03 Fakultät Chemie

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    Stereoselective alkylation of aromatic compounds with threonine trifluoromethanesulfonates
    (1987) Effenberger, Franz; Weber, Thomas
    Alkyl trifluoromethanesulfonates (alkyl triflates) have only been used in special cases for the alkylation of aromatic compounds. We have now succeeded in synthesizing the N-protected triflates of (S)- and (R)-serine methyl ester as well as the N-protected triflates 1 of all diastereomeric threonine methyl esters in very good yields and with complete retention of configuration.
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    Stereoselektive Synthesen von α,α'-Iminodicarbonsäuren (Aminosäuren ; 5)
    (1986) Effenberger, Franz; Burkard, Ulrike
    Die Trifluormethansulfonate 1, 11 bzw. 26 von enantiomerenreinen Milchsäureestern, β-Phenylmilchsäureestern bzw. α-Hydroxyglutarsäure-dimethylester (25) reagieren mit (S)-bzw. (R)-α-Aminosäureestern 2, 5, 8, 14, 17, 20 bzw. 23 diastereoselektiv zu denα,α'-Iminodicarbonsäureestern 3, 6, 9, 12, 15, 16, 18, 21 bzw. 24, die sich ohne Schwierigkeiten zu den entsprechenden α,α'-Iminodicarbonsäuren 4, 7, 10, 13, 19 bzw. 22 verseifen lassen. Verantwortlich für die hohe Diastereomerenausbeute bei diesen Reaktionen ist die große Austrittstendenz des Trifluormethansulfonat-Ions, das auch bei tiefen Temperaturen den Austausch mit den relativ schwach nucleophilen α-Aminosäureestern nach einem reinen SN2-Mechanismus ermöglicht.
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    Designing covalent organic framework‐based light‐driven microswimmers toward therapeutic applications
    (2023) Sridhar, Varun; Yildiz, Erdost; Rodríguez‐Camargo, Andrés; Lyu, Xianglong; Yao, Liang; Wrede, Paul; Aghakhani, Amirreza; Akolpoglu, Birgul M.; Podjaski, Filip; Lotsch, Bettina V.; Sitti, Metin
    While micromachines with tailored functionalities enable therapeutic applications in biological environments, their controlled motion and targeted drug delivery in biological media require sophisticated designs for practical applications. Covalent organic frameworks (COFs), a new generation of crystalline and nanoporous polymers, offer new perspectives for light‐driven microswimmers in heterogeneous biological environments including intraocular fluids, thus setting the stage for biomedical applications such as retinal drug delivery. Two different types of COFs, uniformly spherical TABP‐PDA‐COF sub‐micrometer particles and texturally nanoporous, micrometer‐sized TpAzo‐COF particles are described and compared as light‐driven microrobots. They can be used as highly efficient visible‐light‐driven drug carriers in aqueous ionic and cellular media. Their absorption ranging down to red light enables phototaxis even in deeper and viscous biological media, while the organic nature of COFs ensures their biocompatibility. Their inherently porous structures with ≈2.6  and ≈3.4 nm pores, and large surface areas allow for targeted and efficient drug loading even for insoluble drugs, which can be released on demand. Additionally, indocyanine green (ICG) dye loading in the pores enables photoacoustic imaging, optical coherence tomography, and hyperthermia in operando conditions. This real‐time visualization of the drug‐loaded COF microswimmers enables unique insights into the action of photoactive porous drug carriers for therapeutic applications.
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    Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts
    (2024) Ebert, Max; Lange, Alyna; Müller, Michael; Wuckert, Eugen; Gießelmann, Frank; Klamroth, Tillmann; Zens, Anna; Taubert, Andreas; Laschat, Sabine
    Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.
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    The donor strength of dialkylamino functions : a systematic study of delta H /HMO pi-electron density correlations in aminobenzenes
    (1978) Effenberger, Franz; Fischer, Peter; Schöller, Wolfgang W.; Stohrer, Wolf-Dieter
    From the resonance interaction between different NR2 substituents and the arylic π-system in mono-,1, 3-bis- and 1,3,5-tris(dialkylamino)benzenes, quantitative parameters are derived for the relative donor strength of the pyrrolidino, dimethylamino, piperidino and morpholino group. Towards an uncharged π-system in the ground state, the donor potential decreases in the series Pyr>N(CH3)2>Pip>Mor. The same order, though with somewhat different gradation, is observed for the aminobenzene/trinitrobenzene charge transfer complex absorptions, and for the polarographic oxidation potentials. The detailed analysis of the chemical shift/π-charge density correlations for methoxy and dialkylamino benzenes also reveals that these substituents exert a significant deshielding effect on protons in ortho-position. This additional downfield shift is probably due to steric interactions and strongly increases from the pyrrolidino to the piperidino group.
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    Confirmation of siderazot, Fe3N1.33, the only terrestrial nitride mineral
    (2021) Bette, Sebastian; Theye, Thomas; Bernhardt, Heinz-Jürgen; Clark, William P.; Niewa, Rainer
    Siderazot, the only terrestrial nitride mineral, was reported only once in 1876 to occur as coating on volcanic rocks in a fumarolic environment from Mt. Etna and, to date, has been neither confirmed nor structurally characterized. We have studied the holotype sample from the Natural History Museum, London, UK, originally collected by O. Silvestri in 1874, and present siderazot with epsilon-Fe3N-type crystal structure and composition of Fe3N1.33(7) according to crystal structure Rietveld refinements, in good agreement with electron microprobe analyses. Crystal structure data, chemical composition, and Raman and reflectance measurements are reported. Possible formation conditions are derived from composition and phase stability data according to synthetic samples.
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    Regulation of the catalytic activity and specificity of DNA nucleotide methyltransferase 1
    (2014) Bashtrykov, Pavel; Jeltsch, Albert (Prof. Dr.)
    DNA nucleotide methyltransferase 1 (Dnmt1) is mainly responsible for the maintenance of DNA methylation in mammals and plays a crucial role in the epigenetic control of gene expression. Dnmt1 recognizes and methylates hemimethylated CpG sites formed during DNA replication. In the present work, the mechanistic details of the substrate recognition by the catalytic domain of Dnmt1, the possible role of the CXXC and RFTS domains of Dnmt1 in the regulation of specificity and activity of Dnmt1, and the influence of the Ubiquitin-like PHD and RING finger domain-containing 1 (Uhrf1) protein on the enzymatic properties of Dnmt1 was investigated. Using modified substrates, the functional roles of individual contacts of the Dnmt1 catalytic domain with the CpG site of the DNA substrate were analysed. The data show that the interaction with the 5-methylcytosine:guanine pair is required for the catalytic activity of Dnmt1, whereas the contacts to the non-target strand guanine are not important, since its replacement with adenine increased the activity of Dnmt1. It was proposed that the CXXC domain binding to unmethylated CpG sites increases the specificity of Dnmt1 for hemimethylated DNA. Our data showed that the CXXC domain does not influence the enzyme’s specificity in the full-length Dnmt1. In contrast, mutagenesis in the catalytic domain introducing an M1235S exchange resulted in a significant reduction in specificity. Therefore, the readout for the hemimethylated DNA occurs within its catalytic domain. It was observed in a crystal structure that the RFTS domain of Dnmt1 inhibits the activity of the enzyme by binding to the catalytic domain and blocking the entry of the DNA. By amino acid substitution in the RFTS domain its positioning within the catalytic domain was destabilized and a corresponding increase in the catalytic rate was observed, which supports this concept and suggests a possible mechanism to allosterically regulate the activity of Dnmt1 in cells. Uhrf1 has been shown to target Dnmt1 to replicated DNA, which is essential for DNA methylation. Here it is demonstrated that Uhrf1 as well as its isolated SRA domain increase the activity and specificity of Dnmt1 in an allosteric mechanism. The stimulatory effect was independent of the SRA domain’s ability to bind hemimethylated DNA. The RFTS domain of Dnmt1 is required for the stimulation, since its deletion or blocking of its interaction with the SRA domain, significantly reduced the ability of Uhrf1 to increase the activity and specificity of Dnmt1. Uhrf1, therefore, plays multiple roles that support DNA methylation including targeting of Dnmt1, its stimulation and an increase of its specificity.
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    Highly active cooperative Lewis acid : ammonium salt catalyst for the enantioselective hydroboration of ketones
    (2021) Titze, Marvin; Heitkämper, Juliane; Junge, Thorsten; Kästner, Johannes; Peters, René
    Enantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5-3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working.
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