Katalysatoraktivitāt von Iridiumkomplexen bei der homogenen Hydrierung ungesāttigter Verbindungen in Substanz als Funktion der Typen der Komplexe und Substrate.

Catalytic activity of iridum complexes IrClCO[P(C₆H₁₁)₃]₂, IrCl[PPh₃]₃, IrHCO[PPh₃]₃ and IrH₃[PPh₃]₃ in homogeneous hydrogenation of 17 typical unsaturated substrates was investigated. Experiments were carried out between 95 and 150 °C, 10 and 15 bar and without solvent. Optimal catalyst activity regarding hydrogenation of C  C bonds depends on the type of the catalyst and the type of the substrate. Under proper choice of reaction conditions activities a (mmol product/mmol cat-min) of 100 to 700 could be adjusted. The C  O bond is only hydrogenated by catalyst IrH₃[PPh₃]₃ with moderate activity.     

Synthetic Lugdunin Analogues Reveal Essential Structural Motifs for Antimicrobial Action and Proton Translocation Capability

Lugdunin, a novel thiazolidine cyclopeptide, exhibits micromolar activity against methicillin‐resistant Staphylococcus aureus (MRSA). For structure–activity relationship (SAR) studies, synthetic analogues obtained from alanine and stereo scanning as well as peptides with modified thiazolidine rings were tested for antimicrobial activity. The thiazolidine ring and the alternating d ‐ and l ‐amino acid backbone are essential. Notably, the non‐natural enantiomer displays equal activity, thus indicating the absence of a chiral target. The antibacterial activity strongly correlates with dissipation of the membrane potential in S. aureus. Lugdunin equalizes pH gradients in artificial membrane vesicles, thereby maintaining membrane integrity, which demonstrates that proton translocation is the mode of action (MoA). The incorporation of extra tryptophan or propargyl moieties further expands the diversity of this class of thiazolidine cyclopeptides.     

Stochastic Nonlinear Equations of Schrödinger Type

In this article, the solution for a stochastic nonlinear equation of Schrödinger type, which is perturbed by an infinite dimensional Wiener process, is investigated. The existence of the solution is proved by using the Galerkin method. Moment estimates for the solution are also derived. Examples from physics are given in the final part of the article.     

Reactivity of activated versus nonactivated 2-(bromomethyl)aziridines with respect to sodium methoxide: a combined computational and experimental study

The difference in reactivity between the activated 2-bromomethyl-1-tosylaziridine and the nonactivated 1-benzyl-2-(bromomethyl)aziridine with respect to sodium methoxide was analyzed by means of DFT calculations within the supermolecule approach, taking into account explicit solvent molecules. In addition, the reactivity of epibromohydrin with regard to sodium methoxide was assessed as well. The barriers for direct displacement of bromide by methoxide in methanol are comparable for all three heterocyclic species under study. However, ring opening was found to be only feasible for the epoxide and the activated aziridine, and not for the nonactivated aziridine. According to these computational analyses, the synthesis of chiral 2-substituted 1-tosylaziridines can take place with inversion (through ring opening/ring closure) or retention (through direct bromide displacement) of configuration upon treatment of the corresponding 2-(bromomethyl)aziridines with 1 equiv of a nucleophile, whereas chiral 2-substituted 1-benzylaziridines are selectively obtained with retention of configuration (via direct bromide displacement). Furthermore, the computational results showed that explicit accounting for solvent molecules is required to describe the free energy profile correctly. To verify the computational findings experimentally, chiral 1-benzyl-2-(bromomethyl)aziridines and 2-bromomethyl-1-tosylaziridines were treated with sodium methoxide in methanol. The presented work concerning the reactivity of 2-bromomethyl-1-tosylaziridine stands in contrast to the behavior of the corresponding 1-tosyl-2-(tosyloxymethyl)aziridine with respect to nucleophiles, which undergoes a clean ring-opening/ring-closure process with inversion of configuration at the asymmetric aziridine carbon atom.     

Blood group B galactosyltransferase: insights into substrate binding from NMR experiments

The biosynthesis of human blood group B antigens is accomplished by a highly specific galactosyltransferase (GTB). On the basis of NMR experiments, we propose a "molecular tweezers mechanism" that accounts for the exquisite stereoselectivity of donor substrate selection. Transferred NOE experiments for the first time reveal the bioactive conformation of the donor substrate UDP-galactose (UDP-Gal) and of its enzymatically inactive analogue, UDP-glucose (UDP-Glc). Both bind to GTB in a folded conformation that is sparsely populated in solution, whereas acceptor ligands bind in a conformation that predominates in solution. The bound conformations of UDP-Gal and UDP-Glc are identical within experimental error. Therefore, GTB must discriminate between the two activated sugars on the basis of a hitherto unknown transition state that can only be formed in the case of UDP-Gal. A full relaxation and exchange matrix analysis of STD NMR experiments reveals that acceptor substrates dissociate significantly faster (k(off) > 100 Hz) from the binding pocket than donor substrates (k(off) approximately 10 Hz). STD NMR experiments also directly show that proper recognition of the hexopyranose rings of the UDP sugars requires bivalent metal cations. At the same time, this analysis furnishes the complete three-dimensional structure of the enzyme with its bound donor substrate UDP-Gal on the basis of a prior crystal structure analysis. We propose that, upon acceptor binding, GTB uses the Asp 302 and Glu 303 side chains as "molecular tweezers" to promote bound UDP-Gal but not UDP-Glc into a transition state that leads to product formation.     

Stickstoff- und Sauerstoffbestimmung in Uranmononitrid

Zusammenfassung Es wurden UN-Proben nach einer naßchemischen Methode (Lösen in HCl mit H₂SiF₆) nach Kjeldahl und einem modifizierten Vakuumheißextraktionsverfahren auf ihren Stickstoffgehalt analysiert. Die nach beiden Methoden gefundenen Stickstoff-Werte zeigen eine gute Übereinstimmung, die Standardabweichungen beider Verfahren sind gering. Das BMI-Verfahren (oxydierendes Lösen mit Kupferselenat) ergibt in manchen Fällen zu geringe Stickstoffwerte.Bei der Heißextraktion werden gleichzeitig die Sauerstoffgehalte mit befriedigender Genauigkeit erhalten.

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Summary UN-samples were analyzed with a wet-chemical method (dissolving in HCl with H₂SiF₆) according to Kjeldaiil and with a modified vacuum-fusion method for their nitrogen content.The nitrogen values found by both methods show good agreement, the standard variations of both methods are small. In some cases the BMI-method (oxidizing dissolution with copper selenate) shows too low nitrogen values. In the case of vacuum-fusion method the oxygen values are found at the same time with satisfying precision.

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Wir danken Herrn Dr. H. Wedemeyer für die Herstellung zahlreicher Urannitridproben.