88 search hits
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Biphasic, Homogeneous, and Heterogeneous Hydrocarbon Conversion Reactions with Novel Aluminum Chloride Based Catalyst Systems
(2011)
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Matthias Dötterl
- The aim of this thesis was the development of novel cocatalysts for nickel catalyzed olefin dimerization and oligomerization reactions. For this purpose, cheap and air stable Lewis acidic chloroaluminate ionic liquids were the starting point for our investigations. Chloroaluminate melts are immiscible with hydrocarbon phases, and thus, biphasic oligomerization reactions allow a simple product separation by decantation. Unmodified chloroaluminate melts containing excess aluminum chloride predominantly catalyze non selective, cationic olefin oligomerization reactions. We found that the addition of triphenylamine, triphenylphosphine, or triphenylbismuth donors efficiently suppressed these side reactions to yield an ideal cheap and air stable ionic liquid cocatalyst for common nickel complexes used for olefin dimerization reactions. Especially triphenylbismuth was able to buffer slightly acidic chloroaluminate melts as well as highly acidic compositions. For nickel catalyzed dimerization reactions, we found that high buffering levels led to very high selectivities to give dimers, while systems with lower buffer contents were less selective but extremely active. Further, the melting points of acidic chloroaluminate melts were reduced upon addition of BiPh3. Thus, room temperature ionic liquid compositions derived from 100 different organic halide salts were screened towards their performances in nickel catalyzed selective propene dimerization reactions. Amines and BiPh3 used for such systems can be easily recovered by acid base extraction. Subsequently, an optimized composition was successfully employed to dimerize ethene, propene, 1-butene, and 1-hexene with high activities and selectivities. In order to minimize leaching effects and to investigate the interactions of a donor additive with the ionic liquid, a cationic para-trimethylammonium substituted triphenylphosphine derivative was synthesized and its interaction with the ionic liquid was monitored by means of 31P NMR spectroscopy. The concept of buffering highly Lewis acidic aluminum chloride centers was also transferred to binary homogeneous systems. In combination with stoichiometric amounts of BiPh3 or N-methylpyrrole buffer, aluminum chloride readily dissolved in toluene and methylene chloride to form a highly efficient, cheap and air stable cocatalyst for nickel catalyst precursors. Furthermore, the Lewis acidities of these binary homogeneous cocatalyst solutions could be tuned precisely by the choice of the solvent and the type and amount of buffer. Also, buffer addition efficiently suppressed isomerization reactions of the 1-olefinic products. The interaction of BiPh3 and N-methylpyrrole with AlCl3 was monitored by 27Al NMR spectroscopy. Further, the concept of buffering highly Lewis acidic aluminum species was extended to heterogeneous systems. The Lewis acidities of EtAlCl2 or Et2AlCl modified silicas were reduced by the addition of BiPh3 or N-methylpyrrole buffers. Buffered surface modified silica cocatalysts were successfully employed to activate nickel complexes for highly selective olefin dimerization reactions. Also, surface modified silica proved to be an ideal substrate for the formation of supported ionic liquid phase (SILP) cocatalyst systems used in nickel catalyzed olefin dimerization reactions. Buffered aluminum based cocatalysts were also able to activate a nickel diimine complex for biphasic, homogeneous, and heterogeneous ethene polymerization reactions. The polymer yields, molecular weights and polydispersities of the polyethylenes could be influenced by varying the solvent or the type and amount of buffer. We developed the first air stable cocatalyst systems solely based on aluminum chloride. With the right combination of solvent, type and amount of buffer, and process parameters, tailor-made homogeneous, heterogeneous and biphasic cocatalysts for numerous catalyst precursors can be designed. The systems proved to be highly active, long term stable and very selective for olefin dimerization or polymerization reactions. Due to the cheap components and the possibility to recycle buffers or halide salts, these new systems provide promising alternatives to established alkylaluminum based cocatalyst systems.
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The role of life history traits for coexistence and forest recovery after disturbance – a modelling perspective. Towards a better understanding of species-rich forests
(2011)
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Claudia Dislich
- Tropical forests are well known for their exceptional species richness – high diversity of plant species constitutes the basis for an equivalently rich fauna. An astonishing variety of plant life strategies has evolved, manifesting itself also in different compositions of life history traits in trees. This thesis investigates the role of tree life history traits (growth, mortality and recruitment) on different processes structuring species-rich forests. Our study system is a montane rainforest located in the Tropical Andes hotspot of biodiversity in southern Ecuador. Here, we find a mosaic of steep ridges and deeply incised valleys, covered with predominantly broadleaf forest. Forest structure and species composition differ considerably depending on altitude and topographic position. The forest cover is frequently interrupted by scars of landslides, which constitute an important type of natural disturbance in this ecosystem. We utilize ecological models as tools to gain deeper insights into key processes driving the maintenance of tree species richness and affecting forest recovery after landslides. The first part of this thesis concerns the question of species coexistence. We develop a theoretical model to analyze how different trade-offs between life history traits (tree growth, seed dispersal, tree mortality) affect tree species coexistence. We find that the considered trade-offs alone are not sufficient to explain long-term species coexistence. Additional 'stabilizing' mechanisms seem to be indispensable to facilitate coexistence in species-rich forests. Such mechanisms could result from biotic interactions that alter the relation between inter- and intra-specific competition depending on (local) species abundances (e.g. density-dependent mortality). Other possible coexistence mechanisms likely to be relevant to our particular study system are driven by external, abiotic factors like a complex topography resulting in locally differing habitat types (each supporting a different set of species), or the character of a prevailing disturbance regime (e.g. shallow landslides). In the second part of the thesis, we investigate the growth dynamics of the ridge forest in our study system. To this end, we utilize the process-based forest growth model FORMIND. We show that after calibration, the model successfully reproduces forest dynamics on different levels of complexity (e.g. basal area and stem size distribution). We then use this forest model to investigate the influence of landslide disturbances on forest dynamics both on the local scale of a single landslide and on the landscape scale. On landslide sites, changes in environmental conditions might lead to changes in different tree life history traits. We analyze scenarios with changes in different traits (tree recruitment, tree growth, tree mortality) and find that while tree biomass can recover within the first hundred years after a landslide, the time until forest structure and species composition is restored is considerably longer (approximately 200 years). Changes in different traits result in differing spatial distributions of tree biomass: reduced tree growth leads to a more homogeneous distribution of biomass, whereas reduced recruitment and increased mortality yield a more heterogeneous biomass distribution ('patchy' vegetation). On the landscape level, overall forest biomass is substantially reduced by landslides (8-14%), compared to only 2-3% of the area marked by visible traces of landslides. Thus this particular type of disturbance considerably influences the total forest carbon balance. In a complementary investigation we study abiotic and biotic factors that potentially trigger landslide occurrence in our study system. For this, we develop an extension of a standard physically-based model of slope stability. We find that due to the predominantly shallow tree roots, some of the observed landslides might be triggered by the vegetation itself. This thesis demonstrates that ecological models are useful tools to gain deeper insights into important processes shaping forest communities. They can be applied for theoretical questions such as the question of species coexistence, as well as for more applied, management related questions like predicting forest recovery after disturbances.
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Analysis of the "nurse-tree effect" of exotic shelter trees on the growth of the indigenous Podocarpus falcatus in an Ethiopian montane forest
(2011)
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Simone Strobl
- Ethiopian forests disappear with a rate of 1.1% per year due to the high demand of firewood and timber. To protect the remaining parts of the forests, exotic tree plantations were established 60 years ago. But there are considerable concerns regarding exotic plantations: they have the reputation to cause damage to the ecosystems due to high demand of water and nutrients. Considering the environmental deterioration caused by monotonous plantations of exotic tree species, the chance for indigenous woody plants to rejuvenate naturally in those plantations appears to be very small. But there are observations of indigenous tree species regenerating under the shelter of exotic tree plantations. This enhanced growth of indigenous saplings under the canopy of exotics has been termed “nurse-tree effect”. In the Munessa-Shashamene Forest, a tropical montane forest in Ethiopia consisting of plantations of exotic tree species and remnants of natural forest, regeneration and an enhanced growth of native Podocarpus falcatus saplings under the shelter of exotic tree plantations (Pinus and Eucalyptus) was observed. The focus of this work was to examine the different growth patterns of the saplings in the sites, the effects of the on the photosynthetic performance, and to compare the water relations of the Podocarpus saplings and those of the shelter-trees. The results of the study are summarized as follows: 1. Observations over two years showed that the relative growth rates of the saplings were more than three times higher in the Pinus plantation compared to the natural forest and the Eucalyptus plantation. Relative growth rates during the dry and the rainy season were more or less identical. 2. Investigation of the sub-canopy microclimate proved PAR and VPD as major components with impact on the photosynthetic performance of the saplings. 71% of the variations in photosynthetic carbon uptake could be explained by PAR and 4% by VPD. The Pinus plantation was slightly warmer and drier compared to the other two sites. Also highest PAR values of all sites were recorded in the Pinus plantation. In the Eucalyptus plantation, PAR values were the lowest of the three sites, caused by the two-tired canopy of coppiced and uncoppiced Eucalyptus trees. 3. For an assessment of the photosynthetic efficiency of the light climate, the efficacy of the shares of the irradiation from diffuse light and light flecks were determined from light curves. The time spans and distribution of these shares of the daily accumulated radiation were recorded from the daily courses. In the Pinus plantation, the efficiency of the radiation was relatively low (70%), because of the high intensity of the sunflecks, especially during the dry season. On cloudy days the efficiency was nearly 100% and resulted in an optimum photosynthetic performance of the saplings in the Pinus plantation. In the Eucalyptus plantation, the two-tired canopy resulted in a higher proportion of diffuse radiation and less daily accumulated PAR from sunflecks (46%). Also the efficiency of the actual radiation was the lowest of all sites on cloudy (72%) and sunny (53%) days. Daily accumulated PAR under the canopy of the natural forest was in between the other forest types. Such mid-position was also true for the share of the sunflecks and the CO2 uptake. Efficacy of the radiation was 80% on sunny and 86% on cloudy days. 4. Water relations can substantially affect the photosynthetic performance of plants. Especially in the afternoons of the dry season a decrease of photosynthetic CO2 uptake by the Podocarpus saplings became apparent. Whole-tree water consumption was determined by measuring sap flow with the Granier system. In principle sap flow (and transpiration) followed VPD. Comparison of the daily courses of transpiration and stomatal conductance and sap flow showed an earlier decrease of transpiration by the leaves of the saplings than by the shelter-trees, suggesting slight water shortage especially during the dry season. This interpretation is corroborated by the higher 13C values in the leaf tissue of the saplings from the Pinus plantation. Nevertheless severe drought stress did not occur during the two years of investigation. 5. The literature on the „nurse-tree effect“ mentions in particular Eucalyptus as shelter-tree, a finding which is not in agreement with the data of this study: Neither photosynthesis nor growth was enhanced compared with the control saplings in the natural forest. The discrepancy between this work and the literature can be solved when the management of the Eucalyptus plantation is considered. As long as the Podocarpus saplings grow under the two-tired canopy of the coppiced trees, growth is as slow as in the natural forest. However, after coppicing the light climate for the saplings ameliorates considerably and growth rates increase. Thus, a shelter-tree effect could also be observed under Eucalyptus, but its dynamics is stepwise rather than continuous.
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Composites of Spherical Polyelectrolyte Brushes and Nanoparticles – Synthesis, Characterization and Their Use in Catalysis
(2011)
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Frank Polzer
- The main objective of this thesis was the synthesis of colloidal stable managanese oxide nanoparticles (MnOxNP) for applications as a catalyst in aqueous solution. Spherical polyelectrolyte brushes (SPBs) with poly(2-trimethyl ammonium ethyl methacrylate chloride) (pTMAEMC) chains were used as support particles to stabilize the MnOxNP by immobilization. In a first step we established and investigated the method of the in situ generation of the MnOxNP within SPBs. It was found that no reducing agent is needed for the reduction of the permanganate precursors and that they do not react with the cationic polymer chains of the SPBs. By a combination of powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and cryogenic TEM (cryoTEM) it was found that the platelet-like MnOxNP are of layered topology built up from MnO6 octahedra denoted as birnessite. The PXRD patterns revealed a disorder along the stacking direction of the single layers of hexagonal sheets. Furthermore, the structure of the composite material observed by TEM strongly differs compared to that in cryoTEM micrographs. The composite material was furthermore analyzed by high resolution TEM (HRTEM) and X-ray absorption fine structure (XAFS) analysis. The qualitative X-ray absorption near-edge structure (XANES) analysis using reference compounds confirmed the crystallographic similarity of the MnOxNP to a c-disordered birnessite. The local structure of the MnOxNP was investigated by a quantitative extended X-ray absorption fine structure (EXAFS) analysis that revealed that no significant difference between the MnOxNP@SPB in aqueous solution and in the dried state. In general, the hexagonal sheets of edge-share MnO6 octahedra are compressed along the c-direction, that is, along the stacking direction. Additionally, a new kind of composite material composed of star-shaped pTMAEMC homopolymer and MnOxNP was synthesized and characterized To test the MnOxNP@SPB composite material for its catalytic activity, the oxidation of morin by hydrogen peroxide was established as a model reaction. It could be shown by UV/vis measurements that the rate of the decomposition is highly depending on the ratio between morin and the oxidant H2O2. This finding was modeled by a Langmuir-Hinshelwood reaction mechanism. The study proved the potential application of the composite material as a catalyst especially for water-based catalysis. Furthermore, a detailed kinetic analysis of the reduction of 4-nitrophenol by sodium borohydride using gold and platinum nanoparticles immobilized on SPBs has been conducted. In analogy to the work on the oxidative decomposition of morin, a Langmuir-Hinshelwood model was used for the description of the reaction mechanism. Using this model, the adsorption constants for both reactants as well as the rate constant of the surface reaction could be determined for both noble metal nanoparticles. This showed that the higher catalytic activity of Pt is mainly due to the higher rate constant of the surface reaction. Additionally, the induction period of the reduction of 4-nitrophenol could be assigned to a surface reconstructuring of the nanoparticles due to the adsorption of 4-nitrophenol. Finally, the synthesis of a novel zwitterionic SPB could be realized using aqueous atom transfer radical polymerization. These particles show a surprisingly high colloidal stability in aqueous medium though the poly(2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide) (pMEDSAH) chains are not soluble in pure water. The solution behavior in water was furthermore studied by dynamic light scattering, TEM and cryoTEM proving the collapsed state of the brush layer. The zwitterionic shell undergoes an internal phase separation leading to a surface-near layer whereas only a minor part of the chains reaches further out into the solution. The collapse was explained by the formation of aggregates of monomer units by zwitterionic or hydrophobic interactions. It was shown that the zwitterionic shell swells upon the addition of high amounts of salts and/or upon increasing the temperature due to the presence of an upper critical solution temperature. In conclusion, this thesis presented a new method for the generation and stabilization of MnOxNP of layered topology using cationic SPBs. The mechanism of the in situ generation could be elucidated as well as the microscopic structure of the composite material in the aqueous dispersed state. Using state of the art characterization methods like XAFS, the local environment of the MnOxNP around the Mn absorber could be determined. The catalytic activity of the MnOxNP was studied in detail applying a Langmuir-Hinshelwood model to the catalytic degradation of morin. A similar study gave new insights into the reduction of 4-nitrophenol using noble metal nanoparticles applying a similar model. The synthesis and analysis of zwitterionic SPBs gave important information about their solution behavior.
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Climate change impacts on habitats and biodiversity :
From environmental envelope modelling to nature conservation strategies
(2011)
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Torsten Bittner
- Climate change will pose entirely new challenges for nature conservation. A literature study of 852 publications (between 2003 and 2010) illuminates this topic, examines driving research forces as well as focal points and shows recent research gaps. Here could be shown that changes in species distribution, diverse consequences for habitats, changing communities as well as biotic interactions and general aspects of diversity are the major challenges. The potential climatic modifications can alter deeply the distribution of animals and plants. Range changes due to recent climate change already exist and are traceable. In order to quantify such changes, environmental envelope modelling can be used. In addition to individual species, changes in distribution of more complex units are also conceivable. The present work mainly focuses on habitat types listed in the Annex I of the European Habitats Directive. To reveal the potential range changes of habitat types, two principally different modelling approaches have been developed. An indirect approach modelling the distribution of a habitat type using the distribution of its characteristic plant species and a direct approach, using the distribution of the habitat type itself. These two approaches were tested by modelling five grassland habitat types. Looking at the modelled results all habitats are projected to lose between 22% and 93% of their range in the ‘no dispersal’ scenario. Both approaches produce reasonable results. However, modelling an extensive set of habitat types using the indirect approach is currently not possible, because of the required but actually lacking amount of plant distribution data. Therefore, the direct approach is an appropriate instrument for modelling habitat types. Here, all 127 widespread terrestrial habitat types defined in the Annex I of the Habitats Directive were modelled and, resulting from this, a map of terrestrial habitat type diversity was calculated. Several habitat types are projected to lose many of their actually suitable areas, in particular bogs, rocky habitats, grassland and in part forests. Due to their developmental time or rather due to their special abiotic requirements, bogs and rocky habitats even lose under the assumption of a full dispersal scenario. However, most heath and grassland habitats are also projected to lose in the full dispersal scenario. Pooling all modelled results together, terrestrial habitat type diversity is shifting partly to mountain regions and the atlantic biogeographical region is projected to decrease in habitat type diversity. According to the Habitats Directive habitat types listed in Annex I are protected in ‘sites of community interest’ aiming to maintain or restore them at a favourable conservation status. Due to the projected changes a static nature conservation concept could face problems which particularly concern the coherence of the protected area network. This could lead to a loss of protective goods in spite of protected areas. To illustrate the potential problems and difficulties emerging with respect to spatial coherence of habitat types between protected areas, an analysis of spatial coherence under future conditions for a variety of habitat types in Germany was conducted. Here, a combination of environmental envelope modelling and graph theory is presented to assess the coherence of nature conservation networks. The possible incapacity of species to reach all climatically suitable areas is currently debated. Therefore, spatial scales are not only crucial for the coherence of protected areas but also for the question if future projected suitable areas could be colonized. Dispersal movements of species are only infrequently possible in our highly fragmented landscape. To answer this raising question, Odonata listed in the Habitats Directive with known dispersal distances were contemplated. The species Coenagrion ornatum, C. mercuriale and Ophiogomphus cecilia are projected to lose range when incorporating specific dispersal distances, while they are projected to extend their range in the unrestricted dispersal scenario. Furthermore, suitable climatic conditions tend to decline for Leucorrhinia albifrons and L. caudalis, whereas L. pectoralis is projected to gain distribution area assuming either species-specific or unrestricted dispersal. The nature conservation measure of translocation is an at least 100 years old methodology with pros and cons. In this thesis, the emerging problems and opportunities of this species preservation strategy are presented. Further, a new question about the ‘focal unit’ is pointed out as well as the problem of genetic variability and the aspect of pre-adopted subspecies. Moreover, a selective assisted colonisation not by moving species but ecotypes is referred.
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Applying regional climate change projections for spatio-temporal risk analyses of vector-borne diseases
(2011)
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Dominik Fischer
- Bei vorliegender Dissertation handelt es sich um eine Abhandlung zu vektor-assoziierten Krankheiten in Zeiten des Klimawandels. Bei vektor-assoziierten Krankheiten wird ein Pathogen durch einen Vektor (Überträger), auf ein Wirtstier übertragen. Als solche Vektoren agieren meist Arthropoden. Klimatische Veränderungen beeinflussen vektor-assoziierte Krankheiten insbesondere dadurch, dass Arthropoden ihre Körpertemperatur nicht selbst regeln können und zudem bestimmte Temperaturansprüche zur Pathogenentwicklung im Vektor erfüllt sein müssen. Das Klimaänderungssignal des 21. Jahrhunderts wird von Klimamodellen in verschiedenen räumlichen und zeitlichen Auflösungen wiedergegeben. Die Projektionen beruhen auf Emissionsszenarien klimawirksamer Treibhausgase. In der Arbeit werden die Einsatzmöglichkeiten von regionalen Klimamodellen zur Gefährdungsabschätzung anhand verschiedener Fallbeispiele aufgezeigt. Deren Nutzen und Einsatzmöglichkeiten werden einführend aufgeführt. Für die Risikoanalysen werden regionalen Klimamodelle REMO und COSMO-CLM angewandt, die durch dynamisches „Downscaling“ globaler Modelle generiert wurden. Beide sind in ihrem neuesten Prozesslauf in das globale Modell ECHAM5 eingebettet. Der direkte Übertrag bekannter Temperaturansprüche von Vektor und/oder Pathogen auf künftig zu erwartende Bedingungen stellt den ersten methodologischen Schwerpunkt dieser Arbeit dar. Eine Amplifikation des Dengue-Virus im Überträger der Stechmücke Aedes aegypti könnte demnach zunächst in Südeuropa, im weiteren Verlauf des 21. Jhd. aber auch in weiteren europäischen Regionen möglich sein. Weiterhin verdeutlichen die Ergebnisse, dass sich auch das Zeitfenster einer potentiellen Übertragung des Dengue-Virus verlängern kann. Durch das Überlagern der bekannten Temperaturansprüchen von Sandmücken (Gattung Phlebotomus) und der von ihnen übertragbaren Erreger - Leishmania infantum Komplex - können potentielle Regionen Deutschlands identifiziert werden, in denen einer autochthone Übertragung der Leishmaniose möglich ist. Es ist zu erwarten, dass ein solches Risiko zunächst in südwestlichen und westlichen Regionen Deutschlands, im späteren Verlaufe des des 21. Jhd. jedoch auch für eher nördlich und östlich gelegene Regionen bestehen wird. Der zweite innerhalb dieser Arbeit gewählte methodologische Ansatz zeigt die Einsatzmöglichkeiten regionaler Klimaprojektion für die bioklimatische Nischenmodellierung von Krankheitsüberträgern auf. Die anhand statistischer Verfahren ermittelte bioklimatische Nische der jeweiligen Art wird hierbei auf zukünftig zu erwartende klimatische Bedingungen übertragen. Anhand dieser Analyse kann aufgezeigt werden, dass sich die klimatische Eignung für die invasive Stechmücke Aedes albopictus (Überträger mehrere human-pathogener Viren) ausgehend von westlichen Regionen Europas über Mitteleuropa und schließlich Osteuropas erhöhen wird. Der Transfer der ermittelten spezifischen klimatischen Nische ausgewählter Sandmücken-Arten (u.a. Überträger der zum Leishmania-Komplex zählenenden Pathogenen) auf künftige Bedingungen lässt vermuten, dass deren klimatische Eignung in Mitteleuropa - abgesehen von alpinen Regionen - zunehmen wird. Künftige potenzielle Ausbreitungswege der Sandmücken in einer sich verändernden Umwelt, werden via “least-cost analysis“ ermittelt. Die Ergebnisse deuten darauf hin, dass aufgrund der eingeschränkten natürlichen Ausbreitungsfähigkeit, einige der künftig potenziell geeigneten Lebensräume nicht erreicht werden. In den verschiedenen Fallstudien kann gezeigt werden, dass die zu erwartenden klimatischen Veränderungen im 21. Jhd. eine mögliche Ausbreitung der in dieser Arbeit adressierten Vektoren und vektor-assoziierter Krankheiten in Europa begünstigen werden. Als einheitliche Tendenz kann speziell für Mitteleuropa festgehalten werden, dass sich die Gefährdung, Ende des 21.Jhd. erhöhen wird. Dies begründet sich höchstwahrscheinlich durch die projizierte raschere Erwärmung in der zweiten Jahrhunderthälfte. Abschließend bleibt jedoch festzuhalten, dass es neben klimatischen Veränderungen weitere Faktoren für die Ausbreitung bzw. Neuetablierung von Vektoren und den damit verbundenen übertragbaren Infektionskrankheiten ausschlaggebend sind. Der Einfluss einzelner Faktoren auf die Etablierung bzw. Ausbreitung vektor-assoziierte Krankheiten variiert auf raum-zeitlichen Skalen. Für die ermittelten klimatisch-abgeleiteten Risikogebiete sollten in Folgestudien auf kleineren Skalen wirksam werdenden Faktoren integriert werden. Diese Ergebnisse können wiederum die Entwicklung von Surveillance- und Monitoringprogramme unterstützen, um somit Maßnahmen gegen die Ausbreitung von vektor-assoziierten Krankheiten initiieren zu können.
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Chain-Growth Polymerization of 3-Hexylthiophene Towards Well-Defined Semiconductor Block Copolymers
(2011)
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Ruth Lohwasser
- The central point of this thesis is the control of the P3HT synthesis using Kumada catalyst transfer polymerization (KCTP). Kinetic studies of the active monomer formation via Grignard metathesis revealed that this reaction is rather slow, however, its completion is essential for controlled end groups. It was shown that LiCl accelerates this reaction and allows 100% H/Br end groups without long reaction times. MeOH as a quenching reagent was found to cause chain-chain coupling via disproportionation of the Ni-catalyst species. For non-functionalized P3HTs HCl was identified as the best quenching agent. The identification of these factors now allows the fast synthesis of any molecular weight with 100% homogeneous functionalizable H/Br end groups. In the second part these well-defined P3HTs were used to investigate their equilibrium bulk structures and aggregation in solution. A temperature dependent phase diagram of the crystalline regions in P3HT was obtained in bulk. Further, the order of the P3HT side chains was elucidated for the first time as tilted and non-interdigitated. The analysis of the semicrystalline order of P3HT showed extended chain crystals as the equilibrium structures up to a molecular weight of ~20 kDa. The study of the aggregate formation in solution revealed that the amount of aggregates and their nature depends on the used solvent, the molecular weight and the synthetic method. The maximum fraction of aggregates in solution seemed to indicate the resulting charge carrier mobility measured in organic thin film field effect transistors. For the well-defined P3HTs, even low molecular weight samples allow for high charge carrier mobility. In the third part the bromine end groups were used to introduce valuable carboxylic end groups in one single, selective and quantitative post-polymerization step. Thus, carboxylated polymers with a charge carrier mobility as high as 10 3 cm2/Vs were obtained. These functionalized polymers can be anchored onto surfaces, can coordinate nanoparticles or can be used as end cappers for the formation of block copolymers. Additionally, a correlation of the melting enthalpies with the charge carrier mobilities indicated that the crystallinity is the determinant factor for the molecular weight dependence of the charge carrier mobility. A great challenge for the synthesis of P3HT containing block copolymers is to combine KCTP with other controlled polymerization methods. Especially conjugated blocks with high molecular weights are difficult to incorporate, because of their lower solubility and lower percentage of end groups. Here, it was demonstrated that copper catalyzed azide-alkyne cycloaddition, a so-called click reaction, is a simple and efficient approach to create high molecular weight P3HT macroinitiators for nitroxide mediated radical polymerization. First, the synthesis of alkyne functionalized P3HT was optimized. HCl as a quenching reagent caused hydrohalogenation and hydration reactions and MeOH proved to be the quenching reagent of choice. In the second step, P3HT-alkyne was combined with an azide functionalized alkoxyamine to form the macroinitiator in one single post-polymerization step. Two different types of block copolymers were realized. The first type is amphiphilic P3HT-b-P4VP with a polar coil block, which is able to coordinate to inorganic particles. Thermal analysis showed that the crystallite size of P3HT is hardly influenced by the coil content when such long P3HT rod blocks are used. Preliminary investigations of the self-assembly in solution, demonstrated that colloidal structures with crystalline P3HT cores are obtained in P4VP selective solvents. Thus, P3HT b P4VP is an interesting system for hybrid photovoltaics and for investigating structure formation of rod-coil block copolymers in bulk and solution. The second type of block copolymers are fully-functionalized P3HT-b-PPerAcrs, with PPerAcr as an electron conducting second block. With the new synthetic approach it was possible to get an exclusively P3HT-b-PPerAcr diblock copolymer with a high molecular weight. In this context a high content of PPerAcr of 64wt% was realized which is important for a balanced charge transport in OPVs. The pure diblock, the high molecular weight and, hence, a high chiN parameter enabled for the first time the formation of a hexagonally ordered cylindrical bulk structure for a fully-functionalized crystalline-crystalline block copolymer. Additionally, in thin films, order-disorder transition and microphase separation was observed with domain sizes in the range of the exciton diffusion length. In conclusion, the optimization of the synthesis of P3HT allowed a better understanding of fundamental aspects about its molecular weight dependent structures and the resulting properties. Further, new end groups could be introduced to widen its applications. Additionally, long P3HT blocks were efficiently incorporated into well-defined diblock copolymers.
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Strukturelle Untersuchungen an dem Radikalenzym (R)-2-Hydroxyisocaproyl-CoA-Dehydratase
(2011)
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Stefan Knauer
- In der Natur werden viele chemisch schwierige Reaktionen von Radikalenzymen katalysiert. Die (R)-2-Hydroxyisocaproyl-CoA-Dehydratase katalysiert die Dehydratisierung von (R)-2-Hydroxyisocaproyl-CoA in der Fermentation von L-Leucin im humanpathogenen Bakterium Clostridium difficile. Im Gegensatz zu einer Vielzahl von Radikalenzymen, wie beispielsweise der Coenzym B12-abhängigen bakteriellen Klasse II Ribonukleotid-Reduktase oder der S-Adenosylmethionin-abhängigen Biotin-Synthase, benötigt die (R)-2-Hydroxyisocaproyl-CoA-Dehydratase keine komplexen Kofaktoren zur Erzeugung des katalytisch aktiven Radikals. Stattdessen verwendet sie ein einzelnes, hoch energetisches Elektron. Bei der Dehydratisierung – einer atypischen α/β-Eliminierung – werden reduktiv Ketylradikale im Substrat erzeugt. Die Dehydratase muss zunächst aktiviert werden, indem ein Aktivator, der ein [4Fe-4S]-Zentrum enthält, unter ATP-Hydrolyse ein Elektron auf die Dehydratase überträgt. Für den Aktivator werden im Laufe des Aktivierungsprozesses Nukleotid- und redox-abhängige Konformationsänderungen angenommen. Das hoch reaktive Elektron wird nach jedem Substratumsatz wieder auf die Dehydratase übertragen, so dass bis zu 10.000 Umsätze katalysiert werden können, bevor eine erneute Aktivierung notwendig ist. Die strukturelle Grundlage der Aktivierung sowie die Art und Weise, wie das aktive Elektron in der Dehydratase nach erfolgtem Umsatz wiedergewonnen und vor Verlust durch unspezifische Nebenreaktionen geschützt wird, ist unbekannt. In dieser Arbeit konnte zunächst die Struktur der homodimeren (E)-2-Isocaprenoyl-CoA:2-Hydroxyisocaproat-CoA-Transferase gelöst werden, die in der Fermentation von L-Leucin den zweimaligen Transfer eines Coenzym A-Moleküls katalysiert. Sie gehört zur Familie III der CoA-Transferasen und besitzt deren typische Faltung. Ein Aspartatrest im aktiven Zentrum ist wahrscheinlich an der Ausbildung von kovalenten Intermediaten beteiligt. Es wurden die Strukturen des reduzierten Aktivators ohne Nukleotid, mit ADP und mit dem ATP-Analogon ADPNP bestimmt. Der homodimere Aktivator gehört zur Actin-Faltungsfamilie und die Monomere sind durch ein verbrückendes [4Fe-4S]-Zentrum miteinander verbunden. Jedes Monomer besteht aus zwei Domänen, die in Abwesenheit eines Nukleotids eine offene und in Gegenwart eines Nukleotids eine geschlossene Konformation einnehmen. Die Strukturen des reduzierten Aktivators mit gebundenem ADP und ADPNP zeigten keine signifikaten Konformationsunterschiede. Der Vergleich mit der Struktur eines oxidierten Aktivators ergab, dass auch die Reduktion des [4Fe-4S]-Zentrums offenbar keine strukturellen Änderungen zur Folge hat. Möglicherweise spiegeln die Strukturen, die alle mittels Röntgenkristallographie bestimmt wurden, jedoch nicht die konformationelle Flexibilität in Lösung wider. Erstmals wurde die Komplexbildung zwischen dem Aktivator und der Dehydratase mittels ADPNP nachgewiesen. Die Strukturen des Aktivators legen nahe, dass die Komplexildung über den Prozess der konformationellen Selektion geschieht. Die Struktur der Dehydratase zeigte, dass das heterodimere Protein zwei [4Fe-4S]-Zentren in einem Abstand von 12 Å beinhaltet. Jedes Fe/S-Zetrum ist von drei Cysteinresten und einem terminalen Liganden koordiniert. Das Fe/S-Zentrum in der α-Untereinheit ist Teil des aktiven Zentrums und besitzt in Abwesenheit von Substrat ein Hydroxyidion/Wassermolekül als vierten Liganden. Das Substrat ersetzt diesen Liganden und koordiniert das Fe/S-Zentrum mit dem Carbonylsauerstoffatom der Thioestergruppe. Das [4Fe-4S]-Zentrum in der β-Untereinheit hat eine Sulfhydryl-/Sulfidogruppe als terminalen Liganden und fungiert als Speicher für das Elektron, wenn kein Substrat anwesend ist. Die Struktur der (R)-2-Hydroxyisocaproyl-CoA-Dehydratase dient als Modell für die Familie der 2-Hydroxyacyl-CoA-Dehydratasen und der Gruppe der phylogenetisch verwandten Benzoyl-CoA-Reduktasen.
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Gross N turnover and soil solution chemistry as influenced by fluctuations of soil water potential and water table in a Podzol and a fen soil
(2011)
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Yao-Te Chen
- Given the climate scenarios, the higher frequency of drying/rewetting cycles of soils in the future can be expected. These changes of the meteorological conditions likely result in an increasing frequent and intensive drought periods in summer, causing irregular and extreme drought stress in forest soils or a drawdown of water table in wetland ecosystems, which may influence the turnover of nutrients in soils to a larger extend than previously thought. The question arises how these climate changes will influence N and C turnover in forest and fen soils. A growing number of laboratory studies on drying/rewetting of soils have been published during past decades, but many studies used either disturbed soil samples or intact soil cores in laboratory. Although soil drying is a frequent phenomenon in the field, the long-term effects of drying/rewetting and irrigation on in situ fluxes and concentrations of solutes in forest and fen soils are unclear. Several studies have investigated the influence of soil water content on net N turnover rather than gross rates. Net ammonification and nitrification include two major processes: gross ammonification and gross nitrification on the one side and microbial immobilization on the other side. To identify the response of specific processes to soil drying, gross rates need to be measured. This thesis focused on the impact of changing water potential or water table level on gross N turnover rates and soil solution chemistry in two different ecosystems in South-Eastern Germany. In a Norway spruce forest, the effects of decreasing water potential and prolonged periods of summer drought on soil gross N turnover were investigated by laboratory and field experiments. Soil solutions and throughfall were collected and the cumulative in situ fluxes of DIN, DON and DOC with forest floor percolates were calculated. In a minerotrophic fen, we studied the response of N and C mineralization and soil solution chemistry to water table fluctuations in a laboratory experiment. In the field, we collected the soil pore water in 3 depths to clarify the long-term effects of water table level on the concentrations of solutes. Homogenized soil samples of the Oi+Oe, Oa and EA horizons were taken and adjusted to 6 different water potentials in the laboratory. In the field experiment, throughfall exclusion and irrigation plots were established to simulate different precipitation patterns of a dry and wet growing season. Gross N turnover rates were determined in undisturbed soil cores from Oi+Oe and Oa+EA horizons during the drying period and after rewetting. Soil drying decreased gross ammonification rates in the O horizon. The lowest rates were found at the throughfall exclusion plots but the differences to the irrigation and control plots were not statistically significant. A substantial ammonification rate of 14 mg N kg-1 soil day-1 was observed at 3.2 MPa (pF 4.5). The laboratory study showed that gross nitrification decreased with decreasing water potential and was more sensitive to drying than ammonification in the Oa horizon; however, this was not found in the field experiment. The latter might result from the low rates and huge spatial variation, indicating the difference between disturbed samples and intact soil cores. No rewetting pulse of gross ammonification was observed, probably due to its short duration or due to the slow changes of the water potential during the natural rewetting. Although the in situ fluxes of DIN increased at the throughfall exclusion plots after rewetting, the cumulative DIN flux at the throughfall exclusion plots did not significantly exceed that at the control plots. The lowest fluxes of DON and DOC were observed at the throughfall exclusion plots because of the reduction of input with throughfall. In the studies presented here, extended drought periods caused a reduction of gross N turnover in forest soils but gross ammonification continued at considerable rates at low water potential. The hypothesis of increased N turnover and fluxes of DIN, DON and DOC as a consequence of drying/rewetting was not confirmed. In the fen site, undisturbed soil cores were taken and divided to two treatments of water table: permanently flooded and fluctuated. The later was subjected to flooding, drawdown and re-flooding. The permanently flooding enhanced gross ammonification after a lag phase of about 30 days while CO2 emissions were constantly low. The water table drawdown also increased gross ammonification, but again after a lag phase of about 30 days. The first peak of CO2 emissions appeared immediately after water table drawdown, followed by a decrease and a second peak. The ratio of CO2 emission/gross ammonification were close to 2 under anoxic condition which seems to be caused by fast N turnover in the microbial biomass-N pool and low rates of CO2 production. The changes induced by water table drawdown on the N and C turnover were found reversible after re-flooding. Drainage increases SO42- but decrease Fe, DON and DOC concentrations and vice versa when the soils were flooded. Release of DON and DOC was inhibited by increasing SO42- concentrations. Under field conditions, neither drainage nor flooding had an effect on dissolved inorganic N due to the low concentration, indicating the rapid consumption of mineralized N in the field. In the absence of plant uptake and runoff in the laboratory experiment, however, NH4+ increased during the flooding period. Soil desiccation affects the upper soil layers with largest rates of N turnover. While gross N turnover is reduced by soil desiccation, a substantial rate of ammonification was observed even at low water potentials. Nitrification was found more sensitive to desiccation than ammonification which might change the NH4/NO3 ratio of available N under dry conditions. Rewetting of dry soil does not induce a pulse of N turnover and fluxes of DIN, DON and DOC. Overall, an increasing frequency of drying/rewetting cycles seem to have only moderate effect on the N turnover and on N solute fluxes in forest soils. Fluctuations of water table play an important role for the organic matter mineralization, soil solution chemistry and inorganic N availability in minerotrophic fen soils. Acidification by oxidation of S to SO42- can be expected after water table drawdown, causing inhibition of DON and DOC release. The effect of drainage and flooding on gross mineralization and solute concentrations is reversible within a month period. The effect of changing water table regime on N and C turnover in fen soils seems to depend largely on the time scale of the fluctuations. Short term fluctuations at a daily scale will have little effect on N turnover as compared to longer term changes on a monthly scale, while short term changes seem to trigger C losses by CO2.
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Isolierung, Strukturaufklärung und Totalsynthese von Alkaloiden aus Stenus Käfern und der Seeanemone Heteractis aurora
(2011)
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Tobias Müller
- Die neuen Pyridinalkaloide (Z)- und (E)-3-(2-Methyl-1-butenyl)-pyridin konnten jeweils in den Pygidialdrüsen der Kurzflügelkäfer Stenus solutus und S. similis als Hauptkomponenten nachgewiesen werden. Die Norverbindung 3-(1-Isobutenyl)-pyridin wurde als Minderkomponente in S. solutus, S. cicindeloides, S. binotatus und S. pubescens gefunden. Die Strukturen wurden durch NMR-Spektroskopie und Vergleich mit synthetischen Referenzen aufgeklärt. Die Pyridine wurden durch Umsetzung von Nicotinaldehyd mit dem jeweiligen Phosphoniumsalz nach WITTIG in einer Stufe erhalten. Für das bekannte Spreitungs- und Abwehralkaloid Stenusin, welches in den meisten Käfern der Gattung Stenus vorkommt, wurde eine neuartige Syntheseroute entwickelt. Die Stenusin enthaltenden Steninae zeigen eine charakteristische Zusammensetzung aller vier Stereoisomeren. Alle bislang bekannten stereoselektiven Zugänge zu Stenusin, wie auch die auf der SAMP/RAMP-Methode basierende Varinate nach ENDERS et al., erlauben nur die Darstellung der in der Seitenkette (S)-konfigurierten Alkaloide (2S,3R)- und (2S,3S)-Stenusin. Daher wurde eine neue Synthese entwickelt, die es erlaubt, alle Isomere selektiv darzustellen. Das Stereozentrum im Piperidinring wird durch auxiliarvermittelte asymmetrische Hydrierung erzeugt, während die Seitenkette entweder aus dem chiralen Pool ((S)-2-Methylbutanol) entnommen wird – identisch mit der Variante nach ENDERS – oder mittels einer chemoenzymatischen Synthese erzeugt wird. Insbesondere die (2S,3R)- und (2S,3S)-Isomere lassen sich ausgehend von 3-Brompyridin und (S)-2-Methylbutylbromid in einer sehr kurzen Sequenz von nur 4 Stufen in guter Ausbeute darstellen. Weiterhin erfolgt die gesamte Synthese frei von Schutzgruppen. Das verwendete EVANS-Auxiliar kann nach der Hydrierung unverändert wiedergewonnen werden. Das neue Alkaloid Cicindeloin wurde aus Stenus-Käfern der Art S. cicindeloides isoliert und dessen Struktur durch NMR-Spektroskopie in Verbindung mit EI- und ESI-Massenspektroskopie aufgeklärt. Die Absolutkonfiguration von Cicindeloin konnte durch stereoselektive Totalsynthese und GC-Vergleich mit dem Naturstoff an einer chiralen Phase bestimmt werden. Es wurde ein totalsynthetischer Zugang mit 20% Gesamt¬ausbeute entwickelt, der eine lineare Sequenz von 12 Stufen umfasst, von denen neun ohne Chromatographie auskommen.
