Nostalgia, Home and Be-longing in Contemporary Postapartheid Fiction by Zakes Mda and Ivan Vladislavić
- Nostalgia, Home and Be-Longing in Contemporary South African Fiction: Zakes Mda and Ivan Vladislavić
The dissertation examines the representation of nostalgic longing and the question of home and belonging in selected South African novels by the contemporary writers Zakes Mda and Ivan Vladislavić. I argue that nostalgia is a crucial aspect to be explored in the South African context because it poses uncomfortable questions about one’s sense of belonging in a transforming society and one’s position in the present with regard to a history of colonialism and apartheid. The sentiment of nostalgia and the feeling of displacement bind the selected novels and characters and reveal a deep dissatisfaction with the present. In a comparative study, I show that nostalgia and the question of home have different impacts on the two authors. While the longing for a past home and time supports a reaffirmation of cultural identity in Zakes Mda’s novels, Ivan Vladislavić portrays nostalgia with a critical distance showing that the past is difficult to identify with. Both writers are critical of restorative forms of nostalgia (Boym 2001) and show that home and be-longing are a matter of inclusion and exclusion resulting from a legacy of segregation and Othering. I demonstrate how nostalgia can either contribute to constructively engage with the past and present and envision a future for the South African nation, or how it, on the contrary, can enforce the erection of borders and the (colonial) method of Othering within society.
Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions
- Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging.
The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle.
1. Synchrotron Mössbauer Source
There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy.
However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energy- resolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals.
￼The major technical goals of my PhD work were to (a) construct a device that would be permanently ready for operation, and (b) optimize it to have the best possible resolution and highest possible intensity. In order to achieve these goals theoretical calculations were first conducted to understand how to best improve the performance. Second, several experiments were performed to confirm the theoretically predicted results. Third, several optical schemes of the SMS were tested in order to determine the optimal setup.
As a result of the research and development program a powerful Synchrotron Mössbauer Source (SMS) for high-pressure applications was constructed at the Nuclear Resonance beamline (ID18) of the ESRF. Using results obtained in the combined theoretical/experimental study of angular dependence of energy and temporal distributions of the pure nuclear reflections of iron borate crystal, the SMS was optimized for the highest possible intensity and best possible resolution. The bandwidth of radiation provided by the SMS is between 10-15 neV (2-3 Γ0, where Γ0 is a natural linewidth of Mössbauer resonance for Iron), the intensity is ~2.5×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 μeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of microns. SMS is the in-line monochromator, permanently located in the optics hutch and operational immediately after moving it into the incident beam position. The source can be used with all existing sample environments in the experimental hutches downstream of the beamline.
The implementation of this device opens the possibility for studying systems with complex hyperfine structure utilizing energy-resolved approach under various extreme conditions, for example at high-pressure. Furthermore, the SMS allows for very short collection times of only a few minutes, which enables data to be collected during laser heating. Several high-pressure and high-pressure/high-temperature studies that have already been performed are described in the second part of this Ph.D. thesis. The almost 100% recoilless resonant radiation delivered by the source and its high brightness allow a broad field of SMS applications. The SMS can be utilized in any mode of synchrotron storage ring operation.
2. Study of the spin state of Fe3+ ions in perovskite
Silicate perovskite (Mg,Fe)(Si,Al)O3 is the most abundant phase in the Earth’s lower mantle. Knowledge of its properties is indispensable for understanding lower mantle behavior. Dynamic, thermodynamic, and transport properties of silicate perovskite can be significantly affected by the valence and spin state of iron. Silicate perovskite with 5-10 mol% of Fe (where Fe3+/ΣFe ~50–75% (McCammon et al., ￼1997)) and Al, is dominant phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007). The behavior of Fe electronic properties under conditions close to those of the Earths lower mantle remains strongly controversial.
The second part of my Ph.D. work is dedicated to an investigation of the spin state of iron in Fe3+- rich silicate perovskite at high pressure. Four different silicate perovskite samples with different stoichiometry were studied using the Synchrotron Mössbauer Source. SMS spectra were collected at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples.
The hyperfine parameters, i.e., centre shift and quadrupole splitting, for the same phases, which were extracted from measured spectra for all perovskite samples studied in this work, are the same at each pressure within experimental error. Moreover, there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment. The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo spin crossover within the entire pressure range. All observed changes in the spectra are associated with abrupt changes in the electronic state of Fe2+. The hyperfine parameters of the low QS Fe2+ doublet correspond to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting, whose intensity grows with pressure at the expense of the Fe2+ high-spin state, corresponds either to intermediate-spin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Based on results presented in a work of Narygina (2010), we indentify changes in Fe2+ electronic structure as high-spin to intermediate spin transition. Irrespective of the interpretation of the Fe2+ spin state, conclusions regarding the absence of spin crossover in Fe3+ remain valid.
These results show that the previously reported spin crossover of Fe3+ ions does not occur when Fe3+ occupies the A-site. In both alumina-containing and alumina-free silicate perovskites Fe3+ ions remain in the high-spin state up to at least 122 GPa, i.e., almost up to the pressure corresponding to the lower mantle - outer core boundary. The results also indicate that Fe3+ ions do not diffuse from the A-site to the B-site in perovskite after high-temperature annealing at high pressure, Mössbauer spectra of before and after annealing are identical. There is also no evidence for high-spin to low-spin crossover of Fe3+ ions due to site change. In contrast, the results confirm that Fe2+ ions undergo a transition from a high-spin to an intermediate spin state, without reaching a low-spin state within the studied pressure range at room temperature. These results suggest that the seismic velocity anomalies in the lower mantle cannot be attributed to spin crossover in Fe3+.
Structure-Property Correlation of Electron Transport Materials in Organic Devices
- This dissertation deals with organic semiconductors as electron acceptor
(n-type) materials in bulk heterojunction (BHJ) solar cells. Important features of an electron acceptor are strong visible light absorption, sufficient high electron mobility and appropriate energy levels with respect to the donor. Furthermore, the blend morphology of donor and acceptor is crucial for the device performance. Within this thesis, the synthesis and characterization of novel n-type polymers is reported and various techniques to evaluate the above mentioned parameters for n-type small molecules and polymers are presented. The aim was to investigate the impact of chemical structure on the optical and electronic properties and morphology of these semiconductors. Successful strategies how to control and improve light harvesting, electron mobility, blend morphology and solar cell performance were identified. The fundamental question of charge transport properties of the materials was addressed by fabricating single carrier devices using the SCLC (space-charge limited currents) method. The morphology was primarily investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD).
The first part of this thesis focuses on perylene imide based small molecules and polymers. The side groups of a series of N-substituted perylene bisimides (PBI) were found to play a crucial role on crystallinity and charge transport. The nature of the side groups had great impact on the crystalline structure and electron mobility. When hydrophilic oligoethylenglycol (OEG) side groups were present, the perylene molecules aligned in highly ordered hexagonal or lamellar columns and realized high electron mobilities of up to 7∙10E-3 cm2V-1s-1, while the perylene derivative with only hydrophobic alkyl chains only showed 3∙10E-5 cm2V-1s-1. The substituents at the perylene core also had a major impact on the blend morphology of OPV devices when these materials were used in combination with a donor polymer. Here, we were able to tune the extent of phase separation between donor and acceptor via hydrophilic-hydrophobic interactions of donor polymer and acceptor side groups. To improve light harvesting of perylene compounds, the pi-electron system of PBIs was altered and highly soluble, novel perylene side chain polymers (PPDB and PPDI) were synthesized by nitroxide mediated radical polymerization (NMRP). The pendant perylene moieties were perylene diester benzimidazole (PDB) and perylene diester imide (PDI). Compared to polymers bearing PBI side groups, the visible light absorption of PPDB was broadened and red shifted, whereas a narrower and blue shifted absorption was observed for PPDI. Remarkably, also the electronic nature of the two materials was affected by the modification at the perylene core, as PPDB is an n-type semiconductor and PPDI has a more pronounced p-type character. A comparative study of perylene side-chain polymers synthesized by a combination of NMRP and “click” chemistry revealed that the compound with improved optical properties (PPDEB) exhibited worse charge carrier mobility compared to PPBI. Another striking result was found as an amorphous polymer bearing OEG side chains showed a better electron mobility than the corresponding material with alkyl chains, which was liquid crystalline. A very high electron mobility of 1∙10E-2 cm2V-1s-1 was measured.
The second part of this dissertation addresses fullerene based acceptor materials, among which Phenyl-C61-butyric acid methyl ester (PCBM) is the state-of-the-art n-type semiconductor used in OPV. For two fullerene derivatives, Bis-Phenyl-C61-butyric acid methyl ester (bis-PCBM) and Bis-o-quino-dimethane C60 (bis-oQDMC), the LUMO energy levels were higher compared to PCBM. As a result, improved open circuit voltages (Voc) in BHJ solar cells were obtained. The efficiency however did not improve, because of reduced short circuit current densities (Jsc). We found that for the bis-PCBM system, Jsc was limited by low electron transport, while for the bis-oQDMC system an unfavorable blend morphology hampered the performance. The problem of low electron mobility could be overcome by reducing the thickness of the active layer and higher Jsc and overall device performance could be achieved. A drawback of fullerene small molecules is that diffusion, aggregation and crystallization of these molecules within BHJ blends can often negatively affect the stability of the blend morphology and reduce the device performance. We discovered that aggregate and crystallite formation in novel fullerene side chain polymers could be successfully suppressed, whilst high electron mobility and better film properties were achieved.
Altogether, new insights into structure-property relation of organic electron transport materials are presented in this work. Moreover, the detailed analysis of charge transport helped to understand the performance of solar cells.
Hybrids Based on Layered Silicates
- Novel hybrid nanoparticles were synthesized based on combinations of various layered silicates as inorganic core and well-defined polymer chains as a shell. In all cases positively charged 2-(dimethylamino)ethyl methacrylate (DMAEMA) was incorporated into the polymeric structure to serve as a firm anchor onto the negatively charged clay surface via electrostatic adsorption.
First, hybrid nanofillers were synthesized to improve the mechanical properties of a homopolymer matrix by combining a shear-stiff synthetic K-hectorite with a tailored surface activity. For this, the synthetic fluorohectorite with very high aspect ratios was organophilized with a specifically designed macroinitiator created by statistical Reversible Addition Fragmentation Chain Transfer (RAFT) copolymerization of DMAEMA and the initator-monomer 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM). The copolymer was firmly anchored through multiple cationic charges distributed over the chain while the multiple initiating functions were used to polymerize the monomer of choice via Atom Transfer Radical Polymerization (ATRP). The final hybrid was equipped with a hydrophobic polymeric shell of poly(methyl methacrylate) (PMMA), which enables dispersion in organic solvents. The hybrid particles were compounded into a polymeric matrix of commercial PMMA and tested with regard to its reinforcing properties. The similarity of the polymeric shell to the homopolymer matrix of the chosen sample composite combined with the inherent stiffness of the inorganic core lead to an increase in tensile modulus of up to 84 % at 5% filler content. Further, patchy hybrid nanodiscs based on natural montmorillonite as core and a shell made from compartments of two different polymers were evaluated as cheap and versatile compatibilizers in an immiscible polymer blend. In a simple one-step modification process a shell comprising patches of either of two polymer species (PMMA and polystyrene, PS), each chosen to be similar in polarity to one of the matrix polymers, was attached to the inorganic core via Coulomb interaction. The behaviour of these particles in a solvent-cast blend of 2:1 PS/PMMA was investigated via transmission electron microscopy (TEM) and dynamic-mechanical analysis (DMA). Particles were found distributed in both of the blend’s domains and at the interface and an improvement of the storage module of 17% was found.
Finally, kaolinite was used as a core to create true hybrid Janus nanodiscs, which were applied for compatibilizing an immiscible polymer blend of 2:1 PS/PMMA. It was possible to create two chemically distinct surfaces on the clay particle by addressing each of its two basal surfaces individually via simple, but selective, surface modification. Two diblock copolymers were used to create the Janus structure, each one with a first block consisting of monomer units bearing the anchoring group for the respective surface and a second block, PS or PMMA, tailored to the polarity of the respective matrix polymer. Thus it was possible to direct the Janus particles straight into the interface between the polymeric domains, visualized by TEM images taken from solvent-cast nanocomposite films.
Synthesis and investigation of boron phases at high pressures and temperatures
- Boron, discovered as an element in 1808 and produced in pure form in 1909, still remains one of the most complicated light elements full of surprises. Even the number of pure boron polymorphs is a subject of intensive discussions. It is proven the existence of α-, β- and γ-boron phases. Structural details of the most common boron phase (β-B) are still not fully revealed. For decades boron remained the last stable element in the periodic table, whose ground state was not determined. It has been a subject of a longstanding controversy, whether α-B or β-B is the thermodynamically stable phase at ambient pressure and temperature.
The existence of the α-tetragonal boron phase T-50 has been an open question since its first discovery. It was not clear if T-50 could be synthesized as a pure boron phase or its structure must be stabilized by the presence of carbon or nitrogen. Theorists claimed that T-50 could not exist at all because of its unstable electronic configuration.
We have developed a method of synthesis of single crystals of α-boron. They were crystallized from a boron-platinum melt at high pressures (6-11 GPa) and high temperatures (1450-1875 K). An average size of the as-grown isometric crystals was 60 μm to 80 μm in maximum dimension. The crystal structure is in good agreement with the literature data. Detailed investigation of single crystals of α-boron using Raman spectroscopy was performed under elevated pressures and temperatures. The behaviour of the Raman modes under pressure was studied both theoretically and experimentally. Single crystals of β-boron were grown at temperatures above 1550 K and pressures up to 11 GPa using the similar methodology like that worked out for synthesis of α-boron.
In a series of experiments above 8 GPa we synthesized single crystals of tetragonal δ-boron (also known in literature as α-tetragonal boron or T-50) and refined the crystal structure of this phase based on synchrotron X-ray diffraction data. The purity of δ-boron was confirmed by means of the microprobe analysis and the electron energy loss spectroscopy (EELS).
A new, so far unknown boron phase, ε-boron, was synthesized at pressures of 8-10 GPa and temperatures between 2000-2250 K. The microprobe analysis and EELS revealed that the samples were not contaminated. The crystal structure of the new phase was determined by means of single crystal X-ray diffraction. ε-boron crystallizes in a R-3m space group with the unit cell parameters a = 5.5940(7) Å and c = 12.0756(16) Å (in hexagonal setting). The unit cell contains 15 boron atoms. The structure can be presented by the network of B12 icosahedra with a group of three boron atoms in the inter-icosahedra space. This phase is isostructural to boron carbide B13C2 (if carbon atoms are substituted by boron ones). Measured hardness is ~60 GPa which places ε-boron in the family of superhard materials.
We have demonstrated that δ-boron and ε-boron are metastable polymorphs because (a) they were found only together with other stable boron phases (α-, β-, or γ-B), and (b) upon heating at high pressure, both δ-B and ε-B transform to β- or γ-B, if the PT conditions correspond to the fields of stability of the latter.
Summarising, in the course of the present work the high-pressure high-temperature synthesis of the five boron polymorphs was established as a reproducible, verifiable and well-documented process. Following the synthesis prescription one can grow single crystals of α-B, β-B, γ-B, δ-B, and ε-B phases. Based on results of numerous HPHT experiments, the phase boundaries between the stable boron phases (α-B, β-B, γ-B) were found. Thus, our serial exploration of the pressure-temperature field using the large volume press synthesis technique resulted in establishing the phase diagram of boron (showing also the PT fields of the appearance of its two metastable phases, δ-B and ε-B) in the pressure interval of 3 GPa to 18 GPa at temperatures between 1073 K and 2423 K. Based on our experimental data and linear extrapolation of the α/β phase boundary down to ambient pressure we could resolve a longstanding controversy on the ground state of boron in favour of the α-B phase.
Coarse-grained Modeling of Protein Dynamics using Elastic Network Models
- Dynamics is crucial for the functioning of biological macromolecules. Because of severe limitations in studying protein dynamics experimentally or with all-atom simulations, coarse-grained methods, especially elastic network models (ENMs), are frequently employed. In the last years, studies on various proteins showed that ENMs reliably reproduce experimental data, despite the simplified protein representation and the purely harmonic potential function. This work on two proteins with very different dynamical properties highlights the remarkable success of ENMs and shows which kind of questions can be answered using coarse-grained methods.
The allosteric enzyme aminoglycoside phosphotransferase(3')-IIIa (APH), which confers resistance against a broad range of aminoglycoside antibiotics to pathogenic bacteria, drastically changes its flexibility upon binding of substrates, but without changing its average conformation. In contrast, the homotrimeric vesicular stomatitis virus glycoprotein G (VSV-G), which triggers the pH-dependent fusion of viral and host membrane, undergoes a large structural rearrangement. A striking difference between the two proteins is their shape. VSV-G contains weakly constrained protein segments, the fusion loops, which can undergo large-scale motions at low energetic cost, whereas APH is not obviously arranged into different protein segments. Nevertheless, ENM calculations show that also APH consists of independently moving segments with correlated internal motion, so-called dynamic domains. The concept of dynamic domains can explain the differential effects of ligand binding on the dynamics of APH.
The first chapter of this thesis describes how experimental evidence for the importance of dynamics successively replaced the former idea of static proteins, and explains the dynamic basis of ligand binding, allostery and conformational changes. In the second chapter, theoretical methods for the analysis of protein dynamics are introduced, with emphasis on the ENM-based methods used in my studies. The studies are summarized in the third chapter. In the study on APH, I employ the Gaussian network model to analyze the ligand-dependent dynamics, the broad substrate specificity and the perturbation-sensitivity of the ligand binding sites. In a second study, ENM-based as well as all-atom molecular dynamics simulations are used to analyze the conformational change of VSV-G. Both approaches detect the fusion loops of VSV-G as most flexible parts of the protein, and thus as most likely starting point for the structural rearrangement, but only the all-atom model can generate deviations from the average structure at low pH. The last study describes the implementation and application of a dynamic domain assignment method, called CovarDom, which is based on covariances of residue fluctuations. Calculation of dynamic domains for a large protein set demonstrates the general applicability of CovarDom.
Endokrine Signale in der Kastendetermination der ursprünglichen Termite Mastotermes darwiniensis Froggatt (Isoptera: Mastotermitidae)
- Als „ecosystem engineers“ sind Termiten in subtropischen und tropischen Ökosystemen von wichtiger ökologischer Bedeutung. Gleichzeitig gelten einige Termitenarten als Bauholz- und Agrarschädlinge, die weltweit vorkommen und enorme Schäden anrichten können. Für die Entwicklung effizienter und umweltschonender Insektizide gegen Termiten dient besonders deren Kastendifferenzierung als Ansatzpunkt. Die Mechanismen der Kastendifferenzierung sind noch nicht vollständig aufgeklärt, das Juvenilhormon (JH) soll aber eine entscheidende Rolle spielen. Daher sollte in dieser Studie der Einfluss des Juvenilhormons auf die Kastendifferenzierung bei der basalen Termite Mastotermes darwiniensis untersucht werden. Die Regulation der JH-Biosynthese erfolgt über allatoregulierende Neuropeptide, von denen Allatostatin A (AST-A) das am weitesten verbreitete ist. Die Expression des AST-A Gens bei M. darwiniensis wurde daher ebenfalls analysiert.
Zunächst musste die Sequenz des AST-A Gens von M. darwiniensis und die Organisation des AST-A Prohormonvorläufers identifiziert werden. Das AST-A Gen kodiert einen Prohormonvorläufer mit 14 potentiellen Peptiden, die durch vier saure Spacerregionen zu Clustern zusammengefasst werden. Dabei weisen nicht alle Peptide die charakteristische C-terminale Sequenz Y/FXFGL/I/V-amid der AST-A Peptide auf. Trotzdem zeigt der AST-A Prohormonvorläufer von M. darwiniensis eine starke Ähnlichkeit zu dem ebenfalls in dieser Studie identifizierten AST-A Prohormonvorläufer der nahe verwandten Schabenart Cryptocercus darwini sowie zu den bekannten AST-A Prohormonvorläufern anderer Schaben und einer weiteren Termitenart. Die phylogenetische Analyse der Prohormonvorläufer und des AST-A Gens bestätigte die enge Verwandtschaft der Schaben und Termiten. Durch die zentrale Einordnung der Termiten innerhalb der Schaben konnte zudem die noch umstrittene Paraphylie der Blattaria bestätigt werden. Die Stammbäume, die aus AST-A Sequenzen von acht verschiedenen Insektenordnungen berechnet wurden, zeigten generell eine Übereinstimmung mit der anerkannten Phylogenie der Insekten. Eine unterschiedliche Topologie und teilweise geringe Bootstrap-Werte sprechen allerdings gegen die Eignung von AST-A für phylogenetische Analysen.
Das AST-A Gen wird bei M. darwiniensis überwiegend im Gehirn exprimiert. Eine geringere Expression konnte auch in verschiedenen Teilen des Darms nachgewiesen werden, wobei der Mitteldarm höhere Expressionsraten aufwies als der Vorder- und Hinterdarm. In den verschiedenen Entwicklungsstadien von M. darwiniensis zeigten sich ebenfalls Unterschiede bei der AST-A Expression, generell wiesen „adulte“ Stadien eine höhere Expression auf als juvenile Stadien.
Die Messung des Gehaltes von JH III und Ecdysteroiden bei den verschiedenen Entwicklungsstadien von M. darwiniensis erfolgte mittels HPLC-MS. Ein hoher JH III-Gehalt wurde bei Larven nachgewiesen, dieser lag über dem JH III-Gehalt der Nymphen derselben Altersgruppe. Der hohe JH III-Gehalt der mittleren Larven deutet auf die potentielle Entwicklung dieser Tiere zu Soldaten. Während der Entwicklung von Geschlechtstieren konnte nur ein geringer JH III-Gehalt gemessen werden. Dies galt sowohl für die Entwicklung von primären Geschlechtstieren aus Nymphen als auch für die Entwicklung sekundärer Geschlechtstiere aus Arbeitern. Der hohe JH III-Gehalt während der Soldatenentwicklung ebenso wie der niedrige JH III-Gehalt während der Geschlechtstierbildung stimmt mit dem Modell von Nijhout & Wheeler (1982) zum Einfluss von JH auf die Kastendifferenzierung bei Termiten überein. Dieses bezieht sich allerdings auf Termiten mit linearem Entwicklungsweg, während bei Termiten mit zweiästigem Entwicklungsweg – wie M. darwiniensis – bisher nur wenig zur Rolle von JH auf die Kastendifferenzierung bekannt war.
Es konnte ein signifikanter Zusammenhang zwischen dem JH III-Hämolymphtiter und der Expression des AST-A Gens im Gehirn bei M. darwiniensis nachgewiesen werden. Beide Parameter waren negativ miteinander korreliert, d. h. Entwicklungsstadien mit hohem JH III-Titer zeigten überwiegend geringe AST-A Expression und umgekehrt.
Zur weiterführenden Funktionsanalyse des AST-A Gens wurde zudem versucht, die Expression des AST-A Gens in Arbeitern von M. darwiniensis mittels RNA-Interferenz zu unterdrücken. Durch Injektion von doppelsträngiger RNA (dsRNA) abgeleitet vom AST-A Gen konnte die Expression um bis zu 92% reduziert werden. Die Reduktion war dabei in den verschiedenen Geweben unterschiedlich stark ausgeprägt. Es konnte keine vollständige Suppression der AST-A Expression erreicht werden. Drei Tage nach der Injektion von AST-A dsRNA konnte keine Auswirkung auf den JH III-Titer in der Hämolymphe der Tiere beobachtet werden.
Studie zur Software-Unterstützung für die Raumplanung an deutschen Hochschulen
- Die Studie untersucht die verwendete Software zur Raumplanung an ausgewählten deutschen Hochschulen. Durch einen Fragenkatalog werden Unterschiede in den Planungsstrategien erfragt und analysiert. Das Ergebnis der Studie soll zeigen, dass bisher in Deutschland so gut wie keine echte Planungssoftware zur Unterstützung verwendet wird bzw. existiert.
Dynamic charge densities of amino acids and proteins
Prathapa Siriyara Jagannatha
- This PhD thesis deals with the notion of dynamic electron density and describes the effect of temperature on the electron density distribution by analyzing both static and dynamic densities of crystals. The dynamic electron densities have been successfully computed by inverse Fourier transform of accurately computed structure factors from the structure model by employing the method of fast Fourier transform (FFT). Static and dynamic electron densities corresponding to independent atom models (IAM), structure models based on high-order refinement of the IAM (IAM-HO), invariom (INV) models and multipole (MP) models have been constructed for several molecular crystals. Based on all four structure models, the static and dynamic electron densities have been calculated and compared using the low-temperature (T ≈ 20 K) high-resolution data sets of α-glycine, D,L-serine, L-alanine and L-alanyl-L-tyrosyl-L-alanine (Ala-Tyr-Ala) as well as the protein Crambin (T = 100 K). By using a multi-temperature data set of D,L-serine, the effect of temperature on electron densities have been analyzed. Density values near atomic maxima are found to be much smaller in dynamic than in static electron densities due to the thermal smearing in dynamic densities. The electron densities at bond critical points (BCPs) of covalent bonds obtained from dynamic electron densities possess slightly smaller values in comparison to the static densities. However, rather larger differences have been observed for Laplacians. The discrepancy increases with increasing polarity of the bond and with increasing temperature. Nevertheless, at temperatures below 100 K, topological properties at BCPs of dynamic electron densities provide at least a semi-quantitative estimate of the topological properties of static electron densities. In contrast to covalent bonds, electron densities at BCPs of hydrogen bonds possess slightly larger values in dynamic electron densities compared to static densities.
In case of the protein Crambin, it has been found that the ADPs of Crambin at 100 K are larger or equal to ADPs of D, L-serine at 298 K. In corresponding dynamic densities, bonding features have been found to be attenuated due to the masking effects of large ADPs. As a result, the topological properties obtained from dynamic densities of Crambin at 100 K appear to be similar with the topological properties of small molecules at room temperature.
Calculations using the maximum entropy method (MEM) have been undertaken with all four types of dynamic model densities, mentioned above, as prior densities. Electron density analysis by the MEM has been performed for all small molecules studied in this thesis. It is shown that MEM density maps show a tendency to converge to a density map that is independent of choice of prior. Apart from the MP model as prior, it has been found that a good characterization of chemical bonds, at least in organic molecules, can be obtained by the MEM using the IAM-HO or the INV dynamic model densities as prior, while the IAM dynamic model density as prior leads to slightly inferior MEM densities. It will become especially important for the intended application to large systems (for example proteins) where the free refinement of MP model is not possible.
Functional Triblock Terpolymers for Multicompartment Micelle and Janus Particle Synthesis
- This thesis describes the synthesis of ABC triblock terpolymers with functional moieties via living anionic polymerization, followed by Janus particle (JP) and multicompartment micelle (MCM) synthesis from the as prepared triblock terpolymers.
A synthesis method that can accomplish the tasks of the preparation of spherical as well as non-spherical JPs that are well-defined and in the nanometer size range is based on converting self-assembled triblock terpolymer bulk structures via selective cross-linking of the middle block. Until now such soft JPs were prepared mainly from polystyrene-block-polybutadiene-block-poly(methyl methacrylate) and polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate). However, these polymers do not offer many possibilities of chemical alterations and stimuli-responsive elements.
Therefore, potential new functional monomers for the use in JP synthesis from triblock terpolymer bulk structures were identified and their anionic polymerization examined, p-tert-butoxystyrene (tS) and 4-(dimethylaminomethyl)styrene (DMAMS). Polymers with low polydispersity indices could be prepared from both monomers in tetrahydrofuran (THF) with sec-butyllithium (sec-BuLi) as initiator.
Poly(p-tert-butoxystyrene) (PtS) was hydrolyzed to poly(p-hydroxystyrene) (PHS) which is water-soluble at high pH values, opening the possibility to prepare water-soluble JPs. The pH-responsive behavior of poly(4-(dimethylaminomethyl)styrene) could be confirmed and for the first time an LCST behavior was documented with cloudpoints of 59.3 °C at pH 7 and 28.5 °C at pH 8.
PtS was then used in the preparation of two triblock terpolymers, poly(tert-butoxystyrene)-block-polybutadiene-block-poly(tert-butyl methacrylate) (tSBT) and poly(tert-butoxystyrene)-block-polybutadiene-block-poly(2-(dimethylamino)ethyl methacrylate) (tSBD). tSBT exhibited a lamella-cylinder (lc) bulk morphology with polybutadiene (PB) spheres surrounded by alternating lamellae of PtS and poly(tert-butyl methacrylate) (PtBMA). However, the bulk structure of tSBD consisted of a symmetrical lamella-lamella pattern that is not suitable for JP synthesis.
From tSBT bulk material, three different types of non-spherical JPs could be obtained. Photo-cross-linking of the lamella-cylinder (lc)-morphology by co-casting a radical photo-initiator and UV exposure resulted in the expected Janus cylinders. When the bulk material was first swollen in acetonitrile and cross-linked by cold vulcanization, Janus sheets were obtained. Swelling in acetonitrile/decane emulsion lead to a new type of JPs, Janus ribbons. In both cases a phase transition had occurred; in case of the Janus sheets a thin PB layer had formed between the original PB cylinders, resulting in an undulated-lamella morphology. For the formation of Janus ribbons a connecting PB phase had formed in every second interspace along the major axis of the cylinders. Casting a tSBT film from tert-butanol, a non-solvent for PB, also enabled the synthesis of spherical JPs. This way, the importance and versatile application of swelling agents and cross-linking methods for the preparation of JPs from bulk structures was demonstrated. The obtained Janus cylinders were hydrolyzed to have one PHS and one poly(methacrylic acid) (PMAA) hemicylinder, resulting in water-soluble particles.
Further, solution structures of tSBD and tSBT triblock terpolymers were investigated. In water, tSBD formed core-corona micelles that exhibited pH-responsive and LCST behavior due to the responsive poly(2-(dimethylamino)ethyl methacrylate) corona. Employing the novel method of directed hierarchical self-assembly, “football” MCMs were obtained from tSBD whereas tSBT formed “clover” structures. Cross-linking of the B block in such MCMs and their subsequent dissolution in a solvent for all three blocks can be used to prepare spherical JPs. However, as tSBD MCMs existed in water, no sufficient cross-linking method could be found to cross-link the organic PB phase within the aqueous solution. For tSBT MCMs in ethanol the standard approach of adding a photo-initiator to the MCM solution followed by UV exposure was successfully employed and spherical JPs were obtained. These were again hydrolyzed to acquire water-soluble JPs. Apart from some isolated single Janus spheres, cryogenic transmission electron microscopy mainly revealed the formation of “clover” and “hamburger” oligomers possibly due to the slightly better solubility of PMAA. Here, the applicability of the concept of directed hierarchical self-assembly to create MCMs was demonstrated for two different triblock terpolymers and the corresponding method of solution-based JP synthesis was successfully conducted for tSBT.