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Item Open Access The Bacteroidetes Aequorivita sp. and Kaistella jeonii produce promiscuous esterases with PET-hydrolyzing activity(2022) Zhang, Hongli; Perez-Garcia, Pablo; Dierkes, Robert F.; Applegate, Violetta; Schumacher, Julia; Chibani, Cynthia Maria; Sternagel, Stefanie; Preuss, Lena; Weigert, Sebastian; Schmeisser, Christel; Danso, Dominik; Pleiss, Jürgen; Almeida, Alexandre; Höcker, Birte; Hallam, Steven J.; Schmitz, Ruth A.; Smits, Sander H. J.; Chow, Jennifer; Streit, Wolfgang R.Certain members of the Actinobacteria and Proteobacteria are known to degrade polyethylene terephthalate (PET). Here, we describe the first functional PET-active enzymes from the Bacteroidetes phylum. Using a PETase-specific Hidden-Markov-Model- (HMM-) based search algorithm, we identified several PETase candidates from Flavobacteriaceae and Porphyromonadaceae. Among them, two promiscuous and cold-active esterases derived from Aequorivita sp. (PET27) and Kaistella jeonii (PET30) showed depolymerizing activity on polycaprolactone (PCL), amorphous PET foil and on the polyester polyurethane Impranil® DLN. PET27 is a 37.8 kDa enzyme that released an average of 174.4 nmol terephthalic acid (TPA) after 120 h at 30°C from a 7 mg PET foil platelet in a 200 μl reaction volume, 38-times more than PET30 (37.4 kDa) released under the same conditions. The crystal structure of PET30 without its C-terminal Por-domain (PET30ΔPorC) was solved at 2.1 Å and displays high structural similarity to the IsPETase. PET30 shows a Phe-Met-Tyr substrate binding motif, which seems to be a unique feature, as IsPETase, LCC and PET2 all contain Tyr-Met-Trp binding residues, while PET27 possesses a Phe-Met-Trp motif that is identical to Cut190. Microscopic analyses showed that K. jeonii cells are indeed able to bind on and colonize PET surfaces after a few days of incubation. Homologs of PET27 and PET30 were detected in metagenomes, predominantly aquatic habitats, encompassing a wide range of different global climate zones and suggesting a hitherto unknown influence of this bacterial phylum on man-made polymer degradation.Item Open Access Mechanistic studies on the DNA methyltransferases DNMT3A and DNMT3B(2021) Dukatz, Michael; Jeltsch, Albert (Prof. Dr.)In this work, both regulatory and catalytic mechanisms of de novo methyltransferases were investigated, which include interactions with other proteins and the specific recognition of the substrate sequence. Another part of this work strived to elucidate how enzymatic generation of 3-methylcytosine by DNMT3A can occur.Item Open Access Adjustable polystyrene nanoparticle templates for the production of mesoporous foams and ZnO inverse opals(2020) Abitaev, Karina; Qawasmi, Yaseen; Atanasova, Petia; Dargel, Carina; Bill, Joachim; Hellweg, Thomas; Sottmann, ThomasThe manifold applications of porous materials, such as in storage, separation, and catalysis, have led to an enormous interest in their cost-efficient preparation. A promising strategy to obtain porous materials with adjustable pore size and morphology is to use templates exhibiting the appropriate nanostructure. In this study, close-packed polystyrene (PS) nanoparticles, synthesized by emulsion polymerization, were used to produce porous PS and ZnO inverse opals. The size and distribution of the polystyrene nanoparticles, characterized by dynamic light scattering (DLS), small-angle neutron scattering (SANS), and scanning electron microscopy (SEM), were controlled via the concentration of sodium dodecyl sulfate (SDS). Systematic measurements of the water/styrene-interfacial tension show that the critical micelle concentration (CMC) of the ternary water–styrene–SDS system, which determines whether monodisperse or polydisperse PS particles are obtained, is considerably lower than that of the binary water–SDS system. The assemblies of close-packed PS nanoparticles obtained via drying were then studied by small-angle X-ray scattering (SAXS) and SEM. Both techniques prove that PS nanoparticles synthesized above the CMC result in a significantly unordered but denser packing of the particles. The polystyrene particles were subsequently used to produce porous polystyrene and ZnO inverse opals. While the former consists of micrometer-sized spherical pores surrounded by extended open-cellular regions of mesopores (Rpore ≈ 25 nm), the latter are made of ZnO-nanoparticles forming a structure of well-aligned interconnected pores.Item Open Access Eignung von metallorganischen Gerüstverbindungen als stationäre Phase in der Hochleistungsflüssigchromatographie (HPLC)(2017) Lieder, Christian; Klemm, Elias (Prof. Dr.-Ing.)Anwendung von metallorganischen Gerüstverbindungen als stationäre Phase in der HPLC, Vergleich mit klassischen Silika-Materialien. Synthese der metallorganischen Gerüstverbindungen, Modifizierung. Befüllung chromatographischer Säulen und Gegenüberstellung der Füllmethoden. Methodenentwicklung, Einflüsse auf chromatographische Ergebnisse. Chirale Erkennung, Untersuchung der Wechselwirkungen. Theoretisch chemische Berechnungen der Wechselwirkungen.Item Open Access Synthetic and mechanistic prospects of homogeneous gold catalysis(2009) Pankajakshan, Sreekumar; Hashmi, A. Stephen K. (Prof. Dr.)a) The first chapter of this thesis manifests the exploration of homogeneous gold-catalyzed conversions of furan containing aryl-ynamides and ynol ethers. Enynes boast to be the most explored substrate structures in the realm of homogeneous gold catalysis, whereas the reactivity of ene-ynamides and ene-ynol ethers are much less explored till date.34, 45 Hashmi and co-workers recently reported the homogeneous gold-catalyzed synthesis of phenols from furan containing ynamides/ynol ethers.45e These substrates showed excellent reactivity and selectivity apparently owing to the heteroatom directly attached to the alkyne unit. Impressed with the reactivity and high selectivity of these systems, we decided to explore the catalytic activity of the aryl substituted variants of these compounds. Gold catalysis of aryl-ynamides The aryl-ynamide substrates turned out to be interesting candidates for gold catalysis. The mode of reactivity depended on the tether length. The substrates with two carbon tether underwent a Friedel-Crafts type reaction upon subjected to gold catalysts and furnished benzannulated arenes in moderate to excellent yields (Scheme C). A combination of Ph3PAuCl /AgBF4 (5 mol%, 1:1) in CH2Cl2 or CHCl3 was found to be the best choice of catalyst. The reaction conditions were mild and a broad spectrum of polyarenes and polyheteroarenes were synthesized. The substrates carrying an unactivated arene failed to react even under higher temperature and prolonged reaction time. The gold catalysis of substrates 58r-58t with a longer tether took a completely different pathway and gave rise to cyclopentadiene structures (Scheme D). Similar to shorter tethered substrates, electron withdrawing groups on the arene were not tolerated. b) The second chapter of this thesis deals with the development of gold-catalyzed transformations of furan containing alkynes that prove the potential intermediacy of a cationic intermediate in these types of reactions. The actual electronic state of the intermediate in gold-catalyzed enyne cycloisomerizations is a hot spot in recent scientific reports.20a-c A cyclopropyl "carbene" form of the intermediate has been used by most of the authors. But it is important to note that such a species is yet to be isolated or characterized spectroscopically. Recently Fürstner et al., proposed remarkable experimental evidence for the involvement of the cationic form of this intermediate in gold-catalyzed cyclizations.20a,20c The scenario demands the consideration of the actual electronic state of the intermediate as a mesomeric/tautomeric package of different canonical forms (Scheme H). In the context of this thesis it was found that furyl-alkynes carrying electron donating substituents in the α-position of the tether led to the formation of unexpected five-membered structures (instead of the expected phenols) when subjected to gold catalysts (Scheme I). The observed reactivity pattern for the substrates 116/118 was highly informative and served as an experimental evidence for the significant contribution of the cationic form of the intermediate AO in the mesomeric/tautomeric equilibria shown in the scheme H. A mechanistic proposal based on this intermediate was suggested for this hitherto unknown transformation in the gold catalysis of furyl-alkynes (Scheme J). c) The third chapter of this thesis elaborates the unexpected formation of N,O-acetals from oxanorbornadienes-the potential precursors for phenols- upon exposure to gold and other Lewis acids (Scheme K). AuCl was found to be the best Lewis acid catalyst under the given conditions, and Yb(CF3SO3)3 also showed remarkable reactivity. The generality and the absence of any other products/side products confirmed that the reactions followed general acid catalysis. The observation that there is a net reversal in the connectivity of the tether in the product hinted at a complex mechanism operating in the process. d) The fourth chapter of this thesis describes the investigations carried out on the aerobic oxidation reactions catalyzed by gold. A homogeneous catalytic system comprising of gold(I) chloride, n-butyl lithium and sodium carbonate was developed and was found to oxidize primary aromatic alcohols to aldehydes. The system was poor in activity towards aliphatic and secondary alcohols (Scheme L).Item Open Access Bildung von Bis-(β-chlor-β-alkoxyäthyl)-sulfoxyden durch Addition von Thionylchlorid an Enoläther(1964) Effenberger, Franz; Daub, Jörg-Item Open Access Viskosität von Gemischen aus Ammoniak und Wasser bei 20°C(1986) Pleskov, V. A.; Igamberdyev, I.; Pertschi, Ottmar (Übersetzer)Untersucht wurde die Viskosität des Systems Ammoniak - Wasser bei 20°C. Ermittelt wurde, daß die Viskosität bei einem Wassergehalt von ca. 72 Mol.% ein stark ausgeprägtes Maximum durchläuft.Item Open Access Acid catalyzed cross‐linking of polyvinyl alcohol for humidifier membranes(2021) Michele, Andre; Paschkowski, Patrick; Hänel, Christopher; Tovar, Günter E. M.; Schiestel, Thomas; Southan, AlexanderPolyvinyl alcohol (PVA) is a hydrophilic polymer well known for good film forming properties, high water vapor permeance JW, and low nitrogen permeance. However, depending on molar mass and temperature, PVA swells strongly in water until complete dissolution. This behavior affects the usability of PVA in aqueous environments and makes cross‐linking necessary if higher structural integrity is envisaged. In this work, PVA networks are formed by thermal cross‐linking in the presence of p‐toluenesulfonic acid (TSA) and investigated in a design of experiments approach. Experimental parameters are the cross‐linking period tc, temperature ϑ and the TSA mass fraction wTSA. Cross‐linking is found to proceed via ether bond formation at all reaction conditions. Degradation is promoted especially by a combination of high wTSA, tc and ϑ. Thermal stability of the networks after preparation is strongly improved by neutralizing residual TSA. Humidification membranes with a JW of 6423 ± 63.0 gas permeation units (GPU) are fabricated by coating PVA on polyvinyliden fluoride hollow fibers and cross‐linking with TSA. Summarizing, the present study contributes to a clearer insight into the cross‐linking of PVA in presence of TSA, the thermal stability of the resulting networks and the applicability as selective membrane layers for water vapor transfer.Item Open Access Stereoselective alkylation of aromatic compounds with threonine trifluoromethanesulfonates(1987) Effenberger, Franz; Weber, ThomasAlkyl 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.Item Open Access Stereoselektive Synthesen von α,α'-Iminodicarbonsäuren (Aminosäuren ; 5)(1986) Effenberger, Franz; Burkard, UlrikeDie 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.