Refine
Has Fulltext
- yes (11287) (remove)
Year of publication
- 2024 (198)
- 2023 (725)
- 2022 (1060)
- 2021 (1238)
- 2020 (801)
- 2019 (796)
- 2018 (699)
- 2017 (511)
- 2016 (643)
- 2015 (582)
- 2014 (587)
- 2013 (503)
- 2012 (506)
- 2011 (344)
- 2010 (206)
- 2009 (104)
- 2008 (106)
- 2007 (83)
- 2006 (71)
- 2005 (75)
- 2004 (52)
- 2003 (41)
- 2002 (40)
- 2001 (30)
- 2000 (15)
- 1999 (8)
- 1998 (1)
- 1995 (1)
- 1994 (140)
- 1993 (159)
- 1992 (118)
- 1991 (121)
- 1990 (98)
- 1989 (94)
- 1988 (83)
- 1987 (70)
- 1986 (63)
- 1985 (39)
- 1984 (38)
- 1983 (25)
- 1982 (30)
- 1981 (20)
- 1980 (16)
- 1979 (21)
- 1978 (19)
- 1977 (13)
- 1976 (12)
- 1975 (13)
- 1974 (11)
- 1973 (13)
- 1972 (15)
- 1971 (8)
- 1970 (6)
- 1969 (5)
- 1968 (1)
Document Type
- Journal article (7912)
- Doctoral Thesis (2819)
- Book article / Book chapter (133)
- Conference Proceeding (107)
- Preprint (107)
- Working Paper (67)
- Review (43)
- Report (27)
- Master Thesis (26)
- Book (19)
- Other (19)
- Bachelor Thesis (6)
- Habilitation (1)
- Study Thesis (term paper) (1)
Language
- English (11287) (remove)
Keywords
- Toxikologie (119)
- Medizin (99)
- inflammation (95)
- Psychologie (86)
- Biochemie (85)
- cancer (81)
- Organische Chemie (68)
- gene expression (68)
- Anorganische Chemie (66)
- Maus (64)
Institute
- Theodor-Boveri-Institut für Biowissenschaften (1956)
- Graduate School of Life Sciences (811)
- Physikalisches Institut (606)
- Institut für Psychologie (503)
- Neurologische Klinik und Poliklinik (401)
- Medizinische Klinik und Poliklinik II (391)
- Medizinische Klinik und Poliklinik I (378)
- Institut für Organische Chemie (375)
- Institut für Molekulare Infektionsbiologie (364)
- Institut für Anorganische Chemie (357)
Schriftenreihe
- Cultural Animal Studies, Band 3 (24)
- Berichte aus der Informatik (1)
- Deuterocanonical and Cognate Literature Studies (1)
- Deuterocanonical and Cognate Literature Yearbook (1)
- International Archives of the History of Ideas / Archives internationales d’histoire des idées 242 (1)
- Methods in Molecular Biology 2533 (1)
- Methods in Molecular Biology; 2643 (1)
Sonstige beteiligte Institutionen
- VolkswagenStiftung (24)
- Johns Hopkins School of Medicine (17)
- Helmholtz Institute for RNA-based Infection Research (HIRI) (8)
- IZKF Nachwuchsgruppe Geweberegeneration für muskuloskelettale Erkrankungen (7)
- Clinical Trial Center (CTC) / Zentrale für Klinische Studien Würzburg (ZKSW) (5)
- Fraunhofer-Institut für Silicatforschung ISC (5)
- Johns Hopkins University School of Medicine (5)
- Wilhelm-Conrad-Röntgen-Forschungszentrum für komplexe Materialsysteme (5)
- Bernhard-Heine-Centrum für Bewegungsforschung (4)
- Johns Hopkins School of Medicine, Baltimore, MD, U.S. (4)
ResearcherID
- B-1911-2015 (1)
- B-4606-2017 (1)
- C-2593-2016 (1)
- D-1221-2009 (1)
- D-1250-2010 (1)
- I-5818-2014 (1)
- J-8841-2015 (1)
- M-1240-2017 (1)
- N-2030-2015 (1)
- N-3741-2015 (1)
In this thesis, soft x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) studies of the electronic structure of selected organic molecules and liquids were carried out. The first part focuses on the used experimental techniques and the development of the instrumentation necessary for these studies, namely a soft x-ray spectrometer, and a temperature-controlled flow-through liquid cell. The former was optimized by a special analytical ray tracing method developed exclusively for this purpose. Due to its high transmission, the spectrometer facilitates a novel experimental approach of recording comprehensive 'RIXS maps', which are 2-dimensional plots of x-ray scattering intensities as a function of both, excitation and emission photon energy. The liquid cell extends these possibilities to the study of liquids, especially the interaction of molecules in liquids and their chemical reactions under well-controlled conditions. Organic molecules have attracted considerable attention in the last decade. The intense research activities related to these materials have two main motivations: on the one hand, organic molecules have a technological application as building blocks of organic semiconductors, while, on the other hand, organic molecules are the functional elements in biological systems. In order to cost-effectively produce optimized organic electronic devices, a fundamental knowledge of the electronic properties of the organic molecules interface is necessary. Therefore, many studies of the electronic structure of potential candidates for organic electronics exist. Two of these candidates, namely C60 and well-ordered multilayers PTCDA on a Ag(111) surface are investigated in this thesis. For the study of C60 molecules, a comprehensive 'RIXS map' was recorded and analyzed. The RIXS map taken in only 25 minutes allows a quantitative analysis of energy losses, yielding for example the HOMO-LUMO distance. It also identifies a core-excitonic state and facilitates a quantitative comparison of its binding energy with that of valence excitons in C60. Furthermore, decay channel-selective partial fluorescence yield XAS spectra can be extracted from the RIXS map, yielding information on the population of the core-excitonic state as a function of excitation energy. As a second model system of organic molecules relevant for organic electronics, PTCDA was chosen. The complex electronic structure of the occupied states of a highly ordered, flat-lying PTCDA multilayer on a Ag(111) surface was investigated by symmetry-resolved resonant x-ray emission spectroscopy. The rapidly occurring beam damage effects were characterized on the basis of irradiation-time dependent series of C and O x-ray emission spectra. Upon varying the excitation energy and emission geometry, atom- and symmetry-specific carbon K emission spectra with negligible beam damage effects were obtained that allow to distinguish between electronic states with sigma and pi symmetry. A density functional theory calculation of the PTCDA molecule reproduces the energy positions of the most prominent emission features remarkably well. In addition, the energy positions of the sigma and pi emissions agree well with the calculated energies of the respective orbitals. In order to shed light on the second aspect of organic molecules, namely their role in biological systems, first a detailed investigation of the electronic structure and proton dynamics of liquid water as the medium of most chemical and biochemical reactions was carried out. Therefore, a comprehensive oxygen K RIXS map of liquid water was recorded and analyzed in great detail. A temperature-dependent comparison with XAS and RIXS data of D2O, NaOH, and NaOD leads to the conclusion, that ultra-fast dissociation takes place in liquid water on the timescale of the oxygen 1s core hole lifetime, resulting in a characteristic spectral contribution in the RIXS spectra. The dissociation is promoted by intact hydrogen bonds with neighboring molecules. In consequence, the rate of dissociation directly depends on the initial hydrogen bond configuration. In the next step towards biologically relevant systems, the nitrogen K edges of the amino acids glycine and histidine were investigated in powderous form as well as in their native environment, namely in aqueous solution. X-ray absorption and emission spectra of the aqueous solutions were analyzed at pH-values of 6 and for glycine also at pH 12 and compared to the spectra of powders. A pH-value of 12 causes deprotonation of the amino group, leading to significant changes in the nitrogen spectra as compared to pH 6. The results from these four examples demonstrate that a wealth of novel information can be obtained by using the new experimental tools developed in this thesis, namely a highly sensitive x-ray spectrometer and a flow-through liquid cell.
The pioneer tree Macaranga in SE Asia has developed manyfold associations with ants. The genus comprises all stages of interaction with ants, from facultative relationships to obligate myrmecophytes. Only myrmecophytic Macaranga offer nesting space for ants and are associated with a specific ant partner. The nonmyrmecophytic species are visited by a variety of different ant species which are attracted by extrafloral nectaries (EFN) and food bodies. Transitional Macaranga species like M. hosei are colonized later in their development due to their stem structure. Before the colonization by their specific Crematogaster partner the young plants are visited by different ant species attracted by EFN. These nectaries are reduced and food body production starts as soon as colonization becomes possible. We demonstrated earlier that obligate ant partners can protect their Macaranga plants against herbivore damage and vine cover. In this study we focused on nonspecific interactions and studied M. tanarius and M. hosei, representing a non-myrmecophyte and a transitional species respectively. In ant exclusion experiments both M. tanarius and M. hosei suffered significantly higher mean leaf damage than controls, 37% versus 6% in M. hosei, 16% versus 7% in M. tanarius. M. tanarius offers both EFN and food bodies so that tests for different effects of these two food rewards could be conducted. Plants with food bodies removed but with EFN remaining had the lowest mean increase of herbivore damage of all experimental groups. Main herbivores on M. hosei were mites and caterpillars. Many M. tanarius plants were infested by a shootborer. Both Macaranga species were visited by various ant species. Crematogaster spp. being the most abundant. We found no evidence for any specific relationships. The results of this study strongly support the hypothesis that non-specific, facultative associations with ants can be advantageous for Macaranga plants. Food bodies appear to have lower attractive value for opportunistic ants than EFN and may require a specific dietary adaptation. This is also indicated by the fact that food body production in the transitional M. hosei does not start before stem structure allows a colonization by the obligate Crematogaster species. M. hosei thus benefits from facultative association with a variety of ants until it produces its first domatia and can be colonized by its obligate mutualist.
C. borneensis (Myrmicinae) lives in dose association with several myrmecophytic species of the South East Asian pioneer tree genus Macaranga (Euphorbiaceae). The ants are adapted to the plants so dosely that they do not survive away from it. The only food they utilize is provided as food bodies by the plant and honeydew from specific scale insects kept inside the hollow internodes. The anatomy of the digestive tract is also adapted to life on the host plant: the crop is very sm all and can store only minute food quantities. C. borneensis exdusively colonizes certain Macaranga species. Queens as weIl as workers are able to recognize their host plant species, probably by chemical cues. Colony founding queens swarm throughout the year, mostly during darkness. There is strong competition among queens for host plants. Queens do not carry scale insects on their nuptial flight. Worker ants are active day and night. Most of them patrol and collect food bodies on the younger parts of the host plant. An important characteristic is their deaning behaviour, which results in removal of aIl foreign objects. Even though they are rather smalI, workers respond very aggressively to certain kinds of disturbance of the host plant. The ants attack most phytophagous insects and are especially effective in killing and removing smalI, softbodied herbivores (e.g. caterpillars). They do not possess a functional sting, but apply defensive secretion and-once biting an intruder-will not let go. Their effective alarm system results in a mass attack, which provides adequate defence for the colony and the host plant. A comparison with another Crematogaster species further illustrated the special adaptations of C. borneensis to its host plant.
At early developmental stages (embryonic day 7, E7), chick paravertebral sympathetic ganglia contain a cell population that divides in culture while expressing various neuronal properties. In an attempt to identify factors that control neuronal proliferation, we found that ciliary neurotrophic factor (CNTF) specifically inhibits the proliferation of those cells expressing neuronal markers. In addition, CNTF affects the differentiation of sympathetic ganglion cells by inducing the expression of vasoactive intestinal peptide immunoreactivity (VIP-IR). After 1 day in culture, tyrosine hydroxylase immunoreactivity (TH-I R) was expressed by about 86% of the cells whereas VIP-IR was virtually absent. In the presence of CNTF, 50%-60% of the cells expressed VIP-IR after 4 days in culture; however, none of the cells expressed VIP-IR in the absence of CNTF. These results, and the demonstration of cells that express both VIP and TH-IR, indicate that VIP is induced in cells that initially express tyrosine hydroxylase. The findings suggest a potential role for CNTF as a factor affecting the proliferation and differentiation of developing sympathetic neurons.
Motoneuron diseases represent a m&jor challenge to modern neurology, yet their clinical manifestations ware first described more than hundred years ago, and despite many studies the etiology of these diseases ramd,ns obscure with no effective treatments having been reported. Although progress has been made in establishing genetic linkage in the rare inherited for.ms of these diseases such as familial amyotrophic lateral scleriosisl , spinal mDscular atrophy and X-linked bulbo-spinal-mDscular atrophy, this new information has not yet affected therapeutic techniques. During the last few years several important steps have been taken concerning the physiological mechanisms involved in motoneuron survival during development, after lesion and in animal models of degenerative diseases, the molecular clOning of several new neurotrophic factors (brain-derived neurotrophic factor (BDNP), neurotrophin-3 and-4 (NT-3 and NT-4) and ciliary neurotrophic factor (CNTP)); the identification of a gene family of receptor molecules for same of these factors, progress in the understanding of the effects of polypeptide growth factors on muscle cell differentiation, neuronal sprouting (insulin-like growth factor-I and -11 (IGF-I and IGF-II), and in vitro motoneuronal survival (CNTF, IGF-I and -II and basic FGF). These findings have raised new hopes in that they could lead to a better understanding of the pathophysiological processes underlying these diseases, and that the pharmacological use of same of these newly characterized neurotrophic factors could present new possibilities for the treatment of these diseases.
Nonnucleolar chromatin from interphase nuclei of Physarum polycephalum plasmodia occurs in two different structural configurations as seen in electron microscopic spread preparations. While the majority of the chromatin is devoid of nascent ribonucleoprotein (RNP) fibrils and compacted into nucleosomal particles, a minor proportion (10- 20%) is organized differently and reveals a smooth contour. It is this form of smooth chromatin which is rich in transcription units (mean length: 1.36±0.21 11m). Only occasionally are solitary nascent RNP fibrils observed which are associated with beaded strands of chromatin. In transcribed smooth chromatin nucleosomal particles are not only absent from the transcription units but also from their nontranscribed flan king regions, indicating that this special structural aspect is not merely a direct consequence of the transcriptional process. The existence of ca. 10- 20% of Physarum chromatin in the smoothly contoured form is discussed in relation to reports of a preferential digestibility of a similar proportion of Physarum chromatin by DNAse I (Jalouzot et al. , 1980) and to the altered configuration of "peak A" chromatin subunits after micrococcal nuclease digestion (Johnson et al., 1978a, b).
Oocytes of the water beetle, Dytiscus marginalis, contain large amounts of rDNA most of which is present in the form of rings containing one or several pre-rRNA genes. Electron microscopy of spread preparations of vitellogenic oocytes has shown that the rDNA is extended in chromatin rings with transcribed pre- rRNA genes and is not packed into nucleosomes (Trendelenburg eta!. , 1976). When similar preparations are made from previtellogenic ooytes in which a large proportion of the nuc1eolar chromatin is transcriptionally inactive, a different morphological form of this chromatin is recognized. In contrast to the transcribed chromatin rings the inactive nucleolar chromatin circles show the characteristic beaded configuration, indicative of nucleosomal packing. Nuc1eosomal packing is also indicated by the comparison of the lengths of these chromatin rings with both iso lated rDNA circ1es and transcribed chromatin rings. In addition, these inactive nuc1eofilaments often appear to be compacted into globular higher order structures of diameters from 21 to 34nm, each composed of an aggregate of 6-9 nuc1eosomes. While the estimated reduction of the overall length of rDNA, as seen in our preparations, is, on the average, only 2.2 - 2.4 fold in the nuc1eosomal state it is 10- 13 fold when supranuc1eosomal globules are present. These data show that the extrachromosomal rDNA of these oocytes goes through a cycle of condensation and extensio n, as a function of the specific transcriptional activity, and that the beaded state described here is exc1usively found in the non-transcribed state.
Lengths and patterns of transcriptional units in the amplified nucleoli of oocytes of Xenopus laevis
(1977)
Transcriptionally active chromatin from peripheral amplified nuc1eoli of lampbrush-chromosome stage oocytes of Xenopus laevis was dispersed and spread in various solutions of low salt concentrations (incIuding some with additions of detergents) and examined by electron microscopy. Nucleolar material from oocytes of animals with normal (2-nu) and mutant (I-nu) genetical constitution of nucleolus organizers was compared. Histograms showing the distributions of the lengths of matrix units, apparent spacer intercepts, and the total repeating units of the rDNA containing chromatin axes revealed a significant degree of heterogeneity, with indications of subclasses and predominant repeat unit size c1asses of 3.3 and 3.8 11m length. The correspondence of matrix unit length to the molecular weight of the first stable product of rDNA transcription was studied using gel electrophoresis of labelIed pre-rRNA under non-denaturing and denaturing conditions. Evaluations of individual strands of nucleolar chromatin furt her demonstrated the existence of both (i) strands with obviously homogeneous repeating units and (ii) strands with intra-axial heterogeneity of rDNA subunits. " Preludecomplexes ", i.e. groups of transcriptional complexes in apparent spacer intercepts, were not infrequently noted. The data are compared with the measurements of lengths of repeating units in fragments of rDNA obtained by digestion with EcoRI endonuclease as described by Morrow et al. (1974) and Wellauer et al. (1974, 1976a, b). The results are discussed in relation to problems of variations in the modes of arrangement of the pre-rRNA genes, the state of packing of rDNA during transcription, and possible mechanisms of the amplification process.