79 search hits
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Proceedings of the 12th European Agent Systems Summer School Student Session
(2010)
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Tina Balke
Reda Yaich
- This volume contains the papers presented at the Student Session of the 12th European Agent Systems Summer School (EASSS) held on 25th of August 2010 at Ecole Nationale Superieure des Mines de Saint-Etienne, France. The Student Session, organised by students, is designed to encourage student interaction and feedback from the tutors. By providing the students with a conference-like setup, both in the presentation and in the review process, students have the opportunity to prepare their own submission, go through the selection process and present their work to each other and their interests to their fellow students as well as internationally leading experts in the agent field,both from the theoretical and the practical sector.
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ORMOCER (inorganic-organic hybrid polymer)-zeolite Nanocomposites: Advanced Membrane Materials for Gas Separation
(2010)
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Suresh M. Kumbar
- This work represents a comprehensive study on the applicability of novel ORMOCER® (inorganic-organic hybrid polymer) resins for the fabrication of free-standing mixed matrix membranes. Such mixed matrix membranes comprise molecular sieve entities integrated in ORMOCER® matrix. Mixed matrix membranes have the potential to combine the processability of ORMOCER® materials with the superior transport and ion exchange properties of molecular sieves. The versatility of ORMOCER® materials is a decisive factor in achieving the desired separation properties of a novel mixed matrix membrane material. The glycerine-1,3-dimethacrylateurethanetriethoxysilane (GUS)-based ORMOCER® and dimethylsiloxane modified GUS-ORMOCER® resins were used for the fabrication of freestanding mixed matrix membranes. The zeolite Beta was used as a dispersed phase and its amount was varied between 10 to 40 wt.%. The effect of different type of zeolites was also studied using zeolite 3Å, 4Å, and 5Å. The membranes were prepared by conventional solution casting method followed by UV-curing. The ORMOCER® resins and cured membranes were analysed using FT-IR, liquid-state 29Si NMR, TG, DSC, SEM, N2 sorption and single-gas permeation measurements. The permeation performance of the membranes was examined using H2, He, CO2, O2 and N2 as a test gases at room temperature, the upstream pressure was varied between 1.3 to 2.5 atm. The effect of the amount of dimethylsiloxane moiety, zeolite content, different zeolite type and annealing temperature were systematically investigated in relation to the gas permeation performance of the membranes. The addition of dimethylsiloxane moiety in GUS-based ORMOCER® membrane appears to result in a significant increase in gas permeability, with a correspondingly large decrease in selectivity. The mixed matrix membranes show improved gas permeation performance in comparison to pure ORMOCER® membranes. This study was shown important role of zeolite particles in inorganic-organic hybrid (ORMOCER®) system. The potential usefulness of ORMOCER®-zeolite mixed matrix membranes as gas separation membranes is discussed.
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The 5-th element. A new high pressure high temperature allotrope
(2010)
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Evgeniya Zarechnaya
- Boron is the fifth element in the Periodic Table known for its structural complexity, unusual types of bonding, and for a rich variety of compounds that it forms. Even the number of its existing or hitherto proven elemental modifications is still under discussion. The present study of boron behavior at high pressures and high temperatures (HPHT) comprises various experimental techniques for HP synthesis and material characterization, as well as theoretical modeling. A series of high pressure (up to 20 GPa) and high temperature (up to 1900 °C) synthesis experiments were conducted in multianvil apparatus at Bayerisches Geoinstitut. For testing of the reproducibility of HPHT experiments different types of boron precursors and assemblages for large-volume presses were used. The synthesis products were studied by X-ray diffraction and spectroscopic methods. The diffraction patterns collected from samples synthesized from highly purified boron powders (> 99.99 % purity) resemble those published by R.H. Wentorf in 1965 and described as a new boron form, but later effectively forgotten. To eliminate any possible contaminations that could take place during the synthesis process, the purity of the HP boron samples was confirmed by scanning electron and transmission electron microscopy and electron microprobe analysis. The structure of this HPHT boron polymorph was first unknown, but subsequently determined from our X-ray powder diffraction data and further refined using single-crystal synchrotron diffraction data. The structure was found to be orthorhombic with a Pnnm space group and composed of B12 icosahedra and B2 dumbbells. The unit cell contains 28 atoms (two icosahedra and two dumbbells) and will be called here B28. Atoms in B28 are bonded covalently as revealed experimentally by single-crystal X-ray diffraction studies and calculated Electron Localization Function. The X-ray density of 2.52 g cm-3 of this boron allotrope is the highest among its other known modifications. In order to determine the phase transition boundary between beta-and B28 boron phases, in situ HP laser heating experiments were performed at European Synchrotron Radiation Facilities (ESRF). It was demonstrated that B28 is a stable phase above 9 GPa. Systematic investigations aimed at the development of the technique of the HP single crystal growth of B28 were undertaken. It was found that single crystals of the orthorhombic boron can be grown from metal solutions (Au, Cu, or Pt), i.e. after dissolution in metals at high temperature boron precipitates in form of single crystals with temperature decrease. Experimental products were free from any borides only when Au was used. The maximal length of synthesized B28 crystals was ~100 µm and after their isolation high quality X-ray diffraction data was obtained. Synthesized single crystals of the orthorhombic high-pressure boron phase were studied by means of polarized Raman spectroscopy at ambient conditions. Among all possible 42 Raman-active modes, 32 modes were registered and assigned. Investigation of the high pressure behavior of B28 orthorhombic boron was carried out through combining single crystal X-ray diffraction up to 65 GPa and Raman spectroscopy measurements performed up to 105 GPa. Above 40 GPa discontineous behavior in the mode Grueneisen parameter was detected. At the same pressure an inflection in the dependence of the relative volume of a unit cell versus pressure occurs. However, the structure of the material does not change. It is suggested that B28 undergoes an isostructural phase transformation, probably due to reducing of the polarity of covalent bonds. The orthorhombic B28 boron reveals extraordinary physical properties. Due to very strong covalent bonding it has a very high Vickers hardness HV=58 GPa, making B28 to be the second hardest elemental material after diamond. The electrical resistance and IR-spectroscopic measurements demonstrated the semiconducter nature of B28. Combination of these properties makes the high-pressure boron polymorph a prospective material for industrial applications.
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Advanced Technologies in Education - Developing the Science Classroom of the Future
(2010)
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Sofoklis A. Sotiriou
- Information communication technology (ICT) nowadays provides innovative learning systems which although routinely available needs adjustment to real educational environments. Due to the complexity of the task an appropriate integration into everyday classrooms is an important global research challenges focusing on its utilization and effects in both, classroom and non-classroom settings. By rigorous collecting data on teaching methods, classroom characteristics and students learning effects needs analysis by concentrating on selected variables that may determine effectiveness as well as teachers characteristics such as teacherspreparation and professional development. Therefore, the aim of the four presented research papers focuses on envision the science classroom of the future, by constructing a framework for improving current educational practices and learning processes in science and mathematics through the effective implementation of advanced technological tools and applications. Overall this work presents a vision for the science classroom of the future: It will not be an island, a self-contained campus, a counter-world. The classroom of the future will be able to emit and absorb along different wavelengths, be immersed in contemporary culture, be open to the emotions, facts and news of its time. It will be permeated by society, but not unprotected: the relationship between school and society will be one of osmosis, where the pedagogical tools and applications act as a membrane and interface. For this purpose, four empirical studies were carried out in real school environments, based on the use of advanced educational systems. (i) The first system under study is the COSMOS Portal, which is an educational repository that offers access to a network of robotic telescopes across the world. At the same time it offers access to more than 85,000 educational resources. The behaviour of the teachers who are using this system was mapped through the log files of the system database for a period of one year. (ii) The second system, called Lab of Tomorrow, is a wearable device that allows high school students to use their every day life as the field where they will conduct sophisticated experiments and thus will deepen their understanding of the science concepts involved in the activities. The impact of the system on students learning and to the lesson profile was studied for a period of one school year. (iii) The third system, called CONNECT, is also a wearable device that includes an advanced visualization system that augments additional information to the optical view of the user. The system is used in the framework of educational visits in science centres and museums enriching the experiences of the visitors. The effectiveness of the system in supporting the students conceptual change was studied in this case. (iv) The fourth system, called EXPLOAR, evolved from the described CONNECT system to a much more user-friendly handheld device. Taking into account the school curriculum we have designed a series of scenarios of use of these tools. The scenarios of use include classroom activities, field trips in science centres and museums, informal learning activities, professional development opportunities and community building. In all four studies, students cognitive learning is analysed as well as selected teachers tasks on the job. By applying different assessment methods and tools (questionnaires, video captures of lessons, log files and web based data) we monitored the implementation procedure across different European countries. Our working hypothesis is that amending the traditional scientific methodology for experi¬mentation with visualization applications and model building tools will help students to articulate their mental models, make better predictions, and reflect more effectively. Additionally, working to reconcile the gaps and inconsistencies within their mental models, system models, predictions and results, will provide the learners with a powerful, explicit representation of their misconceptions and a means to repair them. Additionally our aim is to support teachers professional development. Apart from the purely technical training, in order for teachers to introduce ICT-enhanced learning methods into their everyday practice, they will have to perform a change in behaviour and to adapt a new culture and philosophy. The use of the new tools asks for systematic and detailed lesson planning procedures and use of student centred approaches. In our work we are demonstrating methods for involving teachers in this process but also tools to monitor this behavioural change.
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Tropical bracken, a powerful invader of pastures in South Ecuador: Species composition, ecology, control measures, and pasture restoration
(2010)
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Kristin Roos
- Bracken (Pteridium spec.) is one of the most wide-spread weeds, especially where fire has been used for forest clearing or maintenance of agricultural areas. Taxonomically, it is considered an aggregate that separates into a northern hemispherical and a southern, tropical complex. Different from the extensively studied northern bracken, the knowledge of ecology and control measures of the tropical species is still fragmentary. The main aims of my thesis were (1) identification and population structure of bracken, (2) ecology of tropical bracken with particular emphasis on its survival of bushfires, and (3) development of an effective bracken control strategy and subsequent re-pasturisation of abandoned areas. The bracken vegetation of the study area consists of mixed stands of Pteridium arachnoideum (KAULF.) MAXON and P. caudatum (L.) MAXON with a proportion of 3:2, and of a hybrid (ca. 2%). Identification was by leaf morphology, allozyme analysis, comparison of particular chloroplastic DNA sequences, and analysis of four genomic and one plastidic microsatellites. Dominance of P. arachnoideum was explained by the fact that P. caudatum, as a lowland species, reaches its upper altitudinal limit in the research area. Analysis of heterozygosity indicated a higher genetic stability of the diploid P. arachnoideum population as compared to the allotetraploid P. caudatum population. Spatial extension of the individual clones is much smaller than reported for the northern bracken, indicating higher significance of sexual reproduction for the tropical fern in comparison to vegetative propagation by rhizome fragmentation. Four weeks after burning the natural rain forest, vigorously sprouting bracken sporophytes were observed. These developed from gametophytes, which germinated from the wind dispersed spores. Fast growth of the young sporophytes established the fern in the areas. After planting pasture grass, bracken was supported by repeated burning of the areas. In the long run, the grass was outcompeted by the fern possibly due to weakening of its vitality by burning and grazing, and the areas have been abandoned. The density of bracken fronds in a settled bracken area of our research site remained constant over years with small deviations caused by particular weather situations. Since this balance holds also for patchy fern canopies, it is assumed that this is due to nutrient shortage of the soil. Most probably, a new leaf can only develop from the nutrients remobilized from a senescing old leaf. Two to three months after a fire, an explosive emergence of new leaves was observed at rates, which substantially exceeded those under undisturbed growth. The newly formed leaves showed an extended life-span, which was attributed to a better nutrient supply from the ash. Subsequent self-thinning reduced the density of the leaves to a stable level within two years. In a laboratory experiment, the effects of heat pulse by a simulated bushfire on the bracken rhizomes were investigated. Separated long and short shoots were heated for a short time either in a water bath or embedded in soil. Subsequent to this heat pulse, they were cultivated in original soil. Short shoots showed a significantly higher heat resistance (up to 80°C) than long shoots (up to 60°C). In addition, the short shoots showed elongation growth and an enhanced frond production, whereas long shoots were not stimulated by the heat pulse. In a bracken control experiment, thirteen control measures (cutting of the fronds, several herbicides, covering with plastic foil and combinations thereof) were applied over a time-period of 23 months. Each treatment was repeated six times and the effects were recorded monthly. Quarterly cutting of the leaves as well as treatment with a customary herbicide mixture (picloram and metsulforon methyl) were the most effective treatments resulting in a reduction of the standing biomass by 65%. Monthly records of the resprouting bracken was necessary to work out the minimum number of treatments required for a clear control effect. For the five most efficient treatments among two to four applications were necessary. However, complete eradication of bracken was not possible. For re-pasturisation, the common pasture grass Setaria sphacelata was planted on the treated areas within a long-term experiment. After nearly two years of observation, the system had stabilized with a cover of S. sphacelata of 75% and of bracken of below 40%. This result demonstrated that the competitive strength of S. sphacelata was sufficient to control bracken once weakened by control treatments. The long-term experiment and, in addition, an experiment in which a gradually bracken-infested area is subjected to controlled burning, are continued.
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Proton Transfer Networks and the Mechanism of Long Range Proton Transfer in Proteins
(2010)
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Mirco Till
- The main energy providing reaction systems in living cells, for example the photosynthesis or the respiratory chain, are based on long range proton transfer (LRPT) reactions. Even since these LRPT reactions have been heavily investigated in the last decades, the mechanism of these reactions is still not completely understood. The reaction kinetics of the LRPT are under heavy discussion and it is not clear, whether the reorientation of the hydrogen bond network (HBN)or the electrostatic barrier for the charge transfer is rate limiting. The main purpose of this work is to investigate the dynamics of chemical reactions inside of proteins, focused on long range proton transfer reactions. Electron transfer reactions, rotations of water molecules or conformational changes of the protein are also considered. The developed sequential dynamical Monte Carlo (SDMC) method is applicable to almost all kinds of chemical reactions. For all proton transfer reactions, the HBN of a protein plays a major role. Protons are transferred along such hydrogen bonds. Therefore, knowledge about the hydrogen bond network of a protein is crucial for the simulation of LRPT systems. The HBN can be calculated from the protein structure and the rotational state of the amino acid side chains. The reaction rate can be calculated from the electrostatic energies of the participating proton donor and acceptor groups. These two criteria are combined for the decision if a proton transfer between two molecules is possible and how fast this transfer would happen. While the calculation of electrostatic energies of protonatable amino acid side chains or relevant cofactors in proteins (among them also water molecules) is already solved - implemented in various programs - the remaining tasks - calculating the hydrogen bond network followed by calculating the reaction rates - were solved during this work. Before the hydrogen bond network and the electrostatic energies could be calculated, the lack of water positions in many available crystallographically resolved protein structures made it necessary to develop an algorithm to detect internal cavities in proteins and fill these cavities with water molecules. The derived water positions could be included in the electrostatic calculations as well as in the calculation of the HBN. The simulation of the LRPT in Gramicidin A (gA) compared to experimental data of the proton transfer in this polypeptide showed the possibilities of the simulation of the LRPT by the SDMC algorithm. The promising results encouraged us to investigate the mechanism of the LRPT, especially, if the reorientation of the HBN or the electrostatic energy barrier of the charge transfer is rate limiting for the LRPT. The results indicate, that both effects influence the LRPT and none of them is exclusively responsible for the LRPT rate. Further analysis of the hydrogen bond network topology showed that graph algorithms can be used to analyze these networks. Hydrogen bond networks can be clustered into regions which are close connected to each other. On the other hand, residues connecting two or more of these densely connected regions might play an important role for proton transfer pathways since a loss of such residues cuts a proton transfer pathway. A comparison of an analysis of the HBN topology of the photosynthetic reaction center with mutation studies of the same system showed, that residues identified as important for proton transfer by the mutation studies are identified as connection points between clusters by the network analysis. The developed algorithms together with the introduction of a new method for the simulation of the LRPT process (SDMC) improved the picture of the proton transfer processes in proteins. Starting from the protein structure, the developed algorithms cover all steps from the detection of protein cavities, the placement of water molecules in these cavities, the calculation and analysis of the hydrogen bond network, the simulation of the LRPT and the investigation of the reaction kinetics. The analysis of the HBN by graph theoretical methods gives further insight into the HBN topology and identifies residues important for proton transfer pathways and therefore important for the protein activity.
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Die Berücksichtigung inhaltlicher und räumlicher Unschärfe bei der GIS-gestützten Erstellung von Bodenkarten
(2010)
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Jens Hannemann
- Die vorliegende Arbeit beinhaltet die Entwicklung einer Methode zur objektivierten Erzeugung von Flächendaten für Bodenkarten. Ziel ist es, Daten, die sich in Maßstab, Datum, Art, Herkunft und Güte unterscheiden, zu bewerten, zu verknüpfen und unter Berücksichtigung ihres Raumbezuges zu einem Zielthema zusammenzuführen. Es werden zunächst der informationstheoretische Hintergrund, sowie bestehende Regionalisierungsverfahren und -ansätze betrachtet. Eine wichtige Rolle spielt die Auseinandersetzung mit dem Sequenz- und Vergesellschaftungsprinzip, sowie der Maßstabsproblematik. Es wird ausführlich auf die vorhandenen Informations-grundlagen eingegangen. Ausgangspunkt ist, sämtliche zur Verfügung stehenden Datenbestände in die Analyse zu integrieren. Die Beschreibung und das Aufzeigen von Analyse- und Bewertungsmöglichkeiten dieser sehr heterogenen Daten nimmt dabei einen zentralen Platz ein. Eine wichtige Rolle spielt stets die Interaktion von Computer und Experten. Die Methode durchläuft die Kette Aufbereitung, Verarbeitung, Auswertung und Visualisierung bodenkundlicher Informationen. Als Bausteine werden bekannte, aber auch neu entwickelte Methoden verwendet, diskutiert und kombiniert. Die Entwicklung der Gesamtmethode bzw. des methodischen Ablaufs an sich steht dabei im Vordergrund. Hauptziel ist die Ausweisung von Kern- und Übergangsbereichen. Es wird besonderer Wert darauf gelegt, die intrinsischen Unterschiede in den Daten herauszuarbeiten und während des gesamten Methodenablaufs mitzuführen. Dazu wird konsequent das Abstufungsprinzip für die inhaltliche und räumliche Komponente gleichermaßen angewendet. Der Umgang mit der inhaltlichen und räumlichen Unschärfe bildet den Schwerpunkt der Arbeit. Es werden Zielkategorien definiert, die die Bewertungs-grundlage für die räumlich auszuweisenden Bodenformen darstellen. Der gesamte Methodenablauf ist eingebettet in die fuzzy-logischen Arbeitsschritte Fuzzifizierung, Inferenz und Defuzzifizierung. Es wird ein Ansatz vorgestellt, der die Verarbeitung unscharfer bodenkundlicher Informationen zu optimieren versucht. Durch eine gewichtete, inhaltlich-räumliche In-Beziehung-Setzung von Daten, durch eine Analyse von qualitativen und quantitativen Daten und die Einbeziehung sprachlicher Modifikatoren wird eine Konzeptkarte mit objektivierter Arealabgrenzung erarbeitet, die auf einer reproduzierbaren Datenverarbeitung und einer quantifizierten Aussagesicherheit fußt. Datenbank, GIS und Bildverarbeitung bilden dabei softwareseitig die Hauptkomponenten. Zur Ergebnis führenden Ausweisung der Kern- und Übergangsbereiche wurde die Bildverarbeitungsmethode der Segmentierung gewählt. Diese wurde mit der Software eCognition vorgenommen. Inhalt und Abgrenzung der Segmente wurden mit den Punktdaten eines Transektes von der Hochfläche über die Niederung zur Hochfläche überprüft. Im Zusammenhang mit der Darstellung der Aussagesicherheit in Karten wird das Prinzip der Raumübertragung und Übertragungsgütemaße aufgegriffen und diskutiert. Es wird ein Zwei-Karten-Konzept angeregt, das neben der Karte mit der eigentlichen Fachinformation stets eine Karte der Aussagesicherheit vorsieht. Weiterhin wird auf Aspekte der Visualisierung der inhaltlichen und/oder räumlichen Komponente eingegangen. Ziel ist dabei die synoptische Darstellung von Infor-mationsergebnissen für den Nutzer.
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Towards a Better Physical Understanding of Human Hair: Development of Fiber Tribology-Methods at the Micro- and Nanoscale
(2010)
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Eva C. Max
- Die Zielsetzung dieser Arbeit ist die Weiterentwicklung verschiedener physikalischer Messmethodiken zur Untersuchung von Mikrofaseroberflächen. Das zugrunde liegende System hierbei hat das Augenmerk auf die Veränderungen der Oberflächen von Haaren durch applizieren mit unterschiedlichen Shampooformulierungen. Fokussiert werden die physikalischen Veränderungen, einerseits in der Makro - und Mikroskala und andererseits in der Nanoskala. Haarpflegemittel wie Shampoos oder Conditioner spielen eine wichtige Rolle für unser Wohlbefinden und bieten einen weltweit attraktiven Markt für die Industrie. Von Seiten der Forschung wurde die Wirkungsweise und Adsorption von aktiven Wirkstoffen in Haarpflegemitteln deshalb intensiv untersucht. Im Gegensatz dazu stecken quantitative Messungen der Veränderungen in den Reibungs- und Wechselwirkungseigenschaften, die durch die Behandlung erreicht werden noch in den Anfängen. Unser Ziel ist hier, Methoden zu entwickeln, die es erlauben solche Eigenschaften zu untersuchen und damit die Basis für eine gezielte Optimierung von Haarpflegeprodukten zu bieten. Die wichtigsten aktiven Substanzen in Haarpflegemitteln sind kationische Polymere. In dieser Arbeit wurden drei der auf dem Markt käuflich erwerbbaren Polymere (Polyquaternium-10, Polyquaternium-87 und Jaguar C-13S) in Hinblick auf ihr Wechselwirkungsverhalten mit dem negativ geladenem Netzwerk der Haaroberfläche untersucht. Ein modifizierter Aufbau des so genannten Universal Oberflächen Testers lieferte Ergebnisse zu den Reibungseigenschaften von Haaren im Mikrometerbereich. Mit der Entwicklung einer neuen Messzelle und der Kalibrierung eines selbst konstruierten Messkopfes wurde das Instrument an das Haarfasersystem angepasst und optimiert. Das Material des Messkopfes (Styrol-Butadien-Kautschuk) wurde dem Material von herkömmlichen Kämmen angeglichen, um einen direkten Vergleich mit Ergebnissen, die aus der so genannte Kämmkraftmethode hervorgingen, zu ermöglichen. Der Schwerpunkt lag auf der Veränderung der auftretenden Reibungskräfte nachdem die Wirkstoffe auf das Haar aufgebracht wurden. Alle drei Polymere verursachten eine Reduktion der Reibungskräfte, die höchste konnte dabei dem Polymer Polyquaternium-87 zugeschrieben werden. Bei beiden Methoden werden jedoch viele verschiedene Wechselwirkungen detektiert, nämlich die Reibung zwischen den einzelnen Haaren, deren Verwicklungen untereinander, die Reibungen zwischen den kationischen Polymereren mit den Haaren und mit dem Kammmaterial und die Wechselwirkungen der Haare mit dem Styrol-Butadien-Kautschuk. Mit dieser Erkenntnis galt es einen Weg zu finden, der diese auftretenden Multikräfte voneinander separiert. Hierfür wurde der Aufbau eines Rasterkraftmikroskop so modifiziert um die Wechselwirkungen zwischen zwei einzelnen Haaren zu detektieren. In einem Rasterkraftmikroskop dienen mikrofabrizierte Blattfedern (Cantilever) als ultra-sensitive Kraftdetektoren, wodurch Kräfte im Bereich von 10^{-11} bis 10^{-5} N gemessen werden können. Um dieses Werkzeug zur Untersuchung von Wechselwirkungen zwischen einzelnen Haaren zu nutzen, wurden mit einem Laser-Skalpell Haarfragmente passender Größe (~50 µm) zurecht geschnitten und an den Cantilever fixiert. Dadurch konnten die direkten Wechselwirkungs- und Reibungskräfte zwischen einem Haarfragment und einem weiteren, auf der Oberfläche immobilisierten Haar, gemessen werden. Zunächst lag der Fokus darin, die auftretende Adhäsionskraft zu charakterisieren und mit einem statistischen Ansatz zu quantifizieren. Mit einem Wert der Adhäsionskraft von 38.08 ± 11.60 nN war es möglich anhand eines Kontinuumskontaktmechanikmodells nach Johnson, Kendall und Roberts die daraus resultierende Adhäsionsenergie pro Kontaktflächeneinheit zu 0.1124 {mJ/ m² zu ermitteln. Dieser relative kleine Wert kann unter anderem darin begründet liegen, dass die eigentliche Kontaktfläche zwischen dem Haarfragment und dem Substrathaar in Wirklichkeit relativ ungewiss ist, da die Schuppen auf der Haaroberfläche einige Mikrometer lang sind und eine Höhe von mehreren hunderten Nanometern besitzen. Solche Größenordnungen sind für rasterkraftmikroskopische Untersuchungen von Wechselwirkungskräften riesig. es Weiteren wurde unter der eben erwähnten Aufbausmodifikation des Rasterkraftmikroskops ein Schwerpunkt auf die Ermittlung der auftretenden Reibungskräfte zwischen zwei einzelnen Haaren gelegt. Ein entscheidender Unterschied zu den vorangegangenen Messungen liegt darin begründet, dass die beiden Haare emph{in situ} mit den verschiedenen Polymeren behandelt wurden. Hierbei wurde einem Waschzyklus nachempfunden. Somit war es möglich nach der Behandlung mit den verschiedenen Polymeren die Änderung der Reibungskraft festzustellen. Die auftretende Reduktion der Reibung konnte in einem guten Einklang mit den Kämmkraftdaten gebracht werden. Genau wie bei diesen, erzielte das Polymer Polyquaternium-87 ein anderes Verhalten, verglichen zu den anderen beiden Polymeren. Bei den Einzelhaarmessungen wurde mit dem Polymer Polyquaternium-87 die geringste Reibungsreduktion gemessen. Ein Vergleich zwischen sogenannten Haptikprüftests und den Ergebnissen, die von den verschiedenen Messmethodiken dieser Arbeit erzielt wurden, eröffnet neue Wege solche Sinnestests physikochemisch zu quantifizieren.
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Investigations on the Reaction Mechanism of Xenobiotic Reductase A
(2010)
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Olivia Spiegelhauer
- Xenobiotic reductase A (XenA) from Pseudomonas putida 86 is a member of the Old Yellow Enzyme family of FMN containing enzymes. It catalyzes the NADH/NADPH dependent reduction of various substrates, including 2-cyclohexenone, coumarin, 7- and 8-hydroxycoumarin in a two-step mechanism consisting of a reductive and an oxidative half-reaction. The overall structure of the family members is similar but the active site residues show considerable variations. One distinct difference of XenA compared to other members is the presence of a cysteine residue (Cys25) in the active site, where most other members have a threonine. Further, the active site of XenA is lined up by two tyrosine (Tyr27 and Tyr183) and two tryptophan (Trp302 and Trp358) residues. To get a better understanding of the reaction mechanism of XenA we analyzed the enzyme using a combination of transient and steady-state kinetics, redox potentiometry and crystal structure analysis. Thermodynamic and kinetic investigations revealed a preference of XenA for NADPH over NADH. Furthermore, the reaction catalyzed by XenA follows a ping-pong mechanism in which both substrates are bound to the same position in the active site but interact with different amino acids. The crystal structures of XenA without and with coumarin bound to the active site were solved at true atomic resolution. The oxidized complex with coumarin showed a compressed active site geometry in which the isoalloxazine ring of FMN is sandwiched between coumarin and the protein backbone. The crystal structure of reduced XenA showed a distortion of the isoalloxazine ring and the movement of Trp302 into the active site. Furthermore, we analyzed the individual contributions of the five active site residues using site-directed mutagenesis. An exchange of Cys25 against serine shifted the reduction potential of the FMN/FMNH- couple by 82 mV, increased the limiting rate constant of the reductive and decreased the limiting rate constant of the oxidative half-reaction. Therefore we conclude that Cys25 modulates substrate binding and the reduction potential of FMN. Moreover, we revealed that Tyr27 contributes to the stabilization of the transition state during the reductive half-reaction by an interaction of its hydroxyl group with the transferred hydride ion. The exchange of Tyr183 resulted in a decreased affinity of XenA for NADPH and a considerable decrease of the rate of the oxidative half-reaction. These results are in agreement with its function as indispensable proton donor in the oxidative half-reaction. Exchanging Trp302 resulted in multiphasic kinetics for both half-reactions and a decreased affinity of XenA for NADPH. In combination with its movement between the reduced and oxidized state of XenA, we propose a redox dependent shaping of the active site by Trp302. Hence, this residue is responsible for the correct positioning of the substrates in both half-reactions, which is an essential part in the reaction mechanism. The results from the exchange of Trp358 indicated that this residue is involved in the orientation of the nicotinamide ring of NAD(P)H by spatial exclusion. Crystal structures of enzyme substrate complexes are usually determined from non-reactive states. The Y183F variant of XenA, lacking the proton donor of the oxidative half-reaction, allowed us to freeze-trap the true Michaelis complexes of reduced XenA in complex with four different substrates. For the first time we were able to observe 2-cyclohexenone in an active site. Finally, we prove that mode of substrate binding of XenA is redox dependent. In summary our results provide a more detailed description of the reaction mechanism of XenA and offer new insights on how substrates interact with flavoenzymes.
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Self-Organizing n-type Perylene Derivatives for Organic Photovoltaics – Synthesis, Characterization and Application
(2010)
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André Dominik Wicklein
- Diese Dissertation beschäftigt sich mit der maßgeschneiderten Synthese und der Charakterisierung verschiedener selbst organisierender Perylenderivate als n-Typ Halbleiter-Farbstoff zur Anwendung in organischen Photovoltaik-Bauelementen. Dabei liegt der Schwerpunkt auf zwei unterschiedlichen Selbstorganisationsphänomenen von Perylenfarbstoffen mit definierter Molekülstruktur: Zum Einen auf der thermotropen flüssigkristallinen Anordnung diskotisch geformter Moleküle, zum Anderen auf der Selbstanordnung zu Organogelen und nanostrukturierten Netzwerken mit Hilfe von Wasserstoffbrückenbindungen oder geeigneter Lösungsmittel. Beide Formen der Selbstorganisation sind vielversprechende Ansätze zur Erhöhung des Wirkungsgrades von Solarzellen. So werden bei einer kolumnaren flüssigkristallinen Ordnung hohe intrakolumnare Ladungsträgermobilitäten in solch quasi eindimensionalen „Nanodrähten“ ermöglicht. Ein nanostrukturiertes Netzwerk hingegen gewährleistet definierte Ladungstransport-wege sowie eine große Grenzfläche zwischen dem Elektronenakzeptor und einem geeigneten Elektronendonator. Zur zielgerichteten Steuerung dieser Selbstorganisationsprozesse wurden geeignete Substitutionsmuster am Perylengerüst sowie adäquate Syntheserouten entwickelt. In diesem Zusammenhang stellte auch die Erweiterung der Absorption im sichtbaren Wellenlängenbereich mit dem Ziel einer effektiven Lichtsammlung einen wichtigen Aspekt dieser Arbeit dar. Das thermotrope Phasenverhalten der synthetisierten, diskotischen Perylenfarbstoffe wurde durch Differentielle Wärmeflusskalorimetrie (DSC), temperaturabhängige Polarisations-mikroskopie sowie temperaturabhängige Röntgendiffraktometrie umfassend untersucht. Morphologien unterschiedlicher Nanostrukturen wurden mittels der Rasterelektronen-mikroskopie (REM) und der Rasterkraftmikroskopie (AFM) charakterisiert. Zusammenfassend dargestellt präsentiert diese Dissertation das zielgerichtete molekulare Design und die effektive Synthese verschiedenartiger, selbst organisierender n-Typ Perylenderivate mit hervorragenden Absorptionseigenschaften und diversen molekularen Anordnungen. Darüber hinaus wurde gezeigt, wie solch selbst anordnende Farbstoffe geschickt in organischen Solarzellen integriert werden können.
