03 Fakultät Chemie
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Item Open Access Untersuchungen zur Derivatisierung und Charakterisierung Carben-analoger N-heterozyklischer Halogenarsane(2021) Bender, Johannes; Gudat, Dietrich (Prof. Dr. Dr.)Die Chemie neutraler N-heterozyklischer Arsane konnte durch Synthese einer Reihe von Verbindungen mit funktionellen Substituenten am Arsen (Halogeno-, Pseudohalogeno-substituiert) erweitert sowie strukturelle und elektronische Verhältnisse aufgeklärt werden. Aus 2-Chloro-1,3,2-Diazaarsolidinen und -1,3,2-Diazaarsolenen konnten einige neue kationische Arsen-Analoga von N-heterozyklischen Carbenen hergestellt werden. Des Weiteren konnten noch unbekannte 2-Thiolato- und 2-Xanthogenato-1,3,2-Diazaarsolidine und -1,3,2-Diazaarsolene dargestellt und charakterisiert werden.Item Open Access Specific DNMT3C flanking sequence preferences facilitate methylation of young murine retrotransposons(2024) Dossmann, Leonie; Emperle, Max; Dukatz, Michael; de Mendoza, Alex; Bashtrykov, Pavel; Jeltsch, AlbertThe DNA methyltransferase DNMT3C appeared as a duplication of the DNMT3B gene in muroids and is required for silencing of young retrotransposons in the male germline. Using specialized assay systems, we investigate the flanking sequence preferences of DNMT3C and observe characteristic preferences for cytosine at the -2 and -1 flank that are unique among DNMT3 enzymes. We identify two amino acids in the catalytic domain of DNMT3C (C543 and V547) that are responsible for the DNMT3C-specific flanking sequence preferences and evolutionary conserved in muroids. Reanalysis of published data shows that DNMT3C flanking preferences are consistent with genome-wide methylation patterns in mouse ES cells only expressing DNMT3C. Strikingly, we show that CpG sites with the preferred flanking sequences of DNMT3C are enriched in murine retrotransposons that were previously identified as DNMT3C targets. Finally, we demonstrate experimentally that DNMT3C has elevated methylation activity on substrates derived from these biological targets. Our data show that DNMT3C flanking sequence preferences match the sequences of young murine retrotransposons which facilitates their methylation. By this, our data provide mechanistic insights into the molecular co-evolution of repeat elements and (epi)genetic defense systems dedicated to maintain genomic stability in mammals.Item Open Access Redox-acid/base phase diagrams as an entry to computational redox chemistry(2024) Becker, Patrick M.; Heinze, Katja; Sarkar, Biprajit; Kästner, JohannesThe rapid depletion of fossil fuels and the change from conventional energy supply to so‐called sustainable and renewable energy sources have led to a renaissance of electrochemical, photochemical, and photoelectrochemical methods for chemical synthesis. While drastic experimental improvements have been realized in recent years, systematic computational studies of these types of reactions are, however, rather limited caused by a lack of suitable representations. Herein we present a generalized method to investigate and analyze a chemical system with respect to its redox‐ and acid/base‐properties based on Gibbs free‐energy differences. We represent the results in a clear manner by means of redox-acid/base phase diagrams. Motivated by computational needs, the presented method is a direct link between experimentally measurable values and Gibbs free‐energy profiles, connecting experiment and simulation. Thus, it serves as an entry to systematic computational studies of reactions, which involve a combination of electron transfers and acid/base‐chemical reaction steps, because it enables the representation of both thermodynamic and kinetic properties. The presented method is applied to four exemplary systems: Phenol, dicobaltocenium amine as a proton‐coupled electron transfer (PCET) reactant, and two porphyrin Ni II catalysts for the electrocatalytic hydrogen evolution reaction (HER).Item Open Access Aluminum-induced crystallization of semiconductor thin films(2015) Qu, Fei; Schmitz, Guido (Prof. Dr.)Thin film materials of the semiconductors, such as silicon (Si), germanium (Ge) or their alloys, are turning into the most promising functional materials in the energy technology. However, the morphologies of these semiconductor thin films must be varied to be suitable for the different applications, e.g. a large-grained layer as the seed layer of thin film solar cells, a porous structure for anode materials of high energy rechargeable lithium (Li) ion batteries. Due to the collective interdiffusion process during the aluminum (Al)-induced crystallization, in this thesis, the suitable morphologies are achieved for the corresponding applications under the different fabrication conditions. A large-grained Si layer can be formed by the crystallization of Si in a porous Al layer, which is obtained by applying a bias voltage. Since the Al grain boundaries are contaminated by e.g. oxygen (O), the diffusion of Si in the Al grain boundaries is retarded. It can lead to a reduction of the nucleation density of Si. At a certain high temperature, a collective diffusion process of Si in Al is activated. Consequently, a large-grained Si layer with (100) texture can be formed. By purposely interrupting the annealing of nanocrystalline Al/amorphous Si (a-Si) bilayers, a porous structure of the crystallized Si can be developed due to the incomplete intermixing of Si and Al. Due to the different dominant diffusion processes of Si in Al at the different annealing temperatures, the most Si diffuses along the different paths in the Al layer, such as triple junction, grain boundary and Al bulk. Therefore, it can develop the different morphologies of the porous Si layers after the selectively etching of Al. By introducing an amorphous Ge interlayer between the crystalline Al and amorphous Si layer, the Al grain boundaries are not essential for the crystallization of the amorphous Si in contrast to the case in Al/Si bilayer system. Si crystallizes continuously on the pre-crystallized Ge seeds which form initially at the original interface of crystalline Al and amorphous Ge. The thermodynamic models to interpret the fundamentals of these different crystallization behaviors of Si are established based on the change of the interface energy between the different phases of the whole system during the crystallization. Using the effective diffusivity, the dominant diffusion process of Si in Al can be investigated to explore the morphological dependence of the crystallized Si layer on the annealing conditions.Item Open Access Understanding the temperature-induced decomposition of commercial nickel-cobalt-aluminum oxide (LiNi0.8Co0.15Al0.05O2) electrodes(2025) Hölderle, Tobias; Baran, Volodymyr; Schökel, Alexander; Westphal, Lea; Stelzer, Robert U.; Niewa, Rainer; Müller‐Buschbaum, Peter; Senyshyn, AnatoliyThis study addresses the thermal degradation and structural stability of the NCA (nickel-cobalt-aluminum oxide) cathode materials under varying states of charge (SOC)/delithiation and temperature. Using simultaneous thermogravimetric and differential thermal analysis and high‐resolution X‐ray diffraction, the sequential evolution from a layered NaCrS2‐type structure to spinel phases (M3O4‐type and LiM2O4‐type) and finally to a rock salt phase is characterized. Degradation involves cation migration, oxygen release, and lattice instabilities, influenced by SOC/lithium content. Fully lithiated NCA (SOC 0%) exhibits superior thermal stability with a single‐step transition, whereas partially delithiated NCA exhibits a multistep transformation process involving spinel intermediates. These findings highlight the complex interplay between energy density and thermal safety, offering guidance for designing NCA cathodes with optimized performance, safety, and stability for high‐energy lithium‐ion batteries.Item Open Access Interrelationships of microstructure, stress and diffusion(2008) Kuru, Yener; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Extensive research has been performed on thin metal films due to their interesting mechanical, electrical and magnetic properties. They can exhibit very high residual, internal stresses arising from the film growth and/or external effects. Apart from direct mechanical consequences, several processes such as grain growth and diffusion can be affected by these stresses and their gradients. As a result, it is of cardinal importance to measure and control the residual stresses in thin films. X-ray diffraction (XRD) is one of the most frequently used approaches for (residual) stress measurement. It is non-destructive, highly accurate (stress (variation) of some MPa can be detected) and the stress states of all crystalline phases in a layered structure can be obtained separately. Moreover, additional microstructural information, as the crystallographic texture, the density of crystalline defects, such as dislocations, and the crystal size can be acquired from the collected XRD data. This thesis is dedicated to the investigation of microstructural changes, residual stresses and interdiffusion in thin films by in-situ XRD. A focal point of interest is methodological aspects of in-situ measurements, which are discussed in detail in Chapter 2 and come to application in the following Chapters 3 and 4.Item Open Access Microarray and molecular genetic analysis of aberrant splicing in human drug metabolizing cytochromes P450 CYP2D6 and CYP2B6(2008) Hofmann, Marco Hans; Schmid, Rolf (Prof. Dr.)This study was devoted to the detection of alternative splicing within the Cytochrome P450 enzymes 2D6 and 2B6, mapping of the most common splice variants and to draw connections to certain single nucleotide polymorphisms (SNPs) and alleles. For both enzymes a splicing sensitive microarray was developed. The microarray was produced and optimized in all steps including the oligonucleotide probe design, microarray processing and target preparation, optimization of hybridization conditions and the development of a new data quantification method for the used probe design. For the developed splicing platform a design was chosen based on 5 different probes. Within the CYP2D6 gene it was known that the SNP 2988G>A (allele *41) in intron 6 shifts splicing towards a variant lacking exon 6, what explains the intermediate phenotype within allele *41. The splicing platform verified this splicing aberration in allele *41. Using the microarry specific splicing patterns were monitored in human liver tissue within the most common alleles of CYP2D6 *1, *2, *4 and *41. It could be observed that within mRNA from allele *41 carriers additionally to the known transcript variant, which is lacking exon 6, total or partial retention of intron 5 and 6 was enhanced. Transcript patterns of CYP2D6*1 and *2 were similar with 5 times higher amount of the full functional transcript (NP), including all nine exons, compared to allele *41. The splicing array showed to be a valuable tool not only for detection of splicing variants in human liver tissue but additionally for detection for allele specific splicing patterns. The existence of highly homologous Cytochrome P450 pseudogenes, which in some cases, as in CYP2D7 also express alternative splicing variants, results in a major problem of interpreting the data from splicing arrays. The developed splicing platform is the first existing array with which gene and pseudogene specific transcript patterns can be monitored individually. The microarray platform can be easily transferred to other genes as shown for the second gene CYP2B6. Alternative splicing in this gene was so far only reported descriptive. CYP2B6 is a polymorphic human drug metabolizing cytochrome P450 with clinical relevance for several drug substrates including cyclophosphamide, bupropion and efavirenz. The common allele CYP2B6*6 [c. 516G>T, Q172H and c.785A>G, K262R] has previously been associated with lower expression in human liver and with increased plasma levels of efavirenz in HIV patients, but the molecular mechanism has remained unclear. With the developed splicing array for CYP2B6 allele specific splicing patterns were observed comparing CYP2B6*6 and CYP2B6*1. This lead to the idea that alternative splicing might play an important role in allele *6. This was investigated in more detail using RNA originating from well-documented human liver tissue. Analysis of mRNA in this tissue demonstrated that additional unknown splicing variants exist (SV8, SV7, SV9). Investigations in human liver tissue using RT-PCR and sequencing showed that the most common transcript in CYP2B6*6 was not the normal transcript (NP) but an alternative splicing transcript lacking exons 4 to 6 (SV1). SV1 was tightly associated with the allele*6 and apparently also with the rare variant c.777C>A (CYP2B6*3). The observations lead to the assumptions that alternative splicing might explain the decreased function observed in allele CYP2B6*6. Further investigations in this direction were performed by cloning CYP2B6 minigene constructs including all nine exons and additional intronic regions. Minigenes carrying the single c.785A>G polymorphism or the rare c.777C>A variant resulted in normal and intermediate expression phenotypes, respectively. In conclusion, the mechanism of the common allele*6 involves predominantly a pretranslational mechanism resulting in decreased enzyme expression. Aberrant splicing is leading to reduce functional mRNA, protein and activity. These results establish the SNP c.516G>T, a nonsynonymous exonic mutation, as the causal sequence variation for severely decreased expression and function associated with CYP2B6*6. This work emphasizes the role of SNPs in non-consensus splicing elements such as exonic and intronic splicing enhancers as well as the clinical relevance of alternative splicing in context of adverse drug reactions. In both investigated genes CYP2D6 as well as in CYP2B6 there exists a common allele (CYP2D6*41 and CYP2B6*6, respectively) in which aberrant splicing results in reduced amounts of functional transcript, reduced amount of protein and enzyme activity. The findings establishes the SNP c.516G>T as the causal sequence variation that can now be reliably used in pharmacogenetic studies in various clinical settings including prediction of drug plasma concentration, toxicity, drug effectiveness and dose adjustment.Item Open Access Mechanistic insights into the allosteric regulation of the Clr4 protein lysine methyltransferase by autoinhibition and automethylation(2020) Khella, Mina S.; Bröhm, Alexander; Weirich, Sara; Jeltsch, AlbertClr4 is a histone H3 lysine 9 methyltransferase in Schizosaccharomyces pombe that is essential for heterochromatin formation. Previous biochemical and structural studies have shown that Clr4 is in an autoinhibited state in which an autoregulatory loop (ARL) blocks the active site. Automethylation of lysine residues in the ARL relieves autoinhibition. To investigate the mechanism of Clr4 regulation by autoinhibition and automethylation, we exchanged residues in the ARL by site-directed mutagenesis leading to stimulation or inhibition of automethylation and corresponding changes in Clr4 catalytic activity. Furthermore, we demonstrate that Clr4 prefers monomethylated (H3K9me1) over unmodified (H3K9me0) histone peptide substrates, similar to related human enzymes and, accordingly, H3K9me1 is more efficient in overcoming autoinhibition. Due to enzyme activation by automethylation, we observed a sigmoidal dependence of Clr4 activity on the AdoMet concentration, with stimulation at high AdoMet levels. In contrast, an automethylation-deficient mutant showed a hyperbolic Michaelis–Menten type relationship. These data suggest that automethylation of the ARL could act as a sensor for AdoMet levels in cells and regulate the generation and maintenance of heterochromatin accordingly. This process could connect epigenome modifications with the metabolic state of cells. As other human protein lysine methyltransferases (for example, PRC2) also use automethylation/autoinhibition mechanisms, our results may provide a model to describe their regulation as well.Item Open Access Fe-C and Fe-N compound layers : growth kinetics and microstructure(2007) Greßmann, Thomas; Mittemeijer, Eric (Prof. Dr. Ir.)Improvement of surface’s properties of iron and steel work pieces by gas nitriding/nitrocarburising plays an important role in metallurgy. In particular the fatigue, tribological and corrosion properties are enhanced by these processes without changing the properties of the bulk. Gas nitriding is mainly performed in NH3/H2 gas mixtures, whereas by nitrocarburising additionally a carbon delivering species (mostly CO) is present in the gas atmosphere. Typical treatment temperatures range from 773 K to 863 K. If sufficient nitrogen (and carbon) is supplied by the gas phase to the workpiece an iron-(carbo-) nitride compound layer will develop at the surface and the matrix becomes enriched with nitrogen (and carbon). Such compound layers consist in general of a hcp (with respect to Fe) epsilon-Fe3(N,C)1+x layer adjacent to the surface and a fcc-type (with respect to Fe) gamma’-Fe4N layer at the layer/substrate interface. This work addresses (i) growth kinetics studies of Fe-C compound layers produced on pure iron sheets by nitrocarburising as well as (ii) microstructural investigations on iron-nitride compound layers obtained by nitriding of iron. Nitrocarburising of iron usually leads to the formation of epsilon/gamma’ compound layers, where the presence of carbon promotes the epsilon phase which can dissolve considerable amounts of carbon what is not the case for the gamma’ phase. It has been observed previously that additionally to the epsilon and gamma’ phases also some cementite (Fe3C) can form within the compound layer leading to complex microstructures. However, it was found for the first time in this work that it is possible to grow massive Fe3C layers using a certain composition of the gas mixture consisting of CO, H2, NH3 and N2. With this new developed treatment procedure one can also prevent the often observed sooting/graphite formation at the surface, leading in some cases to disintegration of the metastable Fe3C in alpha-Fe and graphite, which is associated with “metal dusting”. The growth kinetics of such Fe3C surface layer is evaluated and discussed. During nitriding a N concentration gradient due to the inwards diffusion of N from the surface to the bulk builds up within the compound layer. Since, especially, the epsilon phase has a wide homogeneity range for N, this concentration gradient leads to a considerable variation of the lattice parameters with depth. Furthermore, macrostresses may build up within the compound layer during growth due to the concentration gradient and after growth during cooling due to different coefficients of thermal expansion of the layer phases and the substrate. High-resolution X-ray diffraction measurements at different sample tilting angles using synchrotron radiation revealed a pronounced anisotropic diffraction-line broadening of the epsilon reflections. The obtained diffraction patterns are successfully described by a newly developed model with which it is possible to fit the evolution of the (strain-free) lattice parameters with depth as well as a stress-depth profile simultaneously. Analysis of gamma’ layers by X-ray stress measurements using several reflections simultaneously revealed a for fcc-type metals unusual elastic anisotropy of gamma’-Fe4N with <100> as stiffest and <111> as most compliant direction. These results are compared with single-crystal elastic constants obtained by ab-initio calculations and are related to the crystal structure of gamma’-Fe4N in order to get a better understanding of the behaviour of the elastic properties of gamma’. The stresses determined on both layers, epsilon and gamma’, can be understood as thermally induced, whereas the stresses in the gamma’ layer (compressive stresses) are much larger than those present in the epsilon layer, which change from tensile at the surface to compressive at the epsilon/gamma’ interface.Item Open Access Binder-free V2O5 cathode for high energy density rechargeable aluminum-ion batteries(2020) Diem, Achim M.; Fenk, Bernhard; Bill, Joachim; Burghard, ZaklinaNowadays, research on electrochemical storage systems moves into the direction of post-lithium-ion batteries, such as aluminum-ion batteries, and the exploration of suitable materials for such batteries. Vanadium pentoxide (V2O5) is one of the most promising host materials for the intercalation of multivalent ions. Here, we report on the fabrication of a binder-free and self-supporting V2O5 micrometer-thick paper-like electrode material and its use as the cathode for rechargeable aluminum-ion batteries. The electrical conductivity of the cathode was significantly improved by a novel in-situ and self-limiting copper migration approach into the V2O5 structure. This process takes advantage of the dissolution of Cu by the ionic liquid-based electrolyte, as well as the presence of two different accommodation sites in the nanostructured V2O5 available for aluminum-ions and the migrated Cu. Furthermore, the advanced nanostructured cathode delivered a specific discharge capacity of up to ~170 mAh g-1 and the reversible intercalation of Al3+ for more than 500 cycles with a high Coulomb efficiency reaching nearly 100%. The binder-free concept results in an energy density of 74 Wh kg-1, which shows improved energy density in comparison to the so far published V2O5-based cathodes. Our results provide valuable insights for the future design and development of novel binder-free and self-supporting electrodes for rechargeable multivalent metal-ion batteries associating a high energy density, cycling stability, safety and low cost.