CH2 and C\(_2\)H\(_4\). Experimental data considering the vibrational motion of CH\(_2\) in connection with the relative position of both surfaces are used to obtain an estimate for the overall barrier of the reaction. The height of the barrier is about 6 kcal/mol, while the barrier of the pure triplet reaction is calculated to be 7-9 kcal/mol.
In this thesis, the photophysics and spin chemistry of donor-photosensitizer-acceptor triads were investigated. While all investigated triads comprised a TAA as an electron donor and a NDI as an electron acceptor, the central photosensitizers (PS) were different chromophores based on the dipyrrin-motif. The purity and identity of all target compounds could be confirmed by NMR spectroscopy, mass spectrometry and elemental analysis.
The first part of the work dealt with dipyrrinato-complexes of cyclometalated heavy transition metals. The successful synthesis of novel triads based on Ir(III), Pt(II) and Pd(II) was presented. The optical and electrochemical properties indicated charge separation (CS), which was confirmed by transient absorption (TA) spectroscopy. TA-spectroscopy also revealed that the process of CS is significantly slower and less efficient for the triads based on Pt(II) and Pd(II) than for the analogous Ir(III) triads. This is mostly due to a much more convoluted reaction pathway, comprising several intermediate states before the formation of the final charge separated state (CSS2). On the other hand, CSS2 exhibits long lifetimes which are dependent on the central metal ion. While the Ir(III) triads show lifetimes of about 0.5 µs in MeCN, the Pt(II) and Pd(II) analogues show lifetimes of 1.5 µs. The magnetic field effect on the charge recombination (CR) kinetics of CSS2 was investigated by magnetic field dependent ns-TA spectroscopy and could be rationalized based on a classical kinetic scheme comprising only one magnetic field dependent rate constant k±. The behavior of k± shows a clear separation of the coherent and incoherent spin interconversion mechanisms. While the coherent spin evolution is due to the isotropic hyperfine coupling with the magnetic nuclei of the radical centers, the incoherent spin relaxation is due to a rotational modulation of the anisotropic hyperfine coupling tensor and is strongly dependent on the viscosity of the solvent. This dependence could be used to measure the nanoviscosity of the oligomeric solvent pTHF, which was found to be distinctly different from its macroviscosity.
The second part of the work dealt with bisdipyrrinato complexes and their bridged porphodimethenato (PDM) analogues. Initially, the suitability of the different chromophores for the use as PS in donor-acceptor substituted triads was tested by a systematic investigation of their steady state and transient properties. While the PDM-complex of Zn(II) and Pd(II) exhibited promising characteristics such as a high exited state lifetime and relatively intense emission, the purely organic parent PDM and the non-bridged bisdipyrrinato-Pd(II) complex were less suitable. The difference between the two Pd(II) complexes could be explained by a structural rearrangement of the non-bridged complex which results in a non-emissive metal centered triplet state with disphenoidal geometry. This rearrangement is prevented by the dimethylmethylene-bridges in the bridged analogue resulting in higher phosphorescence quantum yields and excited state lifetimes.
With the exception of the Zn(II)PDM-complex, the synthesis of novel donor acceptor substituted triads could be realized for all desired central chromophores. They were investigated equivalently to the cyclometalated triads described in the first part. The steady state properties indicate a stronger electronic coupling between the subunits due to the lack of unsaturated bridges between the donor and the central chromophore. Photoinduced CS occurs in all investigated triads. Due to the low exited state lifetimes of the central chromophores, CSS is formed less efficiently for the triads based on the unbridged Pd(II)-complex as well as the purely organic PDM. In the triad based on the bridged Pd(II) complex, the CR of CSS2 is faster than its formation resulting in low intermediate concentrations. For its elongated analogue, this is not the case and CSS2 can be observed clearly. Although the spin-chemistry of the triads based on bisdipyrrinato-Pd(II) and porphodimethenato-Pd(II) is less well understood, first interpretations of the magnetic field dependent decay kinetics gave results approximately equivalent to those obtained for the cyclometalated triads. Furthermore, the MFE was shown to be useful for the investigation of the quantum yield of CS and the identity of the observed CSSs.
In both parts of this work, the influence of the central photosensitizer on the photophysics and the spin chemistry of the triads could be shown. While the process of CS is directly dependent on the PS, the PS usually is not directly involved in the final CSSs. None the less, it can still indirectly affect the CR and spin chemistry of the CSS since it influences the electronic coupling between donor and acceptor, as well as the geometry of the triads.
Tristriazolotriazines (TTTs) with a threefold alkoxyphenyl substitution were prepared and studied by DSC, polarized optical microscopy (POM) and X-ray scattering. Six pentyloxy chains are sufficient to induce liquid-crystalline behavior in these star-shaped compounds. Thermotropic properties of TTTs with varying substitution patterns and a periphery of linear chains of different lengths, branching in the chain and swallow-tails, are compared. Generally, these disks display broad and stable thermotropic mesophases, with the tangential TTT being superior to the radial isomer. The structure–property relationships of the number of alkyl chains, their position, length and structure were studied.
Sekundäre Pflanzenstoffe sind aufgrund ihrer großen Strukturvielfalt sowohl als Leit- und Wirkstoffe für die Pharma- und Pflanzenschutzforschung in den Industrieländern als auch zur unmittelbaren medizinischen Grundversorgung der Entwicklungsländer von herausragender Bedeutung für den Menschen. Eine Klasse pharmakologisch, biogenetisch und chemotaxonomisch interessanter Sekundärmetabolite sind die Naphthylisochinolin-Alkaloide, die ausschließlich in den eng verwandten tropischen Pflanzenfamilien Ancistrocladaceae und Dioncophyllaceae vorkommen. Der Untersuchung der Biosynthese dieser acetogeninen Metabolite (z.B. Dioncophyllin A), einschließlich einiger Vorstufenderivate (z.B. Plumbagin, Droseron und Isoshinanolon), durch die konsequente Etablierung von in-vitro-Systemen sowie der Biologie ihrer pflanzlichen Produzenten am Naturstandort und in Kultur, wurde das Hauptaugenmerk in dieser Arbeit gewidmet. Außerdem wurden die biologischen Aktivitäten der Substanzen getestet. Daneben wurde die Strategie der stabilisotopenmarkierten Vorstufenverfütterung exemplarisch auf eine Art aus den nahe verwandten Nepenthaceen ausgeweitet, indem der natürliche Aufnahmemechanismus der carnivoren Pflanze ausgenutzt wurde. Anhand von Verfütterungsexperimenten mit ebenfalls neu etablierten Zellkulturen konnte außerdem die Struktur eines neuartigen Pyridon-Alkaloids (Antidesmon) aus Antidesma membranaceum, das ursprünglich als Isochinolin beschrieben worden war, revidiert werden und dessen ungewöhnliche Biosynthese aus Acetat und Glycin aufgeklärt werden.
The goal of this thesis was to investigate the influence of rotational restriction between individual parts and of the varying electron density in the bridging unit of D B A systems on the exchange interaction 2J, and thus the electronic coupling between a donor state and an acceptor state. A better understanding of how to influence the underlaying spin dynamics in such donor acceptor systems can open up the door to new technologies, such as modern molecular electronics or optoelectronic devices.
Therefore, three series of molecules consisting of a TAA electron donor, a TTC or ATC bridging unit and a PDI electron acceptor were studied. To investigate the influence of rotational restriction on 2J and the electronic coupling, a series of four rotationally hindered triads (chapter 6) was synthesised. The dihedral angle between the TAA and the TTC as well as between the TTC and the PDI was restricted by ortho methyl groups at the phenylene linkers of the connecting ends to the TTC bridge, producing a twist around the linking single bond which minimises the π overlap. The triads exhibit varying numbers of ortho methyl groups and therefore different degrees of rotational restriction. In order to shine light on the influence of varying electron density on 2J and the electronic coupling, a series of four substituted triptycene triads (chapter 7) was synthesised. The electron density in the TTC bridging unit was varied by electron donating and electron withdrawing groups in 12,13 position of the TTC bridging unit and thus varying its HOMO/LUMO energy. The last series of two anthracene bridge triads (chapter 8) connected both approaches by restricting the rotation with ortho methyl groups and simultaneously by varying the bridge energies.
In order to obtain the electronic properties, steady state absorption and emission spectra of all triads were investigated (chapter 4). Here, all triads show spectral features associated with the separate absorption bands of TAA and the PDI moiety. The reduced QYs, compared to the unsubstituted PDI acceptor, indicate a non radiative quenching mechanism in all triads. The CV data (chapter 5) were used to calculate the energies of possible CSSs and those results were used to assign the CR dynamics into the different Marcus regions. fs TA measurements reveal that all triads form a CSS upon excitation of the PDI moiety. The lifetimes of the involved states and the rate constants were determined by global exponential fits and global target analysis. The CR dynamics upon depopulation of the CSSs were investigated using external magnetic field dependent ns TA spectroscopy. The ns TA maps show that all triads recombine via CRT pathway populating the local 3PDI state in toluene and provided the respective lifetimes. The approximate QYs of triplet formation were determined using actinometry. The magnetic field dependent ns TA data reveal the exchange interaction 2J between singlet and triplet CSS for each triad. Those magnetic field dependent ns TA data in toluene were furthermore treated using a quantum mechanical simulation (done by U.E. Steiner) to extract the rate constants kT and kS for CRT and CRS, respectively. However, the error margins of kS were rather wide. Finally, the electronic couplings between the donor and the acceptor states were obtained by combining the aforementioned experimental results of the rate constants and applying the Bixon Jortner theoretical description of diabatic ET and Andersons perturbative theory of the exchange coupling. Therefore, the experimentally determined values of 2J and the calculated values of kCS and kT were used. The rate constant kS was calculated based on the electronic coupling V1CSS 1S0.
The rotationally hindered triads (chapter 6) show a strong influence of the degree of rotational restriction on the lifetimes and rate constants of the CS processes. The rate constants of CS are increasing with increasing rotational freedom. The magnetic field dependent decay data show that the exchange interactions increase with increasing rotational freedom. Based on the CR dynamics, the calculated electronic couplings of the ET processes reflect the same trend along the series. Here, only singlet couplings turned out to be strongly influenced while the triplet couplings are not. Therefore, this series shows that the ET dynamics of donor acceptor systems can strongly be influenced by restricting the rotational freedom.
In the substituted triptycene triads (chapter 7), decreasing electron density in the bridging unit causes a decrease of the CS rate constants. The magnetic field dependent decay data show that with decreasing electron density in the bridge the exchange interaction decreases. The CR dynamics-based rate constants and the electronic couplings follow the same trend as the exchange interaction. This series shows that varying the HOMO/LUMO levels of the connecting bridge between donor and acceptor strongly influences the ET processes.
In the anthracene bridge triads (chapter 8), the CS process is slow in both triads. The CR was fast in the anthracene triad and is slowed down in the methoxy substituted anthracene bridge triad. The increase of the exchange interaction with increasing electron density in the bridge was more pronounced than in the substituted triptycene triads. Thus, the variation of electron density in the bridge strongly influences the ET processes even though the rotation is restricted.
In this thesis, it was shown that the influence of the rotational hindrance as well as the electron density in a connecting bridge have strong influence on all ET processes and the electronic coupling in donor acceptor systems. These approaches can therefore be used to modify magnetic properties of new materials.
Fundamental studies of functional nucleic acids: aptamers, riboswitches, ribozymes and DNAzymes
(2020)
This review aims at juxtaposing common versus distinct structural and functional strategies that are applied by aptamers, riboswitches, and ribozymes/DNAzymes. Focusing on recently discovered systems, we begin our analysis with small-molecule binding aptamers, with emphasis on in vitro-selected fluorogenic RNA aptamers and their different modes of ligand binding and fluorescence activation. Fundamental insights are much needed to advance RNA imaging probes for detection of exo- and endogenous RNA and for RNA process tracking. Secondly, we discuss the latest gene expression–regulating mRNA riboswitches that respond to the alarmone ppGpp, to PRPP, to NAD+, to adenosine and cytidine diphosphates, and to precursors of thiamine biosynthesis (HMP-PP), and we outline new subclasses of SAM and tetrahydrofolate-binding RNA regulators. Many riboswitches bind protein enzyme cofactors that, in principle, can catalyse a chemical reaction. For RNA, however, only one system (glmS ribozyme) has been identified in Nature thus far that utilizes a small molecule – glucosamine-6-phosphate – to participate directly in reaction catalysis (phosphodiester cleavage). We wonder why that is the case and what is to be done to reveal such likely existing cellular activities that could be more diverse than currently imagined. Thirdly, this brings us to the four latest small nucleolytic ribozymes termed twister, twister-sister, pistol, and hatchet as well as to in vitro selected DNA and RNA enzymes that promote new chemistry, mainly by exploiting their ability for RNA labelling and nucleoside modification recognition. Enormous progress in understanding the strategies of nucleic acids catalysts has been made by providing thorough structural fundaments (e.g. first structure of a DNAzyme, structures of ribozyme transition state mimics) in combination with functional assays and atomic mutagenesis.
Mittels einer fünfstufigen Synthese wurde das 2,2´-Ditetracen als Modellsystem zur Erforschung von singlet fission-Prozessen hergestellt. Die Synthese wurde mit einer Gesamtausbeute von 21 % durchgeführt, wobei der Schlüsselschritt, die Kopplung der beiden Monomere, durch eine Suzuki-Kopplung erfolgte. Das gewünschte Produkt konnte nach gründlicher Reinigung mittels Gradientensublimation als leuchtend rote Einkristalle erhalten werden. Während die Emissionsspektren der Einzelmoleküle nahezu identisch sind, zeigen Untersuchungen mittels Photolumineszenzspektroskopie eine Rotverschiebung im Emissionsspektrum des Dimer-Einkristalls im Vergleich zum Einkristall des Tetracen-Monomers. Durch theoretische Berechnung konnte die Absenkung des S1-Zustands des Dimers im Kristall erklärt werden, wodurch die Energiebedingung für singlet fission (2 E(T1) ≤ E(S1)) nicht mehr erfüllt ist.
Weiterhin wurden mehrere mit Alkylgruppen und Vinylgruppen substituierte Tetracenderivate synthetisiert und diese mittels optischer und elektrochemischer Methoden auf ihre Eigenschaften hin untersucht. Es wurde bei allen synthetisierten Derivaten eine Rotverschiebung der Hauptbanden im Absorptionsspektrum beobachtet, was durch einen kleineren HOMO-LUMO-Abstand im Vergleich zum nicht substituierten Tetracen erklärt wird. Es wurde zudem eine erhöhte Stabilität dieser Derivate gegenüber Umwelteinflüssen wie Licht und Sauerstoff, die die Bildung von Endoperoxiden und Dimeren zur Folge haben, festgestellt. Dies kann auf sterische Effekte sowie die Stabilisierung des biradikalischen Zustands dieser Moleküle durch Hyperkonjugation und Resonanzeffekte zurückgeführt werden.
In the course of this work, a total of three photocatalytically active dyads for proton reduction could be synthesized together with the associated individual components. Two of them, D1 and D2, comprised a [Ru(bpy)3]2+ photosensitizer and D3 an [Ir(ppy)2bpy]+ photosensitizer. A Ppyr3-substituted propyldithiolate [FeFe] complex was used as catalyst in all systems. The absorption spectroscopic and electrochemical investigations showed that an inner-dyadic electronic coupling is effectively prevented in the dyads due to conjugation blockers within the bridging units used. The photocatalytic investigations exhibited that all dyad containing two-component systems (2CS) showed a significantly worse performance than the corresponding bimolecular three-component systems (3CS). Transient absorption spectroscopy showed that the 2CS behave very similarly to the associated multicomponent systems during photocatalysis. The electron that was intended for the intramolecular transfer from the photosensitizer unit to the catalyst unit within the dyads remains at the photosensitizer for a relatively long time, analogous to the 3CS and despite the covalently bound catalyst. It is therefore assumed that this intramolecular electron transfer is likely to be hindered as a result of the weak electronic coupling caused by the bridge units used. Instead, the system bypasses this through an intermolecular transfer to other dyad molecules in the immediate vicinity. In addition, with the help of emission quenching experiments and electrochemical investigations, it could be clearly concluded that all investigated systems proceed via the reductive quenching mechanism during photocatalysis.
Ziel der vorliegenden Arbeit war es zum einen, das Potential von chiralen Eisenporphyrin- und Mangansalen-Katalysatoren zur kinetischen Racematspaltung sekundärer Allylalkohole durch asymmetrische Epoxidierung auszuloten. Zum anderen sollten Untersuchungen zum Mechanismus der Jacobsen-Katsuki-Epoxidierung durchgeführt werden; ein besonderes Augenmerk lag dabei auf der Fragestellung, welche Faktoren dazu führen, dass bei der Umsetzung von cis-Olefinen ein Gemisch aus cis- und trans-Epoxiden erhalten wird. Eine Auswahl arylsubstituierter Allylalkohole IIa-f wurde mit den Katalysatoren Ia und Ib,c und 0.8 bzw. 0.6 Äquivalenten an Iodosobenzol als Sauerstoffdonor umgesetzt (Gl. I), wobei es zu einer kinetischen Racematspaltung kommt. Die Oxidation verläuft für beide Katalysatorsysteme sowohl chemoselektiv (vorwiegend Epoxidierung) als auch diastereoselektiv (dr bis zu > 95:5). Als Hauptprodukte werden für die offenkettigen Allylalkohole IIa,e,f die threo-konfigurierten Epoxyalkohole III erhalten, während die cyclischen Allylakohole IIb-d die entsprechenden cis-Epoxyalkohole III lieferen. 1,1-Dimethyl-1,2-dihydro-2-naphthol (IIc) ist hierbei eine Ausnahme, da die CH-Oxidation dieses Substrats eine beachtliche Nebenreaktion darstellt. Der Hauptunterschied zwischen den Fe- und Mn-Katalysatoren liegt in der Enantioselektivität: Während mit dem Fe(porph*)-Komplex Ia nur Selektivitäten von maximal 43 Prozent ee (krel = 2.7) erzielt werden, erwiesen sich die Mn(salen*)-Komplexe Ib,c als geeignete Katalysatoren, mit denen ee-Werte von bis zu 80 Prozent (krel = 12.9) erreicht werden. Die in der kinetischen Racematspaltung erzielten Selektivitäten können durch ein synergistisches Zusammenwirken von hydroxy-dirigierendem Effekt einerseits und sterischen Wechselwirkungen zwischen Substrat und Eisen-Komplex oder, im Falle des Mangan-Komplexes, Angriff des Olefins entlang der so genannten Katsuki-Trajektorie andererseits erklärt werden. Fazit: Die chiralen Mn(salen*)-Komplexe Ib,c sind wirkungsvolle Katalysatoren für die asymmetrische Epoxidierung racemischer sekundärer Allylalkohole II. In exzellenten Chemo- und Diastereoselektivitäten entstehen die entsprechenden Epoxyalkohole III mit ee-Werten bis zu 80 Prozent. Die zurückbleibenden Allylalkohole werden dabei bis zu 53 Prozent ee angereichert. Im Vergleich dazu weist der Eisenkomplex Ia eine ungleich geringere Enantioselektivität auf. Mechanistische Untersuchungen mit Vinylcyclopropan Va ergeben, dass die Jacobsen-Katsuki-Epoxidierung nicht über ein kationisches, sondern über ein radikalisches Intermediat abläuft. Dies wird anhand von Produktstudien durch reversed phase-HPLC-Analytik belegt. In weitergehenden Untersuchungen mit cis-Stilben (Vb) und cis--Methylstyrol (Vc) als Sonden zur cis/trans-Isomerisierung wurde festgestellt, dass die Diastereoselektivität der Epoxidierung nicht nur vom Gegenion des Mangankatalysators Ib, sondern auch von der eingesetzten Sauerstoffquelle [OxD] abhängt. Daher musste der Katalysezyklus (Schema A) um eine diastereoselektivitäts-bestimmende Gabelung erweitert werden: Das primär entstehende MnIII(OxD)-Addukt kann entweder unter Abspaltung der Fluchtgruppe zum etablierten MnV(oxo)-Komplex reagieren (Weg 1) oder direkt das Olefin epoxidieren (Weg 2). Während die Sauerstoffübertragung durch die Oxo-Spezies stufenweise über ein Radikalintermediat verläuft und damit zu einer Mischung aus cis- und trans-Epoxid führt, erfolgt der Lewisäure-aktivierte Sauerstofftransfer konzertiert. Der Gegenion-Effekt auf die cis/trans-Isomerisierung erklärt sich dahingehend, dass die Natur des Anions (koordinierend oder nicht-koordinierend) die Lebensdauer des Radikalintermediats und/oder die Lage und Selektivität der Energiehyperflächen der verschiedenen Spinzustände des MnV(oxo)-Oxidans beeinflusst. Fazit: In der Jacobsen-Katsuki-Epoxidierung existiert neben dem etablierten MnV(oxo)-Oxidans zumindest noch ein weiteres; dabei handelt es sich um das MnIII(OxD)-Addukt, dessen Sauerstoff Lewissäure-aktiviert übertragen wird. Ein unterschiedlicher Anteil der beiden Reaktionskanäle erklärt die Unterschiede im Ausmaß der cis/trans-Isomerisierung. Auch das Gegenion des Mangan-Komplexes Ib beeinflusst die cis/trans-Diastereoselektivität. Mit koordinierenden Gegenionen dominiert Isomerisierung zum trans-Epoxid, während nicht-koordinierende Gegenionen bevorzugt zum cis-Epoxid führen.
Aufbauend auf einen Datensatz von etwa 70 antimalaria-aktiven Verbindungen wurde mit Hilfe des CoMSIA-Verfahrens ein QSAR(Qantitative Structure Activity Relationship)-Modell erstellt, das in der Lage ist antiplasmodiale Aktivitäten von Verbindungen aus der Substanzklasse der Naphthylisochinolin-Alkaloide vorherzusagen. Da die behandelten Strukturen ein sehr kompliziertes konformatives Verhalten aufweisen, mussten für ein möglichst flexibles Alignment (unter Verwendung von FLEXS und GASP) eigene Abläufe entwickelt werden, die schließlich weitestgehend automatisiert werden konnten. Das erstellte Modell erlaubte es darüber hinaus, die für die Aktivität verantwortlichen strukturellen Merkmale zu identifizieren und so entscheidende Anregungen zur Vereinfachung des relativ komplizierten Grundgerüsts zu geben. Die Vorschläge wurden zu einem großen Teil bereits synthetisch verwirklicht, wobei die anschließend experimentell gefundenen Aktivitäten die vorher berechneten sehr gut bestätigten. Die neu entwickelten Substanzen befinden sich derzeit im Patentprüfungsverfahren.
Die vorliegende Arbeit beschäftigt sich mit der Entwicklung von künstlichen Rezeptoren für biologisch relevante Oligopeptide und besteht aus drei Teilen. Im ersten Teil wurde auf der Basis von computergestützten de novo Berechnungen ein künstlicher Rezeptor für den D-Alanin-D-Alanin-C-Terminus entwickelt. Diese Peptidsequenz befindet sich in bakteriellen Zellwänden und nimmt eine Schlüsselfunktion in der Wirkungsweise des Antibiotikums Vancomycin ein. Zur Entwicklung dieses Rezeptors wurde ein Guanidiniocarbonylpyrrol als Bindungsmotiv für Carboxylate mit einer Cyclotribenzylen-Einheit verknüpft. Letztere ist entsprechend der theoretischen Berechnungen in der Lage, die Methylreste des Alanins größenselektiv durch hydrophobe Wechselwirkungen zu koordinieren. Dieser Rezeptor wurde in umfangreichen Bindungsstudien bezüglich seiner Affinität in Wasser und seiner Substratselektivität untersucht. Zur Erhöhung der Löslichkeit und zur Bestimmung der Komplexstruktur mit NMR-Techniken in Wasser wurde ein weiteres Derivat des Rezeptors synthetisiert, welches in peripherer Position mit Triethylenglykolseitenketten substituiert ist. Auf diese Weise gelang es, einen hoch affinen (log K = 4,7) und hoch selektiven künstlichen Rezeptor für den D-Ala-D-Ala-Terminus darzustellen und umfassend zu charakterisieren. So konnte gezeigt werden, dass ein de novo Design derartiger Rezeptoren prinzipiell möglich ist. In einem weiteren Teilprojekt wurde ein künstlicher Rezeptor für die interne RGD-Peptidsequenz entwickelt. Diese nimmt eine zentrale Funktion in Zell-Zell- und Zell-Matrix-Erkennungsprozessen ein. Dieses Teilprojekt wurde in Zusammenarbeit mit dem Arbeitskreis Schrader (Universität Marburg) durchgeführt. Dazu wurde ein Bindungsmotiv für Alkylguanidine (in der Seitenkette von Arg, R) über einen geeigneten Spacer mit einem Bindungsmotiv für Carboxylate (in der Seitenkette von Asp, D) verknüpft. Nach der Synthese und Charakterisierung einer Reihe von vier Rezeptoren konnte die grundsätzliche Anwendbarkeit dieses Ansatzes bestätigt werden. Dabei konnte gezeigt werden, dass der verwendete Spacer für die Effektivität der Koordinierung von besonderer Bedeutung ist. Auf der Grundlage dieser Ergebnisse wurde in einem dritten Teilprojekt ein kombinatorisches Festphasenprotokoll zur Optimierung derartiger Spacer entwickelt. Dabei wurde das Carboxylat-Bindungsmotiv (ein Guanidiniocarbonylpyrrol) auf einem polymeren Träger immobilisiert. Zu diesem Zweck wurden umfangreiche Studien zur Synthese von Pyrrol-Tricarboxylaten und zur Verwendung verschiedener Schutzgruppen unternommen. Die Eigenschaften von drei Schutzgruppen unterschiedlicher Sensitivität (basisch, stark sauer und photolytisch spaltbar) auf dem Acylguanidin wurden in Lösung und an der festen Phase untersucht. Zu diesem Zweck wurde ein umfangreiches HPLC-Protokoll zur Charakterisierung der Reaktion entwickelt. So gelang die Entwicklung und Etablierung eines universell einsetzbaren Protokolls zur Optimierung derartiger Rezeptoren, womit zahlreiche Anwendungsmöglichkeiten in der kombinatorischen Chemie aber auch in weiteren Teilbereichen wie der Katalyse oder der Chromatographie ermöglicht werden.
Although a broad variety of classes of bioactive compounds have already been isolated from seaweeds of the genus Dictyota, most different species are still chemically and biologically unexplored. Dictyota species are well-known brown seaweeds belonging to the Dictyotaceae (Phaeophyta). The phytochemical composition within the genus Dictyota has recently received considerable interest, and a vast array of components, including diterpenes, sesquiterepenes, sterols, amino acids, as well as saturated and polyunsaturated fatty acids, have been characterized. The contribution of these valued metabolites to the biological potential, which includes anti-proliferative, anti-microbial, antiviral, antioxidant, anti-inflammatory, and anti-hyperpigmentation activities, of the genus Dictyota has also been explored. Therefore, this is the most comprehensive review, focusing on the published literature relevant to the chemically and pharmacologically diverse biopharmaceuticals isolated from different species of the genus Dictyota during the period from 1976 to now.
Artificial light-harvesting (LH) systems have been obtained by self-assembly of naphthalene diimide-functionalized zinc chlorin dyads and triad in nonpolar, aprotic solvents. UV-vis, CD, and steady-state emission spectroscopy as well as atomic force microscopy showed that rod-like structures are formed by excitonic interactions of zinc chlorin units, while the appended naphthalene diimide dyes do not aggregate at the periphery of the cylinders. In all cases, photoexcitation of the enveloping naphthalene diimides at 540 and 620 nm, respectively, was followed by highly efficient energy-transfer processes to the inner zinc chlorin backbone, as revealed by time-resolved fluorescence spectroscopy on the picosecond time-scale. As a consequence, the LH efficiencies of zinc chlorin rod aggregates were increased by up to 63%. The effective utilization of solar energy recommends these biomimetic systems for an application in electronic materials on the nanoscale.
Molecular functionality can be often directly attributed to given properties of the electronic wavefunction. Analogous to the potential energy surface, these properties can be represented as a function of the nuclear coordinates, giving rise to molecular “functional landscapes.” However, so far there has been no possibility for their systematic investigation. This perspective aims to discuss the development of new theoretical methods based on the multistate extension of the metadynamics approach, employing electronic collective variables. This emerging methodology allows to explore functional landscapes and to gain a deeper understanding of the structure–function relation in molecules and complex molecular systems in the ground and excited electronic state.
No abstract available
In this communication we describe a helically chiral push-pull molecule named 9,10-dimethoxy-[7]helicene diimide, displaying fluorescence (FL) and circularly polarised luminescence (CPL) over nearly the entire visible spectrum dependent on solvent polarity. The synthesised molecule exhibits an unusual solvent polarity dependence of FL quantum yield and nonradiative rate constant, as well as remarkable gabs and glum values along with high configurational stability.
This thesis included the synthesis of conformationally stable chiral perylene bisimide (PBI) dyes, the study of their optical properties in solution and their chiral self-sorting behaviour in nonpolar solvents in which dimerization via pi-pi-stacking takes place. Furthermore, the influence of PBI core chirality on the properties of these dyes in the condensed state has been also studied. We have demonstrated and quantified the prevalence of chiral self-recognition over self-discrimination in pi-stacking dimerization of PBIs. It has been shown that this self-recognition event is compromised by the increasing flexibility of the structures related to the size of the OEG bridging units. Moreover, the inherent chirality of these PBIs has been proven to strongly influence their condensed state properties, for which large differences between the pure enantiomers and the racemates were revealed, as well as between the different bridged macrocyclic PBIs.
Two series of organic–inorganic composite materials were synthesized through solvothermal imine condensation between diketopyrrolopyrrole dialdehyde DPP-1 and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) in the presence of varying amounts of either amino- or carboxy-functionalized superparamagnetic iron oxide nanoparticles (FeO). Whereas high FeO loading induced cross-linking of the inorganic nanoparticles by amorphous imine polymers, a lower FeO content resulted in the formation of crystalline covalent organic framework domains. All hybrid materials were analyzed by magnetization measurements, powder X-ray diffraction, electron microscopy, IR, and UV/Vis absorption spectroscopy. Crystallinity, chromophore stacking, and visible absorption features are directly correlated to the mass fraction of the components, thus allowing for a fine-tuning of materials properties.
Inspired by the fact that sufficient solubility in aqueous media can be achieved by functional substitution of perylene bisimides (PBIs) with polar groups, one of the essential aims of this thesis was the design and successful synthesis of the new water-soluble PBI cyclophanes [2PBI]-1m and [2PBI]-1p, which are appended with branched, hydrophilic oligoethylene glycol (OEG) chains. Subsequently, the focus was set on the elucidation of properties of PBI cyclophane hosts which are also of relevance for recognition processes in biological systems. The performance of the new amphiphilic PBI cyclophane [2PBI]-1p as synthetic receptors for various natural aromatic alkaloids in aqueous media was thoroughly investigated. Alkaloids represent a prominent class of ubiquitous nitrogen containing natural compounds with a great structural variety and diverse biological activity. As of yet, no chromophore host acting as a molecular probe for a range of alkaloids such as harmine or harmaline is known. In addition, the self-association behavior of cyclophane host [2PBI]-1m and its reference monomer in water was studied in order to gain insights into the thermodynamic driving forces affecting the self-assembly process of these two PBI systems in aqueous environment. Moreover, the chirality transfer upon guest binding previously observed for a PBI cyclophane was investigated further. The assignment of the underlying mechanism of guest recognition to either the induced fit or conformational selection model was of particular interest.
Für die Aufklärung der absoluten Stereostruktur von chiralen Molekülen, die ein Chromophorsystem besitzen, hat sich die Kombination der experimentellen und theoretischen Spektroskopie des elektronischen Circulardichroismus (ECD) als Methode bewährt. In der vorliegenden Arbeit wurden die chiroptischen Eigenschaften von Bisbibenzyl-Makrocyclen, Mono- und Bis(cycloketo)porphyrinen, der Mohnblütenpigmente Nudicaulin I und II sowie von Bordipyrrol-Dimeren mit quantenchemischen Methoden untersucht. Zu diesem Zweck wurden verschiedene dichtefunktionaltheoretische (DFT) Ansätze und post-HF-Methoden, wie z. B. der Coupled-Cluster-Ansatz RI-CC2, bezüglich ihrer Eignung, die Grund- und angeregten Zustände (UV/vis- und ECD-Eigenschaften) der einzelnen Verbindungen korrekt wiederzugeben, evaluiert. Da bei quantenchemischen UV- und ECD-Rechnungen an ausgedehnten π-Systemen aufgrund energetisch nah beieinander liegender Anregungen die Wahrscheinlichkeit für ghost states und charge-transfer-Übergänge sowie Multireferenz-Problematiken steigt, wurden diese Aspekte genauer betrachtet. Die ersten zwei Phänomene lassen sich bereits auf TD-DFT-Niveau durch genaue Analyse der theoretischen spektroskopischen Daten ermitteln und unter Umständen durch entsprechend korrigierte Funktionale sogar sehr gut beschreiben. Im Gegensatz dazu können Doppelanregungsanteile überhaupt erst durch Rechnungen mit geeigneten Methoden, wie z. B. das hier verwendete SORCI-Verfahren, erfasst werden. Zusätzlich wurde das zur Auswertung von UV und ECD-Daten entwickelte Programm SpecDis um Funktionalitäten erweitert, welche die Berechnung des Übereinstimmungsgrades zweier UV- bzw. ECD-Kurven ermöglichen, und dadurch ein zusätzliches quantitatives Kriterium für die Verlässlichkeit des Spektrenvergleichs und folglich für die Zuordnung der absoluten Konfiguration bieten.
Although solid-state nuclear magnetic resonance (NMR) is a versatile analytical tool to study polymorphs and phase transitions of pharmaceutical molecules and products, this work summarizes examples of spontaneous and unexpected (and unwanted) structural rearrangements and phase transitions (amorphous-to-crystalline and crystalline-to-crystalline) under magic angle spinning (MAS) conditions, some of them clearly being due to the pressure experienced by the samples. It is widely known that such changes can often be detected by X-ray powder diffraction (XRPD); here, the capability of solid-state NMR experiments with a special focus on \(^{1}\)H-\(^{13}\)C frequency-switched Lee–Goldburg heteronuclear correlation (FSLG HETCOR)/MAS NMR experiments to detect even subtle changes on a molecular level not observable by conventional 1D NMR experiments or XRPD is presented. Furthermore, it is shown that a polymorphic impurity combined with MAS can induce a crystalline-to-crystalline phase transition. This showcases that solid-state NMR is not always noninvasive and such changes upon MAS should be considered in particular when compounds are studied over longer time spans.
The focus of this work was the development and application of highly efficient RNA catalysts for the site-specific modification of RNA with special focus on methylation. In the course of this thesis, the first methyltransferase ribozyme (MTR1), which uses m6G as the methyl group donor was developed and further characterized. The RNA product was identified as the natural modification m1A. X-Ray crystallography was used to solve the 3D structure of the ribozyme, which directly suggested a plausible reaction meachnism. The MTR1 ribozyme was also successfully repurposed for a nucleobase transformation reaction of a purine nucleoside. This resulted in a formyl-imidazole moiety directly on the intact RNA, which was directly used for further bioconjugation reactions. Finally, additional selections and reselections led to the identification of highly active alkyltransferase ribozymes that can be used for the labeling of various RNA targets
Deoxyribozymes (DNAzymes) are small, synthetic, single-stranded DNAs capable of catalysing chemical reactions, including RNA ligation. Herein, we report a novel class of RNA ligase deoxyribozymes that utilize 5’-adenylated RNA (5’-AppRNA) as the donor substrate, mimicking the activated intermediates of protein-catalyzed RNA ligation. Four new DNAzymes were identified by in vitro selection from an N40 random DNA library and were shown to catalyze the intermolecular linear RNA-RNA ligation via the formation of a native 3’-5’-phosphodiester linkage. The catalytic activity is distinct from previously described RNA-ligating deoxyribozymes. Kinetic analyses revealed the optimal incubation conditions for high ligation yields and demonstrated a broad RNA substrate scope. Together with the smooth synthetic accessibility of 5’-adenylated RNAs, the new DNA enzymes are promising tools for the protein-free synthesis of long RNAs, for example containing precious modified nucleotides or fluorescent labels for biochemical and biophysical investigations.
RNA-catalysed RNA methylation was recently shown to be part of the catalytic repertoire of ribozymes. The methyltransferase ribozyme MTR1 catalyses the site-specific synthesis of 1-methyladenosine (m\(^1\)A) in RNA, using O\(^6\)-methylguanine (m\(^6\)G) as methyl group donor. Here we report the crystal structure of MTR1 at a resolution of 2.8 Å, which reveals a guanine binding site reminiscent of natural guanine riboswitches. The structure represents the postcatalytic state of a split ribozyme in complex with the m1A-containing RNA product and the demethylated cofactor guanine. The structural data suggest the mechanistic involvement of a protonated cytidine in the methyl transfer reaction. A synergistic effect of two 2'-O-methylated ribose residues in the active site results in accelerated methyl group transfer. Supported by these results, it seems plausible that modified nucleotides may have enhanced early RNA catalysis and that metabolite-binding riboswitches may resemble inactivated ribozymes that have lost their catalytic activity during evolution.
Site-specific introduction of biorthogonal handles into RNAs is in high demand for decorating RNAs with fluorophores, affinity labels or other modifications. Aldehydes represent attractive functional groups for post-synthetic bioconjugation reactions. Here, we report a ribozyme-based method for the synthesis of aldehyde-functionalized RNA by directly converting a purine nucleobase. Using the methyltransferase ribozyme MTR1 as an alkyltransferase, the reaction is initiated by site-specific N1 benzylation of purine, followed by nucleophilic ring opening and spontaneous hydrolysis under mild conditions to yield a 5-amino-4-formylimidazole residue in good yields. The modified nucleotide is accessible to aldehyde-reactive probes, as demonstrated by the conjugation of biotin or fluorescent dyes to short synthetic RNAs and tRNA transcripts. Upon fluorogenic condensation with a 2,3,3-trimethylindole, a novel hemicyanine chromophore was generated directly on the RNA. This work expands the MTR1 ribozyme’s area of application from a methyltransferase to a tool for site-specific late-stage functionalization of RNA.
Nearly all classes of coding and non-coding RNA undergo post-transcriptional modification including RNA methylation. Methylated nucleotides belong to the evolutionarily most conserved features of tRNA and rRNA.1,2 Many contemporary methyltransferases use the universal cofactor S-adenosylmethionine (SAM) as methyl group donor. This and other nucleotide-derived cofactors are considered as evolutionary leftovers from an RNA World, in which ribozymes may have catalysed essential metabolic reactions beyond self-replication.3 Chemically diverse ribozymes seem to have been lost in Nature, but may be reconstructed in the laboratory by in vitro selection. Here, we report a methyltransferase ribozyme that catalyses the site-specific installation of 1-methyladenosine (m1A) in a substrate RNA, utilizing O6-methylguanine (m6G) as a small-molecule cofactor. The ribozyme shows a broad RNA sequence scope, as exemplified by site-specific adenosine methylation in tRNAs. This finding provides fundamental insights into RNA’s catalytic abilities, serves a synthetic tool to install m1A in RNA, and may pave the way to in vitro evolution of other methyltransferase and demethylase ribozymes.
Im Rahmen dieser Arbeit wurden Elektronentransferprozesse in Systemen, die auf Triphenylaminredoxzentren basieren, mit Hilfe spektroskopischer und elektrochemischer sowie spektroelektrochemischer Methoden studiert. Im ersten Teil der vorliegenden Arbeit wurden Bistriarylaminsysteme analog zu N,N,N’,N’-Tetra(4-methoxyphenyl)-1,4-phenylendiamin (1) untersucht, deren Radikalkationen eine für gemischtvalente Systeme typische breite und insbesondere bei 1 stark asymmetrische IVCT-Absorptionsbande zeigen. Die Analyse dieser Banden nach Hush sowie einem modifizierten Modell, das der Vibronic coupling-Theorie angelehnt ist, deutet auf die Abnahme der elektronischen Kopplung mit zunehmender Vergrößerung des zentralen Phenylenspacers durch Naphthalin- (2) bzw. Anthracenspacer (3) und damit größerer sterischer Hinderung hin. Gleichzeitig nimmt aber mit der Vergrößerung des -Systems des Spacers auch die Reorganisationsenergie  ab. Insgesamt verhalten sich alle drei Verbindungen sehr ähnlich, was insbesondere das Verhältnis von Absorptionsmaximum der IVCT-Bande zum zweifachen Wert der elektronischen Kopplung betrifft. Legt man vor allem das modifizierte Vibronic coupling-Modell zugrunde, so liegt dieses Verhältnis bei 1+, 2+ und 3+ sehr nahe bei 1, so daß alle drei Systeme sehr nahe am Übergang von Robin-Day-Klasse II zu Klasse III liegen. Weiterhin wurden über einen 1,4-Diethinylphenyl-Spacer verbrückte Bistriarylaminsysteme untersucht, bei denen durch Variation der Spacereinheit (1,4-Diethinylphenyl (5), 1,4-Diethinylnaphthalin (6), 1,4-Diethinyl-2,5-dimethoxyphenyl (10)) die Energie eines Brückenzustandes im Vergleich zu Zuständen, bei denen das Radikal an einem Triarylaminzentrum lokalisiert ist, schrittweise abgesenkt wird. Die auftretenden Elektronentransferprozesse können mit Hilfe eines Dreiniveaumodells mit zwei voneinander unabhängigen Elektronentransferkoordinaten beschrieben werden. Es zeigt sich, daß bei elektronenarmen Spacern, wie z.B. bei 5+, der Elektronentransfer nach einem Superexchange-Mechanismus erfolgt. Bei der Verwendung einer elektronenreichen Dimethoxy-substituierten Brücke wie in 10+ kann der Elektronentransfer neben dem Superexchange- auch nach einem Hopping-Mechanismus erfolgen. Bei Verbindungen, die einen 9,10-Diethinylanthracenspacer (8+ und 9+) enthalten, liegt der Brückenzustand energetisch sogar deutlich tiefer als der Zustand mit einem oxidierten Triphenylaminredoxzentrum. Im zweiten Abschnitt wurden gerichtete Elektronentransferprozesse an Redoxkaskaden und Dendrimeren, die auf Triarylaminredoxzentren basieren, studiert. Die Möglichkeit, die Redoxpotentiale von Triphenylaminzentren durch Substituenten zu beeinflussen, erlaubt die Synthese von Kaskaden mit einem vorgegebenen Redoxgradienten. Innerhalb einer Kaskade, die ein Acridin-Fluorophor, ein 4-Chlor-substituiertes sowie ein 4-Methoxy-substituiertes Triphenylaminredoxzentrum enthält (18), kann nach Anregung des Acridin-Chromophors in polaren Lösungsmitteln ein ladungsgetrennter Zustand erreicht werden, worauf sowohl statische und zeitaufgelöste Fluoreszenzmessungen als auch transientenspektroskopische Untersuchungen hinweisen. Die Lebensdauer kann durch Verlängerung der Redoxkaskade durch ein weiteres Aminzentrum deutlich vergrößert werden. In unpolaren Lösungsmitteln erfolgt dagegen keine Ladungstrennung über die gesamte Kaskade. Ebenso tritt bei 20 (Kaskade aus Acridin, 4 Methoxy-substituiertem Triphenylamin und 4-Chlor-substituiertem Aminzentrum), wo der Redoxgradient entgegen zu 18 gerichtet ist, kein Ladungstransfer auf. Im dritten Teil dieser Arbeit wurden Verbindungen untersucht, die neben 1,4 Phenylendiamineinheiten in para-Position unsubstituierte Triphenylamine enthalten und sich elektrochemisch polymerisieren lassen. Die Eigenschaften der dotierten redoxaktiven Polymere werden durch die enthaltenen p-Phenylendiamin- und Benzidin-Substrukturen dominiert, wofür hauptsächlich die geringe Wechselwirkung der einzelne Redoxzentren untereinander verantwortlich ist. Impedanzspektroskopische Untersuchungen zeigen eine Zunahme der Leitfähigkeit der dotierten Polymerfilme, wobei der Ladungstransfer vermutlich durch Hopping zwischen den p-Phenylendiamin- und Benzidinuntereinheiten erfolgt.
Two dipolar merocyanines consisting of the same π‐conjugated chromophore but different alkyl substituents adopt very different packing arrangements in their respective solid state with either H‐ or J‐type exciton coupling, leading to ultranarrow absorption bands at 477 and 750 nm, respectively, due to exchange narrowing. The social self‐sorting behavior of these push‐pull chromophores in their mixed thin films is evaluated and the impact on morphology as well as opto‐electronical properties is determined. The implementation of this well‐tuned two‐component material with tailored optical features allows to optimize planar heterojunction organic photodiodes with fullerene (C\(_{60}\)) with either dual or single wavelength selectivity in the blue and NIR spectral range with ultranarrow bandwidths of only 11 nm (200 cm\(^{-1}\)) and an external quantum efficiency of up to 18% at 754 nm under 0 V bias. The application of these photodiodes as low‐power consuming heart rate monitors is demonstrated by a reflectance‐mode photoplethysmography (PPG) sensor.
Tropische Infektionskrankheiten sind noch immer die Haupttodesurache in vielen Ländern der Dritten Welt. Unter ihnen ist Malaria neben der Immunschwächekrankheit AIDS und Tuberkulose am weitesten verbreitet. Laut WHO erkrankten allein im Jahr 2016 rund 216 Millionen Menschen an Malaria und weltweit verstarben 445.000 Menschen an den Folgen dieser Infektion. Solange die Wirksamkeit des Impfstoffs RTS,S/AS01 gegen Malaria noch Schwachstellen aufweist und andere Impfstoff-Kanditaten sich noch in präklinischen Testphasen befinden, ist vor allem die Entwicklung neuer Wirkstoffe, auch im Hinblick auf die rasante Ausbreitung von Resistenzen gegen herkömmliche Medikamente, weiterhin eine dringende Aufgabe.
Eine vielversprechende Wirkstoffklasse mit interessanten strukturellen Eigenschaften und einer ungewöhnlichen Biosynthese aus Acetat-Einheiten sind die Naphthylisochinolin-Alkaloide aus den beiden paläotropischen Pflanzenfamilien der Dioncophyllaceae und Ancistrocladaceae. Die Naphthylisochinolin-Alkaloide sind hervozuheben aufgrund ihrer exzellenten Aktivität gegen Plasmodium falciparum, den Erreger der Malaria tropica, sowie wegen ihrer Wirksamkeit gegen Erreger weiterer Krankheiten wie beispielsweise AIDS, Leishmaniose und Afrikanische Trypanosomiasis. Auch im Kampf gegen maligne Erkrankungen wie Leukämie und pankreatischen Krebs werden sie aufgrund ihrer cytotoxischen Eigenschaften als vielversprechende Leitstrukturen betrachtet.
Die strukturell beeindruckendsten Naphthylisochinolin-Alkaloide sind die dimeren Mbandakamine, die von unserer Arbeitsgruppe vor einiger Zeit aus einer kongolesischen Ancistrocladus-Liane isoliert wurden. Sie besitzen sieben stereogene Elemente und sind die ersten natürlich vorkommenden Dimere mit einer höchst unsymmetrischen 6',1''-gekuppelten zentralen Biarylachse. Diese impliziert eine außergewöhnlich hohe sterische Hinderung an der zentralen Achse, wie sie noch in keinem anderen dimeren Naphthylisochinolin-Alkaloid gefunden wurde. Verbunden mit ihren bemerkenswerten und vielseitigen pharmakologischen Wirkeigenschaften sind sie ausgesprochen interessante Moleküle für eine synthetische Erschließung.
Ziel dieser Arbeit war die erstmalige Totalsynthese von Mbandakamin A und B sowie die Synthese ihrer monomeren Hälften 5-epi-Korupensamin E und 8-O-Methylkorupensamin A. Zudem sollten weitere Naphthylisochinolin-Dimere, die bei der Synthese der Mbandakamine anfallen, isoliert und charakterisiert werden. Alle neuen mono- und dimeren Naphthylisochinoline sollten abschließend am Schweizerischen Tropen- und Public-Health-Institut auf ihre biologische Aktivität getestet werden.
Zusätzlich gelang im Rahmen eines Kooperationsprojekts erstmals die stereochemische Charakterisierung des strukturell ganz neuartigen, inhärent chiralen Wolframbiscorrols durch online HPLC-ECD-Analyse in Kombination mit quantenchemischen Rechnungen.
In terms of the need of environmentally benign renewable and storable energy sources, splitting of water into hydrogen and oxygen by using sunlight is a promising approach. Hereby, water oxidation catalysts (WOCs) are required to perform the water oxidation comprising the transfer of four electrons to provide the reducing equivalents for producing hydrogen. The class of Ru(bda) (bda = 2,2'-bipyridine-6,6'-dicarboxylate) catalysts has proven to be efficient for this reaction.
In this thesis, ligand exchange processes in Ru(bda) complexes have been analyzed and the formation of multinuclear macrocyclic WOCs was studied. Based on the knowledge acquired by these studies, new multinuclear cyclic Ru(bda) complexes have been synthesized and their catalytic efficiencies in homogeneous water oxidation have been investigated. Going one step further for setting up functional devices, molecular WOCs have been immobilized on conducting or semiconducting supporting materials. Direct anchoring on carbon nanotubes generated a promising materials for further applications.
Catalytic water splitting is a viable process for the generation of renewable fuels. Here it is reported for the first time that a trinuclear supramolecular Ru(bda) (bda: 2,2′‐bipyridine‐6,6′‐dicarboxylate) catalyst, anchored on multi‐walled carbon nanotubes and subsequently immobilized on glassy carbon electrodes, shows outstanding performance in heterogeneous water oxidation. Activation of the catalyst on anodes by repetitive cyclic voltammetry (CV) scans results in a catalytic current density of 186 mA cm\(^{−2}\) at a potential of 1.45 V versus NHE. The activated catalyst performs water oxidation at an onset overpotential of 330 mV. The remarkably high stability of the hybrid anode is demonstrated by X‐ray absorption spectroscopy and electrochemically, revealing the absence of any degradation after 1.8 million turnovers. Foot of the wave analysis of CV data of activated electrodes with different concentrations of catalyst indicates a monomolecular water nucleophilic attack mechanism with an apparent rate constant of TOFmax (turnover frequency) of 3200 s\(^{−1}\).
Two di- and tetranuclear Ru(bda) (bda: 2,2′-bipyridine-6,6′-dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di- to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 \(^{-1}\) to 8.7 \(^{-1}\) and 10.5 s\(^{-1}\) in chemical and 0.6 s\(^{-1}\) to 3.3 \(^{-1}\) and 5.8 \(^{-1}\) in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure–activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.
Poorly water-soluble drugs frequently solubilize into bile colloids and this natural mechanism is key for efficient bioavailability. We tested the impact of pharmaceutical polymers on this solubilization interplay using proton nuclear magnetic resonance spectroscopy, dynamic light scattering, and by assessing the flux across model membranes. Eudragit E, Soluplus, and a therapeutically used model polymer, Colesevelam, impacted the bile-colloidal geometry and molecular interaction. These polymer-induced changes reduced the flux of poorly water-soluble and bile interacting drugs (Perphenazine, Imatinib) but did not impact the flux of bile non-interacting Metoprolol. Non-bile interacting polymers (Kollidon VA 64, HPMC-AS) neither impacted the flux of colloid-interacting nor colloid-non-interacting drugs. These insights into the drug substance/polymer/bile colloid interplay potentially point towards a practical optimization parameter steering formulations to efficient bile-solubilization by rational polymer selection.
Water‐soluble multinuclear complexes based on ruthenium 2,2′‐bipyridine‐6,6′‐dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three‐component visible light‐driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co‐solvents and with increasing oligomer length. In‐depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self‐assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s\(^{−1}\)) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)‐based photocatalytic water oxidation systems.
Eine Reihe von Acetylen-verknüpften Perylenbisimid(PBI)-Makrozyklen mit unterschiedlicher Ringgröße wurde durch Palladium-katalysierte Homokupplung synthetisiert und mit Hilfe von Recycling-GPC getrennt. Diese Makrozyklen wurden durch NMR-Spektroskopie und Massenspektrometrie charakterisiert und weiterhin die photophysikalischen Eigenschaften durch UV/Vis-Absorptions- und Fluoreszenzemissions-Messungen untersucht. Die Selbstorganisation dieser PBI-Makrozyklen zu hochgeordneten Nanostrukturen auf HOPG-Oberflächen wurde mittels Rasterkraftmikroskopie untersucht.
It is demonstrated that the di‐\(\pi\)‐methane (DPM) rearrangement of carbonyl‐substituted dibenzobarrelene (9,10‐dihydro‐9,10‐ethenoanthracene) derivatives is induced by visible‐light‐induced triplet photosensitization with Ir(ppy)\(_{3}\), Ir(dFppy)\(_{3}\) or 1‐butyl‐7,8‐dimethoxy‐3‐methylalloxazine as catalysts, whereas derivatives that lack carbonyl substituents are photoinert under these conditions. Notably, the products are formed almost quantitatively.
The effective binding of anions like carboxylates and phosphates in aqueous solutions is of particular interest for various reasons. The natural archetypes of effective anion receptors are enzymes that contain often arginine as relevant amino acid in the binding pocket. For this reason, one class of artificial anion receptors that emerged more than two decades ago mimics the anion binding with the guanidinium group present in the amino acid side chain. In 1999, Schmuck and coworkers developed a new class of guanidinium-based oxo anion receptor that binds carboxylates even in aqueous media. The binding modes of the 2-(guanidiniocarbonyl)-1H-pyrroles are based on individually weak non-covalent interaction between artificial host and substrate like ion pairing and multiple hydrogen bonds. The zwitterionic derivative with substitution of a carboxylate group in position 5 of the pyrrole ring system shows a strong self-assembly to discrete dimers (dimer 1) with an estimated association constant of 170 M-1 even in water. In order to further improve the structure motif for an effective oxo anion binding it is therefore of great interest to quantify the different intermolecular interactions between two monomeric units of 1. Against this background several theoretical ab initio studies were conducted in order to elucidate the influences of intrinsic properties as well as solvent effects on the stability of self-assembled dimers. In chapter 4.1 the molecular interactions in dimer 1 were investigated by comparison to various “knock-out” analogues. In these analogues single hydrogen bonds were switched off by substitution of hydrogen donor atoms with either methylene groups or ether bridges. The calculations were done for vacuum and solvation, as represented by a conductor-like polarizable continuum. It could be shown that the application of a simple continuum solvent model fails to predict the absolute energies of the knock-out analogues in strongly polar solvents. However, the calculated trends can explain the relative stabilities. In chapter 4.2 the structural similarity of arginine with structure 1 was used in order to examine the dependence of self-assembly from the flexibility of the molecular structure. In chapter 4.2.1 new global minimum structures of the canonical and zwitterionic arginine in gas phase were found by means of exhaustive force field based conformational searches in conjunction with ab initio structure optimizations of the lowest energy conformers. Most of the newly identified minimum conformers of both the zwitterionic and canonical tautomer revealed geometrical arrangements with hitherto unreported stacked orientations of the terminal groups. Finally a novel global minimum structure was detected that is more than 8 kJ mol-1 lower in energy than the previously published conformers. The same strategy for finding minimum energy conformers of the arginine monomer has also been employed for the arginine dimer structures. While previous theoretical studies favoured directed hydrogen bonds the new global minimum structure MMFF1 is about 60 kJ mol-1 more stable and exhibits a stacked orientation of the guanidinium and carboxylate groups. The importance of rigidity on the dimer stability was proven by calculations of an artificially stiffened arginine dimer system. The high binding affinity dimer 1 results by about 50% from the rigidity of the monomers which prevents any intramolecular stabilization. In chapter 4.3 novel structure motifs with varying ring systems have been examined on a DFT level of theory in order to make proposals for an improved carboxylate binding motif. The direct dependency of the dimerization energy on an increasing dipole moment was demonstrated by various anellated ring structures. The influence of the delocalization in the monomer on the dimerization energy was examined by variation of the electronic structure of electronically decoupled biphenylenes. With the aid of various substituted 7-guanidinioindole-2-carboxylate derivatives we could show that the carbonyl function is mainly responsible for the advantageous preorganisation, whereas the effect on the acidity seems to be only of minor importance. In the last chapter cooperativity effects in supramolecular assemblies have been investigated. This was achieved by NMR shift calculations of adenosine-carboxylic acid complexes as model systems and comparison to experimental low-temperature NMR studies. We could demonstrate that only by applying vibrational averaged NMR shifts the experimental proton shifts obtained at very low temperatures in the hydrogen bond exchange regime could be reproduced.
Protein-like enwrapped perylene bisimide chromophore as bright microcrystalline emitter material
(2019)
Strongly emissive solid‐state materials are mandatory components for many emerging optoelectronic technologies, but fluorescence is often quenched in the solid state owing to strong intermolecular interactions. The design of new organic pigments, which retain their optical properties despite their high tendency to crystallize, could overcome such limitations. Herein, we show a new material with monomer‐like absorption and emission profiles as well as fluorescence quantum yields over 90 % in its crystalline solid state. The material was synthesized by attaching two bulky tris(4‐tert‐butylphenyl)phenoxy substituents at the perylene bisimide bay positions. These substituents direct a packing arrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts, in which chromophores are rigidly embedded in protein environments.
Wasserstoffbrückengesteuerte Ausrichtung von Merocyaninfarbstoffen für photorefraktive Materialien
(2008)
Merocyaninchromophore spielen eine herausragende Rolle bei der Entwicklung von photorefraktiven Materialien für Anwendungen in der Holographie. Der photorefraktive Effekt beruht auf einer Orientierung der dipolaren Merocyanine in einem elektrischen Feld. Diese können umso effektiver ausgerichtet werden, je größer ihr Dipolmoment ist. Folglich sollten Merocyanine mit sehr großen Dipolmomenten den gewünschten Effekt hervorbringen. Es hat sich jedoch gezeigt, dass solche Merocyanine Dimere mit antiparalleler zentrosymmetrischer Struktur bilden. In dieser Anordnung addieren sich die Dipolmomente destruktiv, so dass die dipolare Eigenschaft des Materials verloren geht. In dieser Arbeit ist es gelungen, Merocyanine über sechsfache Wasserstoffbrückenbindungen zu supramolekularen Strukturen mit großen resultierenden Dipolmomenten zu assoziieren. Diese Komplexe werden in schwach polaren Lösungsmitteln sogar bei sehr niedrigen Farbstoffkonzentrationen gebildet.
In der vorligenden Arbeit wurden Vanadium-abhängige Bromidperoxidase- (VBPO-) Modelle zur stereoselektiven Synthese funktionalisierter O-Heterocyclen entwickelt, die durch Vanadium-katalysierte Oxygenierung von Bishomoallylalkoholen mechanistisch untersucht wurden. Weiterhin wurden Bromcyclisierungen von Bishomoallylalkoholen auf enzymatischem (VBPO), oxidativem und radikalischem Weg für Referenzprodukte einer neuen Variante der Bromcyclisierung durch Vanadium-katalysierte Bromidoxidation durchgeführt. Die Selektivitätsmuster aus den Synthesen ß-hydroxylierter und ß-bromierter Tetrahydrofurane wurden anschließend innerhalb einfacher Naturstoffsynthesen genutzt. Anhand eigener Vorarbeiten wurden neue Vanadium(V)-Komplexe aus Triethoxyvanadat mit tridentaten Schiffbaseliganden, basierend auf Salicylaldehyd und Aminoalkoholen mit strukturell unterschiedlichen Seitenketten in quantitativen Ausbeuten synthetisiert und charakterisiert (51V-NMR, UV und IR). In Test-Cyclisierungen unterschiedlicher Bishomoallylalkohole eignete sich VO(salin)(OEt) mit hohen Umsätzen und guten Regio- wie Stereoselektivitäten am besten. Die relative Geschwindigkeitskonstante (krel = 120±20) der Vanadium-katalysierten Oxidation des Testsystems konnte über Konkurrenzkinetik (Alkenol versus Alken) ermittelt werden und weist deutlich auf eine Bindung des Alkenols an Vanadium während der Oxidation hin. Um die Regio- und Stereoselektivitäten Vanadium-Schiffbase-katalysierter Oxidationen von Bishomoallylalkoholen verstehen zu können, wurden stereochemische Studien anhand des Testsystems durchgeführt. Dessen Oxidation wird demnach im selektivitätsbestimmenden Schritt dem Metallzentrum abgewandt in like-Position bevorzugt gebildet und führt zu dem cis-konfigurierten Tetrahydrofuran als Hauptprodukt. Im Folgenden wurden Vanadium-katalysierte Oxidationen unterschiedlich substituierter bishomoallylischer Alkohole durchgeführt, sämtliche Oxidationen führten regioselektiv zu Tetrahydrofuranen als Hauptprodukte, die Oxygenierung Dimethyl-substituierter Pentenole lieferte durch Substitution an C-1 selektiv cis-konfigurierte Tetrahydrofurane, 2-Substitution führte ebenso wie 3-Substitution zu trans-konfigurierte Oxolanen. Alkohole nicht aktivierter Olefine wurden in der Reihenfolge C-1 ? C-3 mit höherer Selektivität zu trans-konfigurierten Tetrahydrofuranen gebildet. Die Regio- und Stereoselektivitäten der radikalischen Bromcyclisierungen folgen den schon in früheren Arbeiten unserer Arbeitsgruppe aufgestellten Richtlinien (5-exo-trig; 2,5-trans, 2,4-cis und 2,3-trans). Die ionischen Bromcyclisierungen 5,5-dimethylierter Bishomoallyl-alkohole mittels NBS verliefen komplementär zu den Radikal-Cyclisierungen regioselektiv und in Abhängigkeit der Phenylsubstituenten an C-1 - C-3 stereoselektiv zu den 2,5-trans-, 3,5-cis- und 4,5-trans-konfigurierten Tetrahydropyranen. Aus der Bromcyclisierung prochiraler Pentenole in Gegenwart eines Acetonpulvers aus Ascophyllum nodosum (A.n.A.P.) konnte das b-bromierte Tetrahydrofuran racemisch in 87proz. Ausbeute erhalten werden. Ebensowenig führte der Einsatz chiraler Liganden in der Vanadium-katalysierten Oxygenierung prochiraler Penten-1-ole bei guten Ausbeuten (>80%) zu Enantiomeren-angereicherten Tatrahydrofuranen. Innerhalb einfacher Naturstoffsynthesen wurden cis-Pityol, Linalooloxid sowie (-)-epi-Bisabololoxid selektiv unter Standardbedingungen der Vanadium-Schiffbase-katalysierten Oxidationen mit VO(salin)(OEt) und TBHP dargestellt. Die Stereoselektivitäten steigen proportional zu dem Größenunterschied der Substituenten an Position 1 der Bishomoallylalkohole. Abschließend wurde durch Vanadium-katalysierte Bromidoxiadtion mit TBHP eine neue dreistufige Totalsynthese der vier "natürlichen" Muscarin-Isomere ausgearbeitet. Die Gesamtausbeuten dieser Synthesen liegen zwischen 3.0 und 19.9%.
This thesis deals with the synthesis of improved organic semiconductors, the detailed investigation of the molecular properties and the solid state arrangements revealed by single crystal X-ray diffraction and finally the development of structureperformance dependencies by measuring of the charge carrier mobilities of the derivatives in thin film transistors. The two main-goals of this thesis were achieved. Well soluble acene derivatives for spin-coated TFTs were obtained, showing charge carrier mobilities in the range of polymer p-type materials. Novel core-fluorinated perylene bisimide dyes were synthesized particularly and the use of electron deficient substituents lead to PBIs with outstanding air-stable mobilities in thin film transistors prepared by vacuum deposition techniques. The relationship between performance, air stability and solid state packing was elucidated in detail by single crystal X-ray diffraction analysis.
AbstractWater oxidation catalysis is a key step for sustainable fuel production by water splitting into hydrogen and oxygen. The synthesis of a novel coordination oligomer based on four Ru(bda) (bda = 2,2′‐bipyridine‐6,6′‐dicarboxylate) centers, three 4,4′‐bipyridine (4,4′‐bpy) linkers, and two 4‐picoline (4‐pic) end caps is reported. The monodispersity of this tetranuclear compound is characterized by NMR techniques. Heterogeneous electrochemical water oxidation after immobilization on multi‐walled carbon nanotubes (MWCNTs) shows catalytic performance unprecedented for this compound class, with a turnover frequency (TOF) of 133 s\(^{−1}\) and a turnover number (TON) of 4.89 × 10\(^6\), at a current density of 43.8 mA cm\(^{−2}\) and a potential of 1.45 V versus normal hydrogen electrode (NHE).
As viruses are obligatory intracellular parasites, any step during their life cycle strictly depends on successful interaction with their particular host cells. In particular, their interaction with cellular membranes is of crucial importance for most steps in the viral replication cycle. Such interactions are initiated by uptake of viral particles and subsequent trafficking to intracellular compartments to access their replication compartments which provide a spatially confined environment concentrating viral and cellular components, and subsequently, employ cellular membranes for assembly and exit of viral progeny. The ability of viruses to actively modulate lipid composition such as sphingolipids (SLs) is essential for successful completion of the viral life cycle. In addition to their structural and biophysical properties of cellular membranes, some sphingolipid (SL) species are bioactive and as such, take part in cellular signaling processes involved in regulating viral replication. It is especially due to the progress made in tools to study accumulation and dynamics of SLs, which visualize their compartmentalization and identify interaction partners at a cellular level, as well as the availability of genetic knockout systems, that the role of particular SL species in the viral replication process can be analyzed and, most importantly, be explored as targets for therapeutic intervention.
Utilizing Pd‐catalyzed [5+2] annulation a series of heptagon‐extended corannulenes could be synthesized from a borinic acid precursor furnished by C−H borylation strategy. Single‐crystal X‐ray analysis revealed the presence of two conformational enantiomers crystallizing in a racemic mixture. Through their embedded five‐ and seven‐membered rings these polycyclic aromatic hydrocarbons (PAHs) exhibit both negative and positive curvature and UV/Vis/NIR absorption spectroscopy as well as cyclic voltammetry experiments provided insights into the influence of larger flanking aromatic systems and electron‐donating substituents encompassing the heptagonal ring. Through [5+2] annulation of acenaphthylene an azulene‐containing PAH with intriguing optoelectronical properties including a very small bandgap and absorption over the whole visible spectrum could be obtained. Theoretical calculations were employed to elucidate the long‐wavelength absorption and aromaticity.
Palladium‐catalyzed [5+2] annulation of 1‐boraphenalenes with ortho‐dihaloarenes afforded negatively curved π‐extended pleiadienes. Two benzo[1,2‐i:4,5‐i’]dipleiadienes (BDPs) featuring a seven‐six‐seven‐membered ring arrangement were synthesized and investigated. Their crystal structure revealed a unique packing arrangement and theoretical calculations were employed to shed light onto the dynamic behavior of the BDP moiety and its aromaticity. Further, a naphthalene‐fused pleiadiene was stitched together by oxidative cyclodehydrogenation to yield an additional five‐membered ring. This formal azulene moiety led to distinct changes in optical and redox properties and increased perturbation of the aromatic system.
In this thesis, the synthesis and photophysics of a great variety of squaraine dyes are presented. This variety is based on four parent squaraines containing either indolenine or quinoline heterocycles. By a suitable choice of the donor and acceptor unit, the optical properties can already be adapted to the properties desired on the stage of the monomer.
To promote a further derivatisation of these dyes, diverse functional groups are attached to the monomers using transition metal-catalysed C-C coupling reactions. However, this has to be preceded by the synthesis of bromine-functionalised derivatives as a direct halogenation of squaraine dyes is not feasible. Therefore, the halogen function is already introduced in precursor molecules giving rise to a molecular building block system containing bromine-, boronic ester-, and alkyne-functionalised monomer units, which pave the way to a plethora of squaraine oligomers and polymers.
The indolenine homopolymer pSQB-1 as well as the corresponding small molecular weight oligomers dSQB-1 and tSQB were synthesized applying Ni-mediated Yamamoto and Pd-catalysed Suzuki coupling methodologies, respectively. The motivation for this project relied on the fundamental investigations by Völker et al. on pSQB-V. A progressive red-shift of the lowest energy absorption maximum from the dimer to the polymer was observed in CHCl3 compared to the monomer. With increasing number of monomer units, the exciton coupling decreases from the dimer to the polymer. In addition, the shape of the absorption band manifold shows a strong dependence on the solvent, which was also observed by Völker et al. J-type aggregate behavior is found in chlorinated solvents such as CHCl3 and DCM, whereas H-type aggregates are formed in acetone. Temperature-dependent absorption studies in PhCN reveals a reversible equilibrium of diverse polymer conformers, which manifests itself in a gradual change from H-aggregate behavior to a mixture with a more pronounced J-aggregate behavior upon raising the temperature. It isassumed that both characteristic aggregate bands correlate in borderline cases with two polymer structures which can be assigned to a zig-zag and a helical structure. As no experimental evidence for these structures could hitherto be provided by NMR, TD-DFT computations on oligomers (22-mers) can reproduce very closely the characteristic features of the spectra for the two conformational isomers.
The subsequent chapters are motivated by the goal to influence the optical properties through a control of the superstructure and thus of the intramolecular aggregate formation.
On the one hand, bulky groups are implemented in the 3-position of the indolenine scaffold to provoke steric repulsion and thus favoring J-aggregate behavior at the expense of helical arrangements. The resulting homopolymer pDiPhSQB bearing two phenyl groups per indolenine exhibits J-type aggregate behavior with red-shifted absorption maxima in all considered solvents which is explained to be caused by the formation of elongated zig-zag structures. Furthermore, single-crystal X-ray analysis of monomer DiPhSQB-2-Br2 reveals a torsion of the indolenine moieties as a consequence of steric congestion. The twist of the molecular geometry and the resulting loss of planarity leads to a serious deterioration of the fluorescence properties, however a significant bathochromic shift of ca. 1 200 cm-1 of the lowest absorption band was observed compared to parent SQB, which is even larger than the shift for dSQB-1 (ca. 1 000 cm-1).
On the other hand, a partial stiffening of the polymer backbone is attempted to create a bias for elongated polymer chains. In this respect, the synthetic approach is to replace every second biarylaxis with the rigid transoid benzodipyrrolenine unit. Despite a rather low average degree of polymerization < 10, exclusively red-shifted absorption maxima are observed in all solvents used.
In order to complete the picture of intramolecular aggregates through the selective design of H-aggregates, a squaraine-squaraine copolymer was synthesised containing the classic cisoid indolenine as well as the cisoid quinoline building block. Taking advantage of the highly structure directing self-assembly character of the quinoline moiety, the copolymer pSQBC indeed showes a broad, blue-shifted main absorption band in comparison with the monomer unit dSQBC. The shape of the absorption band manifold solely exhibited a minor solvent and temperature dependence indicating a persistent H-aggregate behaviour. Hence, as a proof of concept, it is shown that the optical properties of the polymers (H- and J-aggregate) and the corresponding superstructure can be inherently controlled by an adequate design of monomer precursors.
The last chapter of this work deals, in contrast to all other chapters, with intermolecular aggregates. It is shown that the two star-shaped hexasquarainyl benzenes hSQA-1 and hSQA-2 exhibit a strong propensity for self-organisation. Concentration- and temperature-dependent studies reveal a great driving force for self-assembly in acetone. While the larger hSQA-2 instantaneously forms stable aggregates, the aggregates of hSQA-1 shows a pronounced kinetic stability. Taking advantage of the kinetic persistency of these aggregates, the corresponding kinetic activation parameters for aggregation and deaggregation can be assessed. The absorption spectra of both hexasquarainyl benzenes in the aggregated state reveal some striking differences. While hSQA-1 features an intensive, very narrow and blue-shifted absorption band, two red-shifted bands are observed for hSQA-2, which are closely located at the monomer absorption. The very small bandwidth of hSQA-1 are interpreted to be caused by exchange narrowing and pointed towards highly ordered supramolecular aggregates. The concentration-dependent data of the two hexasquarainyl benzenes can be fitted to the dimer-model with excellent correlation coefficients, yielding binding constants in excess of 10^6 M-1, respectively. Such high binding constants are very surprising, considering the unfavourable bulky 3,3-dimethyl groups of the indolenine units which should rather prevent aggregation. Joint theoretical and NMR spectroscopic methods were applied to unravel the supramolecular aggregate structure of hSQA-1, which is shown to consist of two stacked hexasquarainyl benzenes resembling the picture of two stacked bowls.
No abstract available
The ongoing transition from fossil to renewable feedstocks demands new efficient processes for an economically viable production of biomass‐derived commodities and fine chemicals. Novel energy‐ and material‐efficient product purification and separation will play a crucial role due to altered product and feed composition. The present study comprises the synthesis and tests of cross‐linked p‐vinylphenylboronate polymers for the separation of 18 diols, sugar alcohols, and saccharides, which can be obtained during biomass processing. The separation was based on molecular recognition, that is, esterification of the phenylboronate with vicinal diols. A correlation of the molecular complexation constant, the polymer swelling, and the maximum adsorption capacity was found. The adsorption curves over time were recorded. Preliminary results on competitive adsorption of binary mixtures showed a high potential for the separation of substrates with significantly different complexation constants. Desorption tests implied easier desorption of substrates that only adsorb on the outer polymer shell.
Many dyes suffer from fast non-radiative decay pathways, thereby showing only short-lived excited states and weak photoluminescence. Here we show a pronounced fluorescence enhancement for a weakly fluorescent merocyanine (MC) dye by being co-facially stacked to other dyes in hetero-folda-trimer architectures. By means of fluorescence spectroscopy (lifetime, quantum yield) the fluorescence enhancement was explained by the rigidification of the emitting chromophore in the defined foldamer architecture and the presence of a non-forbidden lowest exciton state in H-coupled hetero-aggregates. This folding-induced fluorescence enhancement (FIFE) for specific sequences of π-stacked dyes points at a viable strategy toward improved fluorophores that relates to the approach used by nature in the green fluorescent protein (GFP).
The thesis discusses aspects of the photocatalytic water oxidation reaction. The first chapter deals with a supramolecular macrocycle which contains three ruthenium metal centers. This novel catalyst shows promising catalytic activity and provides insides into the mechanism of the water oxidation reaction. After this part, the focus lies on the light interacting components of the photocatalytic water oxidation. In this regard, the azabenz-annulated perylene derivatives appeared to be a promising dye class. The combination of these chromophores and metal complexes result in metal organic compounds, which have photosensitizer potential.
Nanodiamant bietet in der Medizin und in der Biologie zahlreiche Anwendungsmöglichkeiten aufgrund der guten Biokompatibilität und geringen Toxizität. Durch die umfangreichen Funktionalisierungsmöglichkeiten der Oberfläche der nanometergroßen Partikel können viele unterschiedliche Wirkstoffe, Rezeptormoleküle oder Peptidsequenzen angebunden werden ,die zusammen mit Nanodiamant ein anderes, durchaus besseres Wirkprofil aufweisen als der Wirkstoff allein. Ziel dieser Arbeit war die Synthese eines pH-labilen Linkersystems, dass hydroxylhaltige Wirkstoffe kovalent bindet und zusammen mit Nanodiamant in die Zelle, in der ein saurer pH-Wert herrscht, eingeschleust werden. Über die Änderung des pH-Wertes in der Zelle soll der Wirkstoff freigesetzt werden und seine Wirkung entfalten können.
Weiterhin wurde ein pH-labiles Linkersystem auf der Basis eines Hydrazons hergestellt. Über das synthetisierte Hydrazinderivat können Wirkstoffe, die über eine Aldehyd- oder Ketonfunktion verfügen angebunden werden und pH-labil in der Zelle freigesetzt werden. Zusätzlich trägt der Nanodiamant ein kovalent angebundenes Targeting-Molekül, welches eine verbesserte Adressierung der Wirkorte gewähr¬leisten soll. Die Freisetzung wurde mittels UV-Vis-Spektroskopie detektiert und ausgewertet.
Neben der spezifischen Funktionalisierung von Nanodiamant besitzt auch die Interaktion der Nanodiamantpartikel mit biologischen Medien eine besondere Bedeutung für zukünftige biomedizinische Anwendungen. Wenn die Partikeloberfläche durch Proteinadsorption gegenüber dem Wirkort abgeschirmt wird, so kann der angebundene Wirkstoff gegebenenfalls nicht freigesetzt werden und somit nicht seine Wirkung entfalten und bleibt letztlich ungenutzt. So war es von besonderem Interesse die Wechselwirkungen von Nanodiamant in Humanserum und auch weiteren physiologischen Medien zu untersuchen. Dabei wurden sowohl freie Nanodiamantpartikel als auch solche, die auf klinisch bereits eingesetzten Gerüstmaterialien im Bereich der Therapie großer Knochendefekte adsorbiert waren, untersucht. Auch wurden die Wechselwirkungen von Nanodiamant mit der physiologischen Umgebung untersucht, die zur Agglomeration der Nanopartikel führen können. Es wurde ein unter¬schiedliches Agglomerationsverhalten der Nanodiamanten in wässriger Umgebung verglichen mit Nanodiamanten in physiologischen Medien sowie deren Stabilität im Serum beobachtet.
Durch die in dieser Arbeit vorgestellten Untersuchungen konnten wichtige Erkenntnisse zur Wechselwirkung verschieden präparierter und funktionalisierter Nanodiamanten mit physiologisch relevanten Umgebungen sowie zu stimuli-responsiven Wirkstofffreisetzung aus Nanodiamant-Konjugaten gewonnen werden. Zudem wurde mit der Untersuchung der angelagerten Proteine um Nanodiamant ein erster Schritt in Richtung eines umfassenden Verständnisses der Wechselwirkung dieses Materials mit biologischen Umgebungen unternommen. Auch wenn diese Wechselwirkungen sehr komplex sind, so sind erste Aussagen bezüglich der Art der angelagerten Proteine möglich. Erste Versuche der Stabilisierung von Nanodiamant in physiologischen Medien wurden ebenfalls erfolgreich durchgeführt und zeigen eine effiziente und einfache Möglichkeit, Nanodiamant in biologischen Medien vor der Agglomeration zu bewahren. Die im Rahmen dieser Arbeit gewonnen Erkenntnisse bezüglich mehrfacher Funktionalisierungsmöglichkeiten von Nanodiamant sowie dessen Stabilisierung in physiologischen Medien zeigen die breite Anwendungsmöglichkeit und das enorme Potential von Nanodiamant im Bereich medizinischer und biologischer Anwendungen auf.
Eine molekulare Fliegenfalle zur Erkennung von biologisch relevanten (poly)-anionischen Substraten
(2006)
Im Rahmen dieser Arbeit konnte ein multivalenter Rezeptor auf Basis von Guanidiniocarbonylpyrrolen zur Komplexierung von biologisch relevanten (poly)- anionischen Substraten wie Citrat, Malat und Tatrat dargestellt werden. Der Rezeptor bindet Citrat selbst in Gegenwart eines 1000fachen Überschusses an NaCl und einem 10fachen Überschuss an Bis-tris Puffer mit einer hohen Bindungskonstante von KAss = 86000 M-1 in Wasser. Wenn man Rezeptoren auf Metall- und Boronsäurebasis nicht berücksichtigt, handelt es sich nach meinem Wissen um den besten Citrat-Rezeptor, der in der Literatur bisher publiziert ist. Außerdem zeigt der Rezeptor mit einem Faktor von mindestens 8 eine hohe Selektivität für Citrat gegenüber anderen biologisch relevanten Dicarboxylaten wie Malat und Tatrat. Mithilfe von Bindungsstudien und Molecular Modeling Rechnungen konnte hergeleitet werden, welchen Einfluss die verschiedenen nicht-kovalenten Wechselwirkungen auf die Bindungskonstante haben. Dabei konnte gezeigt werden, dass Flexibilität, Hydroxy-Funktionen, pi-Stacking und Symmetrie bei den Substraten Einfluss auf die Bindungskonstanten zeigen, wobei vor allem die unpolaren Wechselwirkungen und die zusätzliche Hydroxy-Funktionen einen hohen Anteil an der Bindung zu haben scheinen. Neben der selektiven Erkennung von Citrat durch den Rezeptor konnte zusätzlich ein Sensorsystem mit Carboxyfluorescein auf Basis eines indicator displacement assays entwickelt werden, mit dem die Anwesenheit von Citrat im Gegensatz zu anderen Carboxylaten mit dem bloßen Auge (naked eye) erkannt werden kann. Neben den Polycarboxylaten zeigt der Rezeptor außerdem noch hohe Bindungskonstanten für polyanionische Zucker. So konnten z.B. für Glucophosphate mit UV-Spektroskopie Bindungskonstanten von KAss = ca. 20000 M-1 in dem sehr polaren Lösemittelgemisch 10 % DMSO/Wasser (pH = 4, Acetat-Puffer) gefunden werden.
In the first part of this thesis, the synthesis of a series of bistriarylamine (bisTAA) compounds was presented. On the one hand, the substitution pattern of the TAA at the benzene bridging unit was varied from meta- to para-position (pX and mX), on the other hand, the energetic position of the bridging unit was tuned by use of two electron-donating or electron-accepting substituents X (with X = OMe, Me, Cl, CN, NO2) in 2,5-position. In case of the meta-series, compounds with X in 4,6-position were synthesized (mX46). The photophysical and electrochemical properties of the neutral compounds were investigated.
The cationic mixed valence (MV) bisTAA compounds could be generated by oxidation. Thermally induced hole transfer (HT) in the groud state was investigated by temperature depending ESR spectroscopy. While the HT rate k and HT barrier ΔG in mX are unaffected by the substituents X, k and ΔG in the pX series increase simultaneously with increasing electron-donating strength of X. This, at first contradictory observation can be explained by an increasingly important solvent dynamic effect and an additional, effective barrier. The optically induced HT was examined by UV/Vis/NIR spectroscopy. The pX-series revealed an increase of the electronic coupling V, and correspondingly a decrease of ΔG, with an increase of the electron donating character of X. For mX, a spectroscopic determination of these parameters was not possible. mX46 showed an intermediate behavior, MV compounds with strong electron-donating X, obtained coupling of similar magnitude as pX, which could be explained by means of DFT calculations, with regard to the molecular orbitals.
In the second part of this work, the synthesis of a series of dyads with triarylamine (TAA) as a donor and naphthalene diimide (NDI) as an acceptor was presented. Again, the substitution pattern of the redox centers at the benzene bridging unit was varied in the form of a meta- or para-position (pXNDI or mXNDI) and the energetic position of the bridging unit was varied by X (with X = OMe, Me, Cl, CN, NO2) attached in the 2,5-position. Additionally, compound mOMe46NDI with methoxy substitution in 4,6-position was synthesized. The photophysical and electrochemical properties of these compounds were investigated. The electron transfer (ET) processes of charge separation (CS) and charge recombination (CR) of these were investigated by means of transient absorption (TA) spectroscopy in toluene. This was not possible for the nitro-compounds p-/mNO2NDI, since they decomposed under irradiation. In addition to that, the CR of pXNDI was not detectable by ns-setup, which is why the focus was given to the mXNDI series (with X = OMe–CN).The CS was examined by fs-TA spectroscopy, where the formation of a CS state could be detected. The rise time of the CS states decreases with increasing electron-withdrawing substituents X. CR was examined with ns-TA spectroscopy and shows a biexponential decay behavior, which is caused by singlet-triplet equilibrium in the CS state. By applying an external magnetic field, the decay behavior was decisively changed and the singlet-triplet splitting could be determined. This finding could also be confirmed by simulating the decay curves.
In both parts of this work, the decisive influence of the benzene bridging unit on the appearing ET processes became obvious. For the HT in the ground state of the MV compound, as well as for the ET in the exited states of the DA compounds, the highest transfer rates were found for the para-series pX and pXNDI, and much smaller rates for the meta-series mX and mXNDI. The meta46-compounds mX46 and mOMeNDI46 showed an intermediate behavior in both parts of this work.
A highly strained covalent organic cage compound was synthesized from hexahydroxy tribenzotriquinacene (TBTQ) and a meta-terphenyl-based diboronic acid with an additional benzoic acid substituent in 2’-position. Usually, a 120° bite angle in the unsubstituted ditopic linker favors the formation of a [4+6] cage assembly. Here, the introduction of the benzoic acid group is shown to lead to a perfectly preorganized circular hydrogen-bonding array in the cavity of a trigonal-bipyramidal [2+3] cage, which energetically overcompensates the additional strain energy caused by the larger mismatch in bite angles for the smaller assembly. The strained cage compound was analyzed by mass spectrometry and \(^{1}\)H, \(^{13}\)C and DOSY NMR spectroscopy. DFT calculations revealed the energetic contribution of the hydrogen-bonding template to the cage stability. Furthermore, molecular dynamics simulations on early intermediates indicate an additional kinetic effect, as hydrogen bonding also preorganizes and rigidifies small oligomers to facilitate the exclusive formation of smaller and more strained macrocycles and cages.
The indepth metabolic profiling of the crude extracts of two African Ancistrocladus species viz. A. likoko from Central Africa and A. abbreviatus from West Africa, resulted in a total of 87 alkaloids among them 54 new ones. All of the compounds were intensely elucidated by 1D and 2D NMR, HRESIMS, as well as chemical and chiroptical techniques.
Among the newly discovered compounds are quinoid naphthylisoquinolines with an ortho-diketone in the naphthalene portion, nor-naphthylisoquinoline alkaloid lacking the always present methyl group at C-1, seco-(ring cleaved) naphthylisoquinolines, and a newly discovered class of natural products called the naphthylisoindolinones.
Some of the compounds displayed strong antitumoral activities against human pancreatic cancer cells and leukemia cells in-vitro.
RNA-cleaving deoxyribozymes have found broad application as useful tools for RNA biochemistry. However, tedious in vitro selection procedures combined with laborious characterization of individual candidate catalysts hinder the discovery of novel catalytic motifs. Here, we present a new high-throughput sequencing method, DZ-seq, which directly measures activity and localizes cleavage sites of thousands of deoxyribozymes. DZ-seq exploits A-tailing followed by reverse transcription with an oligo-dT primer to capture the cleavage status and sequences of both deoxyribozyme and RNA substrate. We validated DZ-seq by conventional analytical methods and demonstrated its utility by discovery of novel deoxyribozymes that allow for cleaving challenging RNA targets or the analysis of RNA modification states.
Deoxyribozymes are synthetic enzymes made of DNA that can catalyze the cleavage or formation of phosphodiester bonds and are useful tools for RNA biochemistry. Here we report new RNA-cleaving deoxyribozymes to interrogate the methylation status of target RNAs, thereby providing an alternative method for the biochemical validation of RNA methylation sites containing N\(^6\)-methyladenosine, which is the most wide-spread and extensively investigated natural RNA modification. Using in vitro selection from random DNA, we developed deoxyribozymes that are sensitive to the presence of N\(^6\)-methyladenosine in RNA near the cleavage site. One class of these DNA enzymes shows faster cleavage of methylated RNA, while others are strongly inhibited by the modified nucleotide. The general applicability of the new deoxyribozymes is demonstrated for several examples of natural RNA sequences, including a lncRNA and a set of C/D box snoRNAs, which have been suggested to contain m\(^6\)A as a regulatory element that influences RNA folding and protein binding.
The thesis describes the development of new synthetic strategies towards planar nanometer-sized and electron-deficient polycyclic aromatic dicarboximides, which are rather unexplored compared with the large variety of electron-rich polycyclic aromatic hydrocarbons and nanographenes. Thus, new polycyclic aromatic systems containing a different number of dicarboximide groups were designed since this class of compounds has revealed its significance in the past due to a range of desirable molecular properties and its high thermal and photochemical stability. The synthetic concept towards these systems includes different C–C coupling techniques that were combined within coupling cascade reactions. Therefore, this thesis provides new insights into the reactivity of aromatic substrates and elucidates mechanistic aspects of C–C coupling cascade reactions to facilitate the precise design of new and desirable materials based on polycyclic aromatic dicarboximides. Furthermore, structure-property relationships as well as the optical and electrochemical properties were investigated by UV/Vis absorption and fluorescence spectroscopy and cyclic or square wave voltammetry. Insights into the molecular structures in the solid state were obtained by single-crystal X-ray analysis. In subsequent studies, highly electron-deficient perylene bisimides and their reduced species have been investigated in detail. Thus, core-functionalized perylene bisimides were synthesized and UV/Vis absorption spectroscopy, spectroelectrochemistry and cyclic or square wave voltammetry were used to determine their optical properties and the stability of the individual reduced species.
Herein, we report the one-pot synthesis of an electron-poor nanographene containing dicarboximide groups at the corners. We efficiently combined palladium-catalyzed Suzuki-Miyaura cross-coupling and dehydrohalogenation to synthesize an extended two-dimensional pi-scaffold of defined size in a single chemical operation starting from N-(2,6-diisopropylphenyl)-4,5-dibromo-1,8-naphthalimide and a tetrasubstituted pyrene boronic acid ester as readily accessible starting materials. The reaction of these precursors under the conditions commonly used for Suzuki-Miyaura cross-coupling afforded a C\(_{64}\) nanographene through the formation of ten C-C bonds in a one-pot process. Single-crystal X-ray analysis unequivocally confirmed the structure of this unique extended aromatic molecule with a planar geometry. The optical and electrochemical properties of this largest ever synthesized planar electron-poor nanographene skeleton were also analyzed.
Beyond the four canonical nucleosides as primary building blocks of RNA, posttranscriptional modifications give rise to the epitranscriptome as a second layer of genetic information. In eukaryotic mRNA, the most abundant posttranscriptional modification is N6-methyladenosine (m6A), which is involved in the regulation of cellular processes. Throughout this thesis, the concept of atomic mutagenesis was employed to gain novel mechanistic insights into the substrate recognition by human m6A reader proteins as well as in the oxidative m6A demethylation by human demethylase enzymes. Non-natural m6A atomic mutants featuring distinct steric and electronic properties were synthesized and incorporated into RNA oligonucleotides. Fluorescence anisotropy measurements using these modified oligonucleotides revealed the impact of the atomic mutagenesis on the molecular recognition by the human m6A readers YTHDF2, YTHDC1 and YTHDC2 and allowed to draw conclusions about structural prerequisites for substrate recognition. Furthermore, substrate recognition and demethylation mechanism of the human m6A demethylase enzymes FTO and ALKBH5 were analyzed by HPLC-MS and PAGE-based assays using the modified oligonucleotides synthesized in this work.
Modified nucleosides not only expand the genetic alphabet, but are also extensively researched as drug candidates. In this thesis, the antiviral mechanism of the anti-SARS-CoV-2 drug remdesivir was investigated, which causes delayed stalling of the viral RNA-dependent RNA polymerase (RdRp). Novel remdesivir phosphoramidite building blocks were synthesized and used to construct defined RNA-RdRp complexes for subsequent studies by cryogenic electron microscopy (cryo-EM). It was found that the 1'-cyano substituent causes Rem to act as a steric barrier of RdRp translocation. Since this translocation barrier can eventually be overcome by the polymerase, novel derivatives of Rem with potentially improved antiviral properties were designed.
N\(^6\)-methyladenosine (m\(^6\)A) is an important modified nucleoside in cellular RNA associated with multiple cellular processes and is implicated in diseases. The enzymes associated with the dynamic installation and removal of m\(^6\)A are heavily investigated targets for drug research, which requires detailed knowledge of the recognition modes of m\(^6\)A by proteins. Here, we use atomic mutagenesis of m\(^6\)A to systematically investigate the mechanisms of the two human m\(^6\)A demethylase enzymes FTO and ALKBH5 and the binding modes of YTH reader proteins YTHDF2/DC1/DC2. Atomic mutagenesis refers to atom-specific changes that are introduced by chemical synthesis, such as the replacement of nitrogen by carbon atoms. Synthetic RNA oligonucleotides containing site-specifically incorporated 1-deaza-, 3-deaza-, and 7-deaza-m\(^6\)A nucleosides were prepared by solid-phase synthesis and their RNA binding and demethylation by recombinant proteins were evaluated. We found distinct differences in substrate recognition and transformation and revealed structural preferences for the enzymatic activity. The deaza m\(^6\)A analogues introduced in this work will be useful probes for other proteins in m\(^6\)A research.
A series of monomeric chirally substituted indolenine squaraine monomers were successfully synthesized and utilized for the construction of various oligo- and polymers, in order to study their chiroptical properties in terms of exciton chirality. The quaternary carbon atom at the 3-position of the indolenine subunit, as well as the alkyl side chain attached to the indolenine nitrogen were selected as the most suitable site for chiral functionalization.
For the C(3)-chiral derivatives, two synthetic routes depending on the desired substitution at the stereogenic center were established. The chiral side chains were prepared via Evans asymmetric alkylation where the resulting branching point at the 2 position constituted the chiral center. While the chiral substitution only had minor effects on the linear optical properties and geometric structure of the chromophore, all compounds exhibited a distinct and measurable CD signal that correlated with the distance of the chiral center to the central chromophore.
Polymers bearing chiral side chains exhibited a solvent- and temperature-dependent helix-coil equilibrium, which was influenced by the type of side chain used. CD spectroscopy revealed the helical conformation to possess a preferred twist sense, and temperature-dependent measurements showed the degree of homohelicity to be nearly complete in certain cases. Furthermore, a CPL signal was able to be obtained for the helical conformer of one polymer.
Various (co)oligo- and polymers comprising the C(3)-chiral monomers only displayed a solvent-independent J-type absorption behavior and thus did not form helical conformations in solution. CD spectroscopy revealed a solvent-dependent adoption of quasi-enantiomeric conformers, which was elucidated by quantum chemical TDDFT calculations.
Die Chlorophylle stellen in der Natur die wichtigsten Pigmente dar, weil sie verantwortlich für die Photosynthese sind und hierbei vielfältige Funktionen wahrnehmen, die sich aus ihrer Selbstassemblierung sowie den vorteilhaften optischen und Redox-Eigenschaften ergeben. Die in dieser Arbeit untersuchten semisynthetischen Zinkchlorine stellen Modellverbindungen des natürlichen Bacteriochlorophylls c (BChl c) der Lichtsammelsysteme (light-harvesting: LH) in Chlorosomen von Bakterien, jedoch ohne Proteingerüst, dar. Die entscheidenden Vorteile dieser Zinkchlorine (ZnChl) gegenüber den natürlichen BChls bestehen im einfachen semisynthetischen Zugang ausgehend von Chlorophyll a (Chl a), ihrer gesteigerten chemischen Stabilität sowie der Möglichkeit ihre Selbstassemblierung durch gezielte chemische Modifizierung der Seitenketten in der Peripherie zu steuern. Während bereits mehrfach über die vielversprechenden Redox- und excitonischen Eigenschaften von Aggregaten von ZnChl und natürlichem BChl c und den damit verbundene Voraussetzungen für Excitontransport über große Distanzen berichtet wurde, sind die Ladungstransporteigenschaften von Aggregaten der biomimetischen ZnChl bis heute unerforscht. Die vorliegende Arbeit beschäftigt sich mit der Aufklärung der Struktur von Aggregaten einer Vielzahl von semisynthetischen Zinkchlorophyllderivaten im Feststoff, in Lösung und auf Oberflächen durch die Kombination verschiedenster spektroskopischer, kristallographischer und mikroskopischer Techniken an die sich Untersuchungen zum Ladungstransport in den Aggregaten anschließen. Schema 1 zeigt die verschiedenen, in dieser Arbeit synthetisierten ZnChls, die entweder mit einer Hydroxy- oder Methoxygruppe in der 31-Position funktionalisiert sind sowie Substituenten unterschiedlicher Art, Länge und Verzweigung an der Benzylestergruppe in 172-Position tragen.Die Packung dieser Farbstoffe hängt entscheidend von ihrer chemischen Struktur ab. Während die ZnChls 1a, 2a, 3 mit 31-Hydroxygruppe und Alkylseitenketten (Dodecyl bzw. Oligoethylenglykol) gut lösliche stabförmige Aggregate bilden, lagern sich die analogen Verbindungen mit 31-Methoxygruppe (1b, 2b) zu Stapeln in Lösung und auf Oberflächen zusammen. Diese supramolekularen Polymere wurden im Detail in Kapitel 3 mit Hilfe von UV/Vis- und CD-Spektroskopie (circular dichroism: CD) sowie dynamische Lichtstreuung (dynamic light scattering: DLS) untersucht. Darüber hinaus lieferten temperaturabhängige UV/Vis- in Kombination mit DLS-Messungen wertvolle Informationen über die Aggregationsprozess dieser beiden Sorten von Aggregaten. Während sich die ZnChl 1a mit 31 Hydroxygruppe entsprechend dem isodesmischen Modell zu röhrenförmigen Aggregaten zusammenlagern, bilden sich die stapelförmigen Aggregate von 1b nach einem kooperativen Keimbildungs-Wachstums-Mechanismus (nucleation-elongation mechanism). Detaillierte elektronenmikroskopische Studien lieferten erstmals überzeugende Beweise für röhrenförmige Nanostrukturen der Aggregate des wasserlöslichen 31-Hydroxy Zinkchlorin 3. Die gemessenen Durchmesser der Röhren von ~ 5-6 nm dieser Aggregate liegen in hervorragender Übereinstimmung mit den Elektronenmikroskopie-Daten von BChl c Stabaggregaten in Chlorosomen (Chloroflexus aurantiacus, Durchmesser ~ 5-6 nm) und entsprechen damit dem von Holzwarth und Schaffner postulierten röhrenförmigen Modell... Im Einklang mit ihren hoch geordneten, robusten Strukturen, die sich eindimensional in einer Größenordnung von Mikrometeren erstrecken, sowie ihrer Fähigkeit zum effizienten Ladungs-trägertransport stellen diese selbstassemblierten Nanoröhren von ZnChls vielversprechende Ausgangsmaterialien für die Fertigung supramolekularer elektronischer Bauteile dar. Wissenschaftliche Bemühungen einige dieser Moleküle und ihre entsprechenden supramolekularen Polymere für die Fertigung von (opto-)elektronischen Bauteilen wie organischen Feldeffekttransistoren zu benutzten, stellen lohnende Aufgaben für die Zukunft dar...
Bedingt durch ihre strukturelle Diversität und biologischen Eigenschaften sind Naturstoffe seit jeher Quelle und Inspiration für Arzneimittel vor allem im therapeutischen Bereich der Onkologie und der Infektionskrankheiten. Ihr einzigartiges pharmakologisches Potenzial wird durch die selektive Interaktion mit einer Vielzahl von Zielmolekülen hervorgerufen. Aufgrund der zentralen Bedeutung von Naturstoffen in der Entdeckung und Entwicklung von neuen Arzneimitteln sind nach wie vor die Isolierung und Strukturaufklärung, die totalsynthetische Darstellung und Derivatisierung sowie die Identifizierung der Zielmoleküle und die Aufklärung des Wirkmechanismus dieser natürlichen Wirkstoffe unabdingbar.
Die kleine, aber spannende Klasse der Naphthylisochinolin-Alkaloide, die ausschließlich aus den beiden Pflanzenfamilien der Dioncophyllaceae und der Ancistrocladaceae gewonnen werden, zeichnet sich mit ihren mehr als 200 Vertretern nicht nur durch ihre strukturelle Vielfalt aus, sondern zeigt vor allem pharmakologisch interessante Wirksamkeiten. Neben ausgeprägten In-vitro-Aktivitäten gegen protozoische Erreger wie Leishmanien, Plasmodien und Trypanosomen besitzen die Vertreter dieser einzigartigen Naturstoffklasse nach neuesten Untersuchungen auch vielversprechende antitumorale Aktivitäten. Für deren Weiterentwicklung zu möglichen Arzneistoffen ist es daher unabdingbar, ihr pharmakologisches Potenzial tiefergehend zu untersuchen.
Ziel der vorliegenden Dissertation war die Entwicklung totalsynthetischer Zugänge zu biologisch interessanten Naphthylisochinolin-Alkaloiden mit Hilfe unterschiedlicher Synthesestrategien. Ebenfalls sollten durch die Darstellung strukturell vereinfachter Derivate sowie markierter Naturstoffe in Zusammenarbeit mit Kooperationspartnern mögliche Zielmoleküle identifiziert und Beiträge zum Wirkmechanismus untersucht werden.
Our research group focusses on the isolation, structural elucidation, and synthesis of bioactive natural products, among others, the naphthylisoquinoline alkaloids from tropical lianas. This intriguing class of compounds comprises representatives with activities against, e.g. P. falciparum, the cause of Malaria tropica, against the neglected disease leishmaniasis, and, as discovered more recently, against different types of cancer cells. Based on the high potency of theses extraordinary secondary metabolites, this thesis was devoted to the total synthesis of bioactive natural products and closely related analogs.
The objective of this thesis focuses on the development of strategies for precise control of perylene bisimide (PBI) self-assembly and the in-depth elucidation of structural and optical features of discrete PBI aggregates by means of NMR and UV/Vis spectroscopy. The strategy for discrete dimer formation of PBIs is based on delicate steric control that distinguishes the two facets of the central perylene surface. The strategy applied in this thesis for accessing discrete PBI quadruple and further oligomeric stacks relies on backbone-directed PBI self-assembly. For this purpose, two tweezer-like PBI dyads bearing the respective rigid backbones, diphenylacetylene (DPA) and diphenylbutydiyne (DPB), were synthesized. The distinct aggregation behavior of these structurally similar PBI dyads can be ascribed to the intramolecular distance between the two PBI chromophores imparted by the DPA and DPB spacers.
Squaraine dyes have attracted more attention in the past decade due to their strong and narrow absorption and fluorescence along with the easily functionalized molecular structure. One successful approach of core functionalization is to replace one oxygen of the squaric carbonyl group with a dicyanomethylene group, which shifts the absorption and emission into the near infrared (NIR) region and at the same time leads to a rigid, planar structure with C2v symmetry. However, such squaraines tend to aggregate cofacially in solution due to dispersion forces and dipole-dipole interactions, usually leading to H-type exciton coupling with undesired blue-shifted spectrum and quenched fluorescence. Therefore, the goal of my research was the design of dicyanomethylene-substituted squaraine dyes that self-assemble into extended aggregates in solution with J-type coupling, in order to retain or even enhance their outstanding optical properties. Toward this goal, bis(squaraine) dyes were envisioned with two squaraine units covalently linked to trigger a slip-stacked packing motif within the aggregates to enable J-type coupling.
In my first project, bis(squaraine) dye BisSQ1 was synthesized, in which two dicyanomethylene squaraine chromophores are covalently linked. Concentration and temperature-dependent UV/Vis/NIR spectroscopy experiments reveal that BisSQ1 undergoes cooperative self-assembly resulting in J-type aggregates in a solvent mixture of toluene/1,1,2,2-tetrachloroethane (TCE) (98:2, v/v). The J type exciton coupling is evident from the significantly red shifted absorption maximum at 886 nm and the fluorescence peak at 904 nm. In conclusion, this was a first example to direct squaraine dye aggregation in solution to the more desired slip-stacked packing leading to J-type exciton coupling by simply connecting two dyes in a head-to-tail bis chromophore structure.
Connecting two squaraine dyes with an additional phenylene spacer (BisSQ2) leads to two different polymorphs with very distinct absorption spectra upon cooling down a solution of BisSQ2 in a solvent mixture of toluene/TCE (98:2, v/v) with different rates. Accordingly, rapid cooling resulted in rigid helical nanorods with an absorption spectrum showing a panchromatic feature, while slow cooling led to a sheet-like structure with a significant bathochromic shift in the absorption spectrum.
It was discovered that the conventional molecular exciton model failed to explain the panchromatic absorption features of the nanorods for the given packing arrangement, therefore more profound theoretical investigations based on the Essential States Model (ESM) were applied to unveil the importance of intermolecular charge transfer (ICT) to adequately describe the panchromatic absorption spectrum. Moreover, the red-shift observed in the spectrum for the sheet-like structure can be assigned to the interplay of Coulomb coupling and ICT-mediated coupling.
Furthermore, the same bis-chromophore strategy was adopted for constructing an NIR-II emitter with a bathochromically-shifted spectrum. In chloroform, BisSQ3 exhibits an absorption maximum at 961 nm with a significant bathochromic shift (1020 cm−1) compared to the reference mono-squaraine SQ, indicating intramolecular J-type coupling via head-to-tail arrangement of two squaraine dyes. Moreover, BisSQ3 shows a fluorescence peak at 971 nm with a decent quantum yield of 0.33%. In less polar toluene, BisSQ3 self-assembles into nanofibers with additional intermolecular J-type coupling, causing a pronounced bathochromic shift with absorption maximum at 1095 nm and a fluorescence peak at 1116 nm. Thus, connecting two quinoline-based squaraines in a head-to-tail fashion leads to not only intra-, but also intermolecular J-type exciton coupling, which serves as a promising strategy to shift the absorption and emission of organic fluorophores into the NIR-II window while retaining decent quantum yields.
In conclusion, my research illustrates based on squaraine dyes how a simple modification of the molecular structure can significantly affect the aggregation behavior and further alter the optical properties of dye aggregates. Elongated supramolecular structures based on dicyanomethylene substituted squaraine dyes were successfully established by covalently linking two squaraine units to form a bis-chromophore structure. Then, a simple but efficient general approach was established to direct squaraine dye aggregation in solution to the more desired slip-stacked packing leading to J-type exciton coupling by directly connecting two squaraine dyes in a head-to-tail fashion without spacer units. Moreover, the additional spacer between the squaraine dyes in BisSQ2 allowed different molecular conformations, which leads to two different morphologies depending on the cooling rates for a hot solution. Hence, this is a promising strategy to realize supramolecular polymorphism.
In general, it is expected that the concept of constructing J-aggregates by the bis-chromophore approach can be extended to entirely different classes of dyes since J-aggregates possess a variety of features such as spectral shifts into the NIR window, fluorescence enhancement, and light harvesting, which are commonly observed and utilized for numerous fundamental studies and applications. Moreover, the insights on short-range charge transfer coupling for squaraine dyes is considered of relevance for all materials based on alternating donor-acceptor π-systems. The panchromatic spectral feature is in particular crucial for acceptor-donor-acceptor (ADA) dyes, which are currently considered as very promising materials for the development of bulk heterojunction solar cells.
A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.
Nucleic acids are one of the important classes of biomolecules together with carbohydrates, proteins and lipids. Both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are most well known for their respective roles in the storage and expression of genetic information.
Over the course of the last decades, nucleic acids with a variety of other functions have been discovered in biological organisms or created artificially. Examples of these functional nucleic acids are riboswitches, aptamers and ribozymes. In order to gain information regarding their function, several analytical methods can be used.
Electron paramagnetic resonance (EPR) spectroscopy is one of several techniques which can be used to study nucleic acid structure and dynamics. However, EPR spectroscopy requires unpaired electrons and because nucleic acids themselves are not paramagnetic, the incorporation of spin labels which carry a radical is necessary.
Here, three new spin labels for the analysis of nucleic acids by EPR spectroscopy are presented. All of them share two important design features. First, the paramagnetic center is located at a nitroxide, flanked by ethyl groups to prevent nitroxide degradation, for example during solid phase synthesis. Furthermore, they were designed with rigidity as an important quality, in order to be useful for applications like pulsed electron double resonance (PELDOR) spectroscopy, where independent motion of the spin labels relative to the macromolecule has a noticeable negative effect on the precision of the measurements.
Benzi-spin is a spin label which differs from most previous examples of rigid spin labels in that rather than being based on a canonical nucleoside, with a specific base pairing partner, it is supposed to be a universal nucleoside which is sufficiently rigid for EPR measurements when placed opposite to a number of different nucleosides. Benzi-spin was successfully incorporated into a 20 nt oligonucleotide and its base pairing behavior with seven different nucleosides was examined by UV/VIS thermal denaturation and continuous wave (CW) EPR experiments. The results show only minor differences between the different nucleosides, thus confirming the ability of benzi-spin to act as a universally applicable spin label.
Lumi-spin is derived from lumichrome. It features a rigid scaffold, as well as a free 2'-hydroxy group, which should make it well suited for PELDOR experiments once it is incorporated into RNA oligonucleotides.
EÇr is based on the Ç family of spin labels, which contains the most well known rigid spin labels for nucleic acids to this day. It is essentially a version of EÇm with a free 2'-hydroxy group. It was converted to triphosphate EÇrTP and used for primer extension experiments to test the viability of enzymatic incorporation of rigid spin labels into oligonucleotides as an alternative to solid-phase synthesis. Incorporation into DNA by Therminator III DNA polymerase in both single-nucleotide and full-length primer extensions was achieved.
All three of these spin labels represent further additions to the expanding toolbox of EPR spectroscopy on nucleic acids and might prove valuable for future research.
The objective of this thesis was the synthesis and characterisation of two linear multifunctional PEG-alternatives for bioconjugation and hydrogel formation: i) Hydrophilic acrylate based copolymers containing peptide binding units and ii) hydrophilic polyether based copolymers containing different functional groups for a physical crosslinking.
In section 3.1 the successful synthesis of water soluble and linear acrylate based polymers containing oligo(ethylene glycol) methyl ether acrylate with either linear thioester functional 2-hydroxyethyl acrylate, thiolactone acrylamide, or vinyl azlactone via the living radical polymerisation technique Reversible Addition Fragmentation Chain Transfer (RAFT) and via free-radical polymerisation is described. The obtained polymers were characterized via GPC, 1H NMR, IR and RAMAN spectroscopy.
The RAFT end group was found to be difficult to remove from these short polymer chains and accordingly underwent the undesired side reaction aminolysis with the peptide during the conjugation studies. Besides that, polymers without RAFT end groups did not show any binding of the peptide at the thioester groups, which can be improved in future by using higher reactant concentrations and higher amount of binding units at the polymer. Polymers containing the highly reactive azlactone group showed a peptide binding of 19 %, but unfortunately this function also underwent spontaneous hydrolysis before the peptide could even be bound. In all cases, oligo(ethylene glycol) methyl ether acrylate was used with a relatively high molecular weight (Mn = 480 Da) was used, which eventually was efficiently shielding the introduced binding units from the added peptide. In future, a shorter monomer with Mn = 300 Da or less or hydrophilic N,N’-dialkyl acrylamide based polymers with less steric hindrance could be used to improve this bioconjugation system. Additionally, the amount of monomers containing peptide binding units in the polymer can be increased and have an additional spacer to achieve higher loading efficiency.
The water soluble, linear and short polyether based polymers, so called polyglycidols, were successfully synthesized and modified as described in section 3.2. The obtained polymers were characterized using GPC, 1H NMR, 31P{1H} NMR, IR, and RAMAN spectroscopy. The allyl groups which were present up to 20 % were used for radical induced thiol-ene chemistry for the introduction of functional groups intended for the formation of the physically crosslinking hydrogels. For the positively charged polymers, first a chloride group had to be introduced for the subsequent nucleophilic substitution with the imidazolium compound. There, degrees of modifications were found in the range 40-97 % due to the repulsion forces of the charges, decreased concentration of active chloride groups, and limiting solution concentrations of the polymer for this reaction. For the negatively charged polymers, first a protected phosphonamide moiety was introduced with a deprotection step afterwards showing 100 % conversion for all reactions. Preliminary hydrogel tests did not show a formation of a three-dimensional network of the polymer chains which was attributed to the short backbone length of the used polymers, but the gained knowledge about the synthetic routes for the modification of the polymer was successfully transferred to longer linear polyglycidols. The same applies to the introduction of electron rich and electron poor compounds showing π-π stacking interactions by UV-vis spectroscopy.
Finally, long linear polyglycidyl ethers were synthesised successfully up to molecular weights of Mn ~ 30 kDa in section 3.3, which was also proven by GPC, 1H NMR, IR and RAMAN spectroscopy. This applies to the homopolymerisation of ethoxyethyl glycidyl ether, allyl glycidyl ether and their copolymerisation with an amount of the allyl compound ~ 10 %. Attempts for higher molecular weights up to 100 kDa showed an uncontrolled polymerisation behaviour and eventually can be improved in future by choosing a lower initiation temperature. Also, the allyl side groups were modified via radical induced thiol-ene chemistry to obtain positively charged functionalities via imidazolium moieties (85 %) and negatively charged functionalities via phosphonamide moieties (100 %) with quantitative degree of modifications. Hydrogel tests have still shown a remaining solution by using long linear polyglycidols carrying negative charges with long/short linear polyglycidols carrying positive charges. The addition of calcium chloride led to a precipitate of the polymer instead of a three-dimensional network formation representing a too high concentration of ions and therefore shielding water molecules with prevention from dissolving the polymer. These systems can be improved by tuning the polymers structure like longer polymer chains, longer spacer between polymer backbone and charge, and higher amount of functional groups.
The objective of the thesis was partly reached containing detailed investigated synthetic routes for the design and characterisation of functional polymers which could be used in future with improvements for bioconjugation and hydrogel formation tests.
Obligate human pathogenic Neisseria gonorrhoeae are the second most frequent bacterial cause of sexually transmitted diseases. These bacteria invade different mucosal tissues and occasionally disseminate into the bloodstream. Invasion into epithelial cells requires the activation of host cell receptors by the formation of ceramide-rich platforms. Here, we investigated the role of sphingosine in the invasion and intracellular survival of gonococci. Sphingosine exhibited an anti-gonococcal activity in vitro. We used specific sphingosine analogs and click chemistry to visualize sphingosine in infected cells. Sphingosine localized to the membrane of intracellular gonococci. Inhibitor studies and the application of a sphingosine derivative indicated that increased sphingosine levels reduced the intracellular survival of gonococci. We demonstrate here, that sphingosine can target intracellular bacteria and may therefore exert a direct bactericidal effect inside cells.
This work is concerned with the syntheses and photophysical properties of para-xylylene bridged macrocycles nPBI with ring sizes from two to nine PBI units, as well as the complexation of polycyclic aromatic guest compounds.
With a reduced but substantial fluorescence quantum yield of 21% (in CHCl3) the free host 2PBI(4-tBu)4 can be used as a dual fluorescence probe. Upon encapsulation of rather electron-poor guests the fluorescence quenching interactions between the chromophores are prevented, leading to a significant fluorescence enhancement to > 90% (“turn-on”). On the other hand, the addition of electron-rich guest molecules induces an electron transfer from the guest to the electron-poor PBI chromophores and thus quenches the fluorescence entirely (“turn-off”). The photophysical properties of the host-guest complexes were studied by transient absorption spectroscopy. These measurements revealed that the charge transfer between guest and 2PBI(4-tBu)4 occurs in the “normal region” of the Marcus-parabola with the fastest charge separation rate for perylene. In contrast, the charge recombination back to the PBI ground state lies far in the “inverted region” of the Marcus-parabola.
Beside complexation of planar aromatic hydrocarbons into the cavity of the cyclophanes an encapsulation of fullerene into the cyclic trimer 3PBI(4-tBu)4 was observed. 3PBI(4-tBu)4 provides a tube-like structure in which the PBI subunits represent the walls of those tubes. The cavity has the optimal size for hosting fullerenes, with C70 fitting better than C60 and a binding constant that is higher by a factor of 10. TA spectroscopy in toluene that was performed on the C60@3PBI(4-tBu)4 complex revealed two energy transfer processes. The first one comes from the excited PBI to the fullerene, which subsequently populates the triplet state. From the fullerene triplet state a second energy transfer occurs back to the PBI to generate the PBI triplet state.
In all cycles that were studied by TA spectroscopy, symmetry-breaking charge separation (SB-CS) was observed in dichloromethane. This process is fastest within the PBI cyclophane 2PBI(4-tBu)4 and slows down for larger cycles, suggesting that the charge separation takes place through space and not through bonds. The charges then recombine to the PBI triplet state via a radical pair intersystem crossing (RP-ISC) mechanism, which could be used to generate singlet oxygen in yields of ~20%.
By changing the solvent to toluene an intramolecular folding of the even-numbered larger cycles was observed that quenches the fluorescence and increases the 0-1 transition band in the absorption spectra. Force field calculations of 4PBI(4-tBu)4 suggested a folding into pairs of dimers, which explains the remarkable odd-even effect with respect to the number of connected PBI chromophores and the resulting alternation in the absorption and fluorescence properties. Thus, the even-numbered macrocycles can fold in a way that all chromophores are in a paired arrangement, while the odd-numbered cycles have open conformations (3PBI(4-tBu)4, 5PBI(4-tBu)4, 7PBI(4-tBu)4) or at least additional unpaired PBI unit (9PBI(4-tBu)4).
With these experiments we could for the first time give insights in the interactions between cyclic PBI hosts and aromatic guest molecules. Associated with the encapsulation of guest molecules a variety of possible applications can be envisioned, like fluorescence sensing, chiral recognition and photodynamic therapy by singlet oxygen generation. Particularly, these macrocycles provide photophysical relaxation pathways of PBIs, like charge separation and recombination and triplet state formation that are hardly feasible in monomeric PBI dyes. Furthermore, diverse compound specific features were found, like the odd-even effect in the folding process or the transition of superficial nanostructures of the tetrameric cycle influenced by the AFM tip. The comprehensive properties of these macrocycles provide the basis for further oncoming studies and can serve as an inspiration for the synthesis of new macrocyclic compounds.
Cofacial positioning of two perylene bisimide (PBI) chromophores at a distance of 6.5 angstrom in a cyclophane structure prohibits the otherwise common excimer formation and directs photoexcited singlet state relaxation towards intramolecular symmetry-breaking charge separation (τ\(_{CS}\) = 161 +/- 4 ps) in polar CH\(_2\)Cl\(_2\), which is thermodynamically favored with a Gibbs free energy of ΔG\(_{CS}\) = -0.32 eV. The charges then recombine slowly in τ\(_{CR}\) = 8.90 +/- 0.06 ns to form the PBI triplet excited state, which can be used subsequently to generate singlet oxygen in 27% quantum yield. This sequence of events is eliminated by dissolving the PBI cyclophane in non-polar toluene, where only excited singlet state decay occurs. In contrast, complexation of electron-rich aromatic hydrocarbons by the host PBI cyclophane followed by photoexcitation of PBI results in ultrafast electron transfer (<10 ps) from the guest to the PBI in CH\(_2\)Cl\(_2\). The rate constants for charge separation and recombination increase as the guest molecules become easier to oxidize, demonstrating that charge separation occurs close to the peak of the Marcus curve and the recombination lies far into the Marcus inverted region.
Electroactive Conjugated Polymers as Charge-Transport Materials for Optoelectronic Thin-Film Devices
(2005)
In this work the electrochemical and spectroelectrochemical properties of a series of pi-conjugated organic polymers were studied. The polymers were deposited on platinum electrodes or ITO-coated glass substrates by potentiodynamic electro-polymerisation of the corresponding monomeric precursor molecules. The electro-chemical and photophysical properties of the triarylborane monomers were studied in detail in order to estimate possible influences on the behaviour of the corresponding polymer. The first part of this work aimed at the synthesis and investigation of conjugated donor–acceptor polymers which combine the prerequisites of an OLED within one material: the transport of positive and negative charges and the formation of emissive excited states. With the carbazole-substituted oxadiazoles 1–3 it was shown that on the one hand the carbazole functionality is suitable for enabling the electrochemical polymerisation of the monomers and on the other hand it facilitates reversible p-doping of the resultant polymers. Although n-doping of poly-1–poly-3 is possible due to the electron-deficient oxadiazole rings, it causes the continuous degradation of these electron-acceptor units. Interestingly, this process does not influence the capability of p-doping of the polymers. With respect to its electrochemical and spectroelectrochemical properties the behaviour of the borane polymer poly-4 is absolutely identical with that of the oxadiazole polymers. Moreover, the optical excitation of poly-4 in the solid state leads to the emission of blue-green light which suggests that this polymer might also possess electroluminescent properties. AFM-measurements of poly-4 films on ITO-coated glass substrates revealed, that the film thickness can be controlled to a certain extent by the number of polymerisation redox cycles. It was shown from the electrochemical and photophysical properties of the triarylboranes 4–6 that the pi–pi-interaction between boron and nitrogen atoms is comparably weak in these molecules. This leads to an unexpected ground-state polarisation with a partially positive boron atom and a partially negative nitrogen atom. Moreover, it was found that TAB 4 possesses a lower symmetry than D3 in solution and that excitation energy can be transferred amongst the three subchromophores of 4. By titration experiments it was also demonstrated that TAB 4 can reversibly bind fluoride ions and that the binding event significantly influences the optical absorption characteristics of the chromophore. It can be assumed, that the above mentioned properties, which have a profound influence on the photophysical behaviour of these triarylborane chromophores, also determine the behaviour of the corresponding polymer in a solid state environment. The aim of the second part of this work was the investigation of purely n-conducting materials based on electron-deficient borane and viologen polymers. The corresponding precursor molecules should be polymerised on platinum electrodes by reductive electropolymerisation. However, a reductive polymerisation was not possible for the borane monomer 19 which is thought to be due to a strong localisation of the unpaired electron on the central boron atom of the radical anion. An electropolymerisation of the cyano-substituted bispyridinio-compound 17 failed because of the poor quality of CN– as a leaving group. Thus, a synthesis of the analogous isomer 18 was developed, in which the cyano-substituents were exchanged by the better leaving group Cl–. The viologen polymer poly-18, which can be regarded as an electron-deficient iso-electronic analogue of poly(para-phenylene), was successfully deposited on a platinum electrode by reductive electropolymerisation of 18. Poly-18 can be reversibly n-doped at comparably low potentials; however, at higher potentials the polymer is overcharged and destroyed irreversibly. As the synthetic strategy for 18 allows the variation of both spacer unit and leaving group in the last two steps of the reaction sequence, a series of analogous compounds can be easily synthesised using this route.
Results ofan ab initio study ofthe hyperfine structure of the X\(^2\)A', A\(^2\) A" ( 1\(^2 \Pi\)) system ofthe formyl radical are presented. Special attention is paid to the analysis of the interplay between the vibronic and magnetic hyperfine etfects. The results of computations are in very good agreement with the available experimental findings. The values for the hyperfine coupling constants in lower bending Ievels of both electronic species are predicted.
No abstract available
The focus of this work was the investigation of energy transfer between charge transfer states. For this purpose the multidimensional chromophores HAB-S, HAB-A, B1 and B2 were synthesised, each consisting of three electron donor and three electron acceptor redox centres linked symmetrically or asymmetrically by the hexaarylbenzene framework. Triarylamines represent in all these compounds the electron donors, whereas the electron poor centres were triarylboranes in B1 and B2 and PCTM centres in HAB-S and HAB-A, respectively. The hexaarylbenzenes were obtained by cobalt catalysed cyclotrimerisation of the respective tolan precursors. In addition, Star was synthesised, which consists of a central PCTM linked to three triarylamin centres by tolan bridging units in a star-like configuration. The hexaarylbenzene S1a/b substituted with six squaraine chromophores could not be realised. It is assumed that the cyclotrimerisation catalyst Co2(CO)8 does not tolerate the essential hydroxyl groups in the tolan precursor S2a. The alternative reaction pathway to execute the cyclotrimerisation reaction first and introduce the hydroxyl groups thereafter failed as well, because the required hexaarylbenzene substituted by six semisquaric acid moieties could not be synthesised. However, energy transfer interactions could be investigated in the tolan precursor S2a with two squaraine units to obtain information about the electronic coupling provided by the tolan bridge. For all multidimensional compounds model molecules were synthesised with only a single donor-acceptor pair (B3, Star-Model and HAB-Model). This allows a separate consideration of energy and charge transfer processes. It has to be stressed that in all before mentioned multidimensional compounds the “through bond” energy transfer interaction between neighbouring IV-CT states is identical to a transfer of a single electron between two redox centres of the same kind (e.g. TAA -> TAA+). The latter can be analysed by electron transfer theory. This situation is observed when the two IV-CT states transferring energy share one redox centre.
All compounds containing PCTM centres were characterised by paramagnetic resonance spectroscopy. Thereby, a weak interaction between the three PCTM units in HAB-S and HAB-A was observed. In addition, when oxidising Star-Model, a strongly interacting singlet or triplet state was obtained. In contrast, signals corresponding to a weakly interacting biradical were obtained for HAB-Model+. This indicates a strong electronic coupling between the redox centres provided by the tolan bridge and a weak coupling when linked by the hexaarylbenzene. This trend is supported by UV/Vis/NIR absorption measurements. The analysis of the observed IV-CT absorption bands by electron transfer theory reveals a weak electronic coupling of V = 340 cm-1 in HAB-Model and a distinctly stronger coupling of V = 1190-2900 cm-1 in Star-Model. In the oxidised HAB-S+, Star+ and Star-Model+ a charge transfer reversed from that of the neutral species, that is, from the PCTM radical to the electron poorer cationic TAA centre, was observed by spectroelectrochemistry. The temporal evolution of the excited states was monitored by ultrafast transient absorption measurements. Within the first picosecond stabilisation of the charge transfer state was observed, induced by solvent rotation. Anisotropic transient absorption measurements revealed that within the lifetime of the excited state (tau = 1-4 ps) energy transfer does not occur in the HABs whereas in the star-like system ultrafast and possibly coherent energy redistribution is observed. Taken this information together the identity between energy transfer and electron transfer in the specific systems were made apparent. It has to be remarked that neither energy transfer nor charge transfer theory can account for the very fast energy transfer in Star.
The electrochemical and photophysical properties of B1 and B2 were investigated by cyclic voltammetry, absorption and fluorescence measurements and were compared to B3 with only one neighbouring donor-acceptor pair. For the asymmetric B2 CV measurements show three oxidations as well as three reduction peaks whose peak separation is greatly influenced by the conducting salt due to ion-pairing and shielding effects. Consequently, peak separations cannot be interpreted in terms of electronic couplings in the generated mixed valence species. Transient absorption, fluorescence solvatochromism and absorption spectra show that charge transfer states from the amine to the boron centres are generated after optical excitation. The electronic donor-acceptor interaction is weak though as the charge transfer has to occur predominantly through space. The electronic coupling could not be quantified as the CT absorption band is superimposed by pi-pi* transitions localised at the amine and borane centres. However, this trend is in good agreement to the weak coupling measured for HAB-Model. Both transient absorption and fluorescence upconversion measurements indicate an ultrafast stabilisation of the charge transfer state in B1- B3 similar to the corresponding observations in HAB-S and Star. Moreover, the excitation energy of the localised excited charge transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within fluorescence lifetime (ca. 60 ns). This was proved by steady state fluorescence anisotropy measurements, which further indicate a symmetry breaking in the superficially symmetric HAB. Anisotropic fluorescence upconversion measurements confirm this finding and reveal a time constant of tau = 2-3 ps for the energy transfer in B1 and B2. It has to be stressed that, although the geometric structures of B1 and HAB-S are both based on the same framework and furthermore the neighbouring CT states show in both cases similar Coulomb couplings and negligible “through bond” couplings, very fast energy transfer is observed in B1 whereas in HAB-S the energy is not redistributed within the excited state lifetime. To explain this, it has to be kept in mind that the energy transfer and the relaxation of the CT state are competing processes. The latter is influenced moreover by the solvent viscosity. Hence, it is assumed that this discrepancy in energy transfer behaviour is caused by monitoring the excited state in solvents of varying viscosity. Adding fluoride ions causes the boron centres to lose their acceptor ability due to complexation. Consequently, the charge transfer character in the donor-acceptor chromophores vanishes which could be observed in both the absorption and fluorescence spectra. However, the fluoride sensor ability of the boron centre is influenced strongly by the moisture content of the solvent possibly due to hydrogen bonding of water to the fluoride anions.
UV/Vis/NIR absorption measurements of S2a show a red-shift by 1800 cm-1 of the characteristic squarain band compared to the model compound S20. From exciton theory a Coulomb coupling of V = 410 cm-1 is calculated which cannot account for this strong spectral shift. Consequently, “through-bond” interactions have to contribute to the strong communication between the two squaraine chromophores in S2a. This is in accordance with the strong charge transfer coupling calculated for the tolan spacer in Star-Model.
Der bekannteste Vertreter der Bispidine ist das Lupinenalkaloid (–)-Spartein, das vor allem in enantioselektiven Deprotonierungen aber auch Übergangsmetall-katalysierten asymmetrischen Reaktionen als Ligand der Wahl eingesetzt wird. Daneben gibt es nur wenige weitere synthetische Vertreter, da keine flexiblen Darstellungsmethoden zu enantiomerenreinen Bispidinen mit variablen Substituenten in 2-endo-Position existieren. Ein zielgerichtetes Design solcher Verbindungen war daher bisher nur eingeschränkt möglich. So sollte in dieser Arbeit eine neue Substanzklasse als chirale Liganden in der asymmetrischen Synthese etabliert werden: 2-endo-substituierte 9-Oxabispidine. Das Brücken-Sauerstoffatom sollte die Synthese stark vereinfachen. Insgesamt wurden drei strategisch unterschiedliche Methoden zur enantioselektiven Synthese von 2-endo-substituierten 9-Oxabispidinen entwickelt. Zunächst ist die sehr kurze Route zu 2-endo-Phenyl-substituierten Derivaten mit diversen Resten R' an den Stickstoff-Funktionen zu nennen. Ausgehend von käuflichem (R,R)-Phenylglycidol wurde (S,S)-3-Benzylamino-3-phenyl-1,2-propandiol dargestellt, das in einer Dreistufen-Eintopf-Reaktion mit (S)-Epichlorhydrin kondensiert und zum all-cis-konfigurierten 2,6-Dimesyloxymethyl-3-phenylmorpholin mesyliert wurde. Die finale Cylisierung erfolgte mit primären Aminen zu verschiedenen 2-endo-Phenyl-9-oxabispidinen in insgesamt drei bis fünf Stufen. Die Darstellung des tricyclischen 9-Oxa-Derivats eines bekannten (+)-Spartein-Ersatzstoffs gelang nach einem verwandten Syntheseprotokoll. Für eine effiziente Variation des 2-endo-Substituenten auf einer späten Synthesestufe wurde zunächst enantiomerenreines 3N-Boc-7N-Methyl-9-oxabispidin-2-on als Schlüsselintermediat ausgewählt, das aus (R)-Epichlorhydrin und racemischem Glycidsäuremethylester dargestellt wurde. Die weitere Überführung in die 9-Oxabispidine wurde durch Grignard-Addition, Abspaltung der N-Boc-Gruppe vom resultierenden, ringoffenen N-Boc-Aminoketon, Cyclisierung zum Imin und abschließende, exo-selektive Reduktion oder Hydrierung erreicht. So wurden bi- und tricyclische 9-Oxabispidine in nur drei Stufen und 51% Ausbeute synthetisiert. Ein größeres Potenzial besitzt jedoch der primär von David Hein parallel zu den eigenen Arbeiten entwickelte Zugang über ein cis-konfiguriertes 6-(N-Boc-Aminomethyl)morpholin-2-carbonitril als zentrale Zwischenstufe, das auch im 10-g-Maßstab problemlos erhältlich war. Die allgemeine Anwendbarkeit und Flexibilität dieser Methode wurde anhand der Darstellung einer Reihe an 9-Oxabispidinen demonstriert. Die dargestellten chiralen 9-Oxabispidine wurden erstmalig in den folgenden fünf unterschiedlichen Gebieten der asymmetrischen Synthese getestet: Organolithium- und Organomagnesium-vermittelte Umsetzungen sowie Pd(II)-, Cu(II) und Zink(II)-katalysierte Reaktionen. Für enantioselektive Deprotonierungen schwach C-H-acider Verbindungen mit sBuLi erwiesen sich die 9-Oxabispidine als ungeeignet, da sie selbst in Brückenkopfposition lithiiert wurden. Die Stabilität der resultierenden -Lithioether war unerwartet hoch; sie ließen sich beispielsweise bei -78 °C in guten Ausbeuten mit Elektrophilen abfangen. Umlagerungen traten erst beim Aufwärmen ein, wenn kein Elektrophil als Reaktionspartner zur Verfügung stand. Als definierte Produkte wurden dabei Ring-kontrahierte N,O-Acetale erhalten. Zusammen mit den weniger basischen Grignard-Reagenzien wurden die 9-Oxabispidine erfolgreich zur Desymmetrisierung von meso-Anhydriden verwendet. Bei Pd(II)-katalysierten oxidativen kinetischen Racematspaltungen sekundärer Alkohole konnten mit einem 9-Oxabispidin-Pd(II)-Katalysator Selektivitätsfaktoren s vergleichbar zu denen mit (–)-Spartein erzielt werden. Die Ursache für die geringere Reaktivität der 9-Oxabispidin-Komplexe liegt gemäß quantenchemischen Berechnungen in der Elektronegativität des Brücken-Sauerstoffatoms, was die Elektronendichte am Palladiumatom verringert. In Kooperation mit David Hein wurde ein von einem tricyclischen 9-Oxabispidin abgeleiteter Cu-Katalysator entwickelt, der bei der Addition von Nitromethan an zahreiche aromatische, heteroaromatische und aliphatische Aldehyde exzellente Enantioselektivitäten im Bereich von 91–97% lieferte. Mit bicyclischen, 2-endo-substituierten 9-Oxabispidinen als chiralen Liganden wurden hingegen lediglich 33-57% ee erreicht bemerkenswerterweise entstanden hierbei bevorzugt die enantiokomplementären β-Nitroalkohole. In Zusammenarbeit mit Janna Börner aus der Arbeitsgruppe von S. Herres-Pawlis wurde der erste chirale, neutrale Diamin-Zink(II)-Katalysator für die Ringöffnungs-Polymerisation von D,L-Lactid entwickelt. Die Reaktion benötigte kein weiteres anionisches Additiv und konnte ohne Lösungsmittel mit nicht-aufgereinigtem, käuflichem Lactid durchgeführt werden.
Die Produktion von Abwehr-, Signal- und Botenstoffen sichert vielen Pflanzen und Mikroorganismen das Überleben in einer sich ständig wandelnden Umwelt mit zahlreichen Konkurrenten und Feinden. Diese Sekundärmetabolite können oft medikamentös gegen Pathogene eingesetzt werden, die den Menschen befallen und Krankheiten verursachen. Die Herausforderung besteht dabei in der selektiven und sensitiven Detektion, der schonenden Isolierung und der richtigen und kompletten Strukturaufklärung dieser Moleküle, sowie der eventuellen synthetischen Modifikation, um eine bessere Verträglichkeit oder Wirkung für den menschlichen Körper zu erreichen. Leistungsfähige chromatographische Instrumente zur Trennung wie HPLC und CZE, emfindliche Detektoren wie UV- und Massenspektrometer, sowie aussagekräftige Messverfahren zur Charakterisierung struktureller Merkmale wie NMR- und CD-Spektroskopie und quantenchemische Rechnungen sind dabei von essentieller Bedeutung.
Mit diesen – und weiteren – Methoden gelang in der vorliegenden Arbeit die Detektion, Isolierung und Strukturaufklärung neuer Naphthylisochinolin-Alkaloide aus zwei tropischen Ancistrocladus-Lianen, die Charakterisierung von bekannten und neuen Polyketiden aus einem Pilz der Gattung Streptomyces, sowie die Analyse von Glucosinolaten im Phloemsaft der Modellpflanze Arabidopsis thaliana.
Fluorogenic Aptamers and Fluorescent Nucleoside Analogs as Probes for RNA Structure and Function
(2020)
RNA plays a key role in numerous cellular processes beyond the central dogma of molecular biology. Observing and understanding this wealth of functions, discovering new ones and engineering them into purpose-built tools requires a sensitive means of observation. Over the past decade, fluorogenic aptamers have emerged to fill this niche. These short oligonucleotides are generated by in vitro selection to specifically interact with small organic fluorophores and can be utilized as genetically encoded tags for RNAs of interest.
The most versatile class of fluorogenic aptamers is based on derivatives of hydroxybenzylidene imidazolone (HBI), a conditional fluorophore mimicking the chromophore structure found in green and red fluorescent proteins. The respective aptamers are well-known by the “vegetable” nomenclature, including Spinach, Broccoli and Corn, and have found numerous applications for studying RNA function in vitro and in cells.
Their success, however, is somewhat overshadowed by individual shortcomings such as a propensity for misfolding, dependence on unphysiologically high concentrations of magnesium ions or, in the case of Corn, dimerization that might affect the function of the tagged RNA. Moreover, most fluorogenic aptamers exhibit limited ligand promiscuity by design, thereby restricting their potential for spectral tuning to a narrow window of wavelengths.
This thesis details the characterization of a new fluorogenic aptamer system nicknamed Chili. Chili is derived from an aptamer that was originally selected to bind 4-hydroxy-3,5-dimethoxy¬hydroxy-benzylidene imidazolone (DMHBI), resulting in a green fluorescent complex. Unlike other aptamers of its kind, Chili engages in a proton transfer cycle with the bound ligand, resulting in a remarkably large Stokes shift of more than 130 nm.
By means of an empirical ligand optimization approach, several new DMHBI derivatives were found that bind to Chili with high affinity, furnishing complexes up to 7.5 times brighter compared to the parent ligand. In addition, Chili binds to π-extended DMHBI derivatives that confer fluorescence in the yellow–red region of the visible spectrum. The highest affinity and degree of fluorescence turn-on for both green and red fluorogenic ligands were achieved by the incorporation of a unique, positively charged substituent into the HBI scaffold.
Supplemented by NMR spectroscopy, kinetic and thermodynamic studies showed that the binding site of Chili is loosely preorganized in the absence of ligand and likely forms a G-quadruplex upon ligand binding.
To showcase future applications, Chili was incorporated into a FRET sensor for monitoring the cleavage of an RNA substrate by a 10-23 DNAzyme.
Besides aptamers as macromolecular fluorescent complexes, fluorescent nucleobase analogs are powerful small isomorphic components of RNA suitable for studying structure and folding. Here, the highly emissive nucleobase analog 4-cyanoindole (4CI) was developed into a ribonucleoside (r4CI) for this purpose. A new phosphoramidite building block was synthesized to enable site-specific incorporation of 4CI into RNA.
Thermal denaturation experiments confirmed that 4CI behaves as a universal nucleobase, i.e. without bias towards any particular hybridization partner. Photophysical characterization established r4CI as a generally useful fluorescent ribonucleoside analog. In this work, it was employed to gain further insight into the structure of the Chili aptamer. Using several 4CI-modified Chili–HBI complexes, a novel base–ligand FRET assay was established to obtain a set of combined distance and orientation restraints for the tertiary structure of the aptamer.
In addition to their utility for interrogating structure and binding, supramolecular FRET pairs comprising a fluorescent nucleobase analog donor and an innately fluorogenic acceptor hold great promise for the construction of color-switchable RNA aptamer sensor devices.
Structure-fluorescence activation relationships of a large Stokes shift fluorogenic RNA aptamer
(2019)
The Chili RNA aptamer is a 52 nt long fluorogen-activating RNA aptamer (FLAP) that confers fluorescence to structurally diverse derivatives of fluorescent protein chromophores. A key feature of Chili is the formation of highly stable complexes with different ligands, which exhibit bright, highly Stokes-shifted fluorescence emission. In this work, we have analyzed the interactions between the Chili RNA and a family of conditionally fluorescent ligands using a variety of spectroscopic, calorimetric and biochemical techniques to reveal key structure - fluorescence activation relationships (SFARs). The ligands under investigation form two categories with emission maxima of ~540 nm or ~590 nm, respectively, and bind with affinities in the nanomolar to low-micromolar range. Isothermal titration calorimetry was used to elucidate the enthalpic and entropic contributions to binding affinity for a cationic ligand that is unique to the Chili aptamer. In addition to fluorescence activation, ligand binding was also observed by NMR spectroscopy, revealing characteristic signals for the formation of a G-quadruplex only upon ligand binding. These data shed light on the molecular features required and responsible for the large Stokes shift and the strong fluorescence enhancement of red and green emitting RNA-chromophore complexes.
RNA aptamers form compact tertiary structures and bind their ligands in specific binding sites. Fluorescence-based strategies reveal information on structure and dynamics of RNA aptamers. Here we report the incorporation of the universal emissive nucleobase analog 4-cyanoindole into the fluorogenic RNA aptamer Chili, and its application as a donor for supramolecular FRET to bound ligands DMHBI+ or DMHBO+. The photophysical properties of the new nucleobase-ligand-FRET pair revealed structural restraints for the overall RNA aptamer organization and identified nucleotide positions suitable for FRET-based readout of ligand binding. This strategy is generally suitable for binding site mapping and may also be applied for responsive aptamer devices.
In this work, two new quadrupolar A-π-D-π-A chromophores have been prepared featuring a strongly electron- donating diborene core and strongly electron-accepting dimesitylboryl F(BMes2) and bis(2,4,6-tris(trifluoromethyl)phenyl)boryl (BMes2) end groups. Analysis of the compounds by NMR spectroscopy, X-ray crystallography, cyclic voltammetry and UV-vis-NIR absorption and emission spectroscopy indicated that the compounds possess extended conjugated π-systems spanning their B4C8 cores. The combination of exceptionally potent π-donor (diborene) and π- acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO-LUMO gaps, resulting in strong absorption in the near-IR region with maxima in THF at 840 and 1092 nm, respectively, and very high extinction coefficients of ca. 120,000 M-1cm-1. Both molecules also display weak near-IR fluorescence with small Stokes shifts.
This work encompasses three parts. The first part provides a concise review of the most prominent metaheuristic concepts currently available and gives essential preliminaries together with definition of the combinatorial optimization problems. It substantiates the choice of the investigation direction and basis idea of the developed methods. In the second part the new nonlinear global optimization routines based on the TS strategy are described. The new approaches are the Gradient Tabu Search (GTS), the Gradient Only Tabu Search (GOTS), and the Tabu Search with Powell’s Algorithm (TSPA). In the last part of the work the GOTS is applied for such chemical optimization problems. The chapter provides a systematic approach how the variables are chosen and the adjustable parameters are set. As test cases the global minimum energy conformation of some amino acids, of two angiotensin converting enzyme (ACE) inhibitors, of 2-acetoxy-N,N,N-trimethylethanaminium, and of a HIV-1 protease inhibitor is determined.
The subject of this thesis is the synthesis and characterization of PBI-based fluorescent metallosupramolecular polymers and cyclic arrays. Terpyridine receptor functionalized PBIs of predesigned geometry have been used as building blocks to construct desired macromolecular structures through metal-ion-directed self-assembly. These metallosupramolecular architectures have been investigated by NMR, UV/Vis and fluorescence spectroscopy, mass spectrometry, and atomic force microscopy.
As central components of life, DNA and RNA encode the genetic information. However, RNA performs several functions that exceed the competences stated in the ‘central dogma of life‘. RNAs undergo extensive post-transcriptional processing like chemical modifications. Among all classes of RNA, tRNAs are the most extensively modified. Their modifications are chemically diverse and vary from simple methylations (e.g. m3C, m6A) to more complex residues, like isopentenyl group (e.g. i6A, hypermodifications: e.g. ms2i6A) or even amino acids (e.g. t6A). Depending on their location within the overall structure, modifications can have an impact on tRNA stability and structure, as well as affinity for the ribosome and translation efficiency and fidelity. Given the importance of tRNA modifications new tools are needed for their detection and to study their recognition by proteins and enzymatic transformations.
The chemical synthesis of these naturally occurring tRNA modifications as phosphoramidite building blocks is a prerequisite to incorporate the desired modification via solid-phase synthesis into oligonucleotides. With the help of the m3C, (ms2)i6A, and t6A oligonucleotides, the importance and impact of tRNA modifications was investigated in this thesis. To this end, the role of METTL8 as the methyltransferase responsible for the installation of the methyl group at C32 for mt-tRNAThr and mt-tRNASer(UCN) was resolved. Thereby, the respective adenosine modification on position 37 is essential for the effectiveness of the enzyme. Besides, by means of NMR analysis, CD spectroscopy, thermal denaturation experiments, and native page separation, the impact of m3C32 on the structure of the tRNA ASLs was shown. The modification appeared to fine-tune the tRNA structure to optimize mitochondrial translation. To investigate the regulation of the dynamic modification pathway of m3C, demethylation assays were performed with the modified tRNA-ASLs and the (α-KG)- and Fe(II)-dependent dioxygenase ALKBH1 and ALKHB3. A demethylation activity of ALKBH3 on the mt-tRNAs was observed, even though it has so far only been described as a cytoplasmic enzyme. Whether this is physiologically relevant and ALKBH3 present a mitochondrial localization needs further validation. In addition, ALKBH1 was confirmed to not be able to demethylate m3C on mt-tRNAs, but indications for a deprenylation and exonuclease activity were found. Furthermore, the aforementioned naturally occurring modifications were utilized to find analytical tools that can determine the modification levels by DNAzymes, which cleave RNA in the presence of a specific modification. Selective DNA enzymes for i6A, as well as the three cytidine isomers m3C, m4C, and m5C have been identified and characterized.
Besides the naturally occurring tRNA modifications, the investigation on artificially modified nucleosides is also part of this thesis. Nucleosides with specific properties for desired applications can be created by modifying the scaffold of native nucleosides.
During the pandemic, the potential of antiviral nucleoside analogues was highlighted for the treatment of the SARS-CoV-2 infection. For examinations of the potential drug-candidate Molnupiravir, the N4-hydroxycytidine phosphoramidite building block was synthesized and incorporated into several RNA oligonucleotides. A two-step model for the NHC-induced mutagenesis of SARS-CoV-2 was proposed based on RNA elongation, thermal denaturation, and cryo-EM experiments using the modified RNA strands with the recombinant SARS-CoV-2 RNA-dependent RNA polymerase. Two tautomeric forms of NHC enable base pairing with guanosine in the amino and with adenosine in the imino form, leading to error catastrophe after the incorporation into viral RNA. These findings were further corroborated by thermal melting curve analysis and NMR spectroscopy of the NHC-containing Dickerson Drew sequence. In conclusion, the anti-amino form in the NHC-G base pair was assigned by NMR analysis using a 15N-labeld NHC building block incorporated into the Dickerson Drew sequence.
This thesis also addressed the synthesis of a 7-deazaguanosine crosslinker with a masked aldehyde as a diol linker for investigations of DNA-protein interactions. The diol functional group can be unmasked to release the reactive aldehyde, which can specifically form a covalent bond with amino acids Lys or Arg within the protein complex condensin. The incorporation of the synthesized phosphoramidite and triphosphate building blocks were shown and the functionality of the PCR product containing the crosslinker was demonstrated by oxidation and the formation of a covalent bond with a fluorescein label.
The development of assays that detect changes in this methylation pattern of m6A could provide new insights into important biological processes. In the last project of this thesis, the influence of RNA methylation states on the structural properties of RNA was analyzed and a fluorescent nucleoside analog (8-vinyladenosine) as molecular tools for such assays was developed. Initial experiments with the fluorescent nucleoside analog N6-methyl-8-vinyladenosine (m6v8A) were performed and revealed a strong fluorescence enhancement of the free m6v8A nucleoside by the installation of the vinyl moiety at position 8.
Overall, this thesis contributes to various research topics regarding the application of naturally occurring and artificial nucleoside analogues. Starting with the chemical synthesis of RNA and DNA modifications, this thesis has unveiled several open questions regarding the dynamic (de-)methylation pathway of m3C and the mechanism of action of molnupiravir through in-depth analysis and provided the basis for further investigations of the protein complex condensin, and a new fluorescent nucleoside analog m6v8A.