Bestellpolitik in Handelsbetrieben — Eine erneute Prämissenkritik

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Spin dynamics for 2d Heisenberg antiferromagnets with combined easy-plane and Dzyaloshinsky interactions

We investigate the spin dynamics of the classical two-dimensional easy-plane Heisenberg antiferromagnet with an additional Dzyaloshinsky interaction, which serves as a simple model for Langmuir-Blodgett films of Mn (C₁₈H₃₅O₂)₂. By mapping the system onto a pure easy-plane model we discuss the corresponding spin wave and vortex dynamics. The additional Dzyaloshinsky interaction forces all spins to cant in a certain direction, which is the same for neighboring spins on different sublattices. This canting causes the presence of a second spin wave peak in the dynamical in-plane correlation function below the Kosterlitz-Thouless transition temperature T_KT and a second vortex central peak above T_KT. Using a vortex gas approach we explicitly calculate the contribution of the free vortices to several dynamical correlation functions. These results are compared to a combined Monte Carlo-Molecular Dynamics simulation on square lattices with different sizes. We also discuss the relevance of this simple model for describing the spin dynamics of Mn (C₁₈H₃₅O₂)₂.     

INTERACTION OF HYDROXYLAMINE WITH THE WATER-OXIDIZING ENZYME INVESTIGATED VIA PROTON RELEASE*

Abstract— Photosynthetic water oxidation is a four-step redox process which is driven by a one-quantum-one-electron reaction center. Stepwise electron Abstract—ion from the water-oxidizing enzyme is accompanied by stepwise proton release with the following stoichiometric pattern at given half-rise times: 1 H⁺ (250 μs, S₀→ S₁):0 H⁺(S₁→ S₂): 1 H⁺ (200 μs, S₂→ S₃): 2 H⁺ (1.2 ms, S₃→ S₄→ S₀). (Förster and Junge, 1985, preceding article in this issue). Hydroxylamine at low concentrations (?100 μ M) appears to compete with water at the active site of the water- oxidizing enzyme. Its interference shifts the dark state of the water-oxidizing enzyme by two steps backwards (Bouges, 1971). We found that the hydroxylamine-induced shift was also reflected in the stoichiometric pattern and in the kinetics of proton-release. In the presence of hydroxylamine, two protons per reaction center were released with a half-rise time of ? 2 ms upon the first exciting flash given to dark adapted thylakoids. This was slower than observed for each of the protons released during unperturbed water oxidation. One proton was released upon the second flash. The half-rise time of the main component observed upon the second flash in hydroxylamine-treated samples agreed with the one observed upon the fourth flash in the absence of hydroxylamine, which had been attributed to the S₀→ S₁ transition. The two protons which were observed upon the first flash in hydroxylamine-treated thylakoids may be due to hydroxylamine oxidation or to the association of water to the catalytic manganese center after detachment of oxidized hydroxylamine from its binding site.     

Metal-free catalysts for the hydrolysis of RNA derived from guanidines, 2-aminopyridines, and 2-aminobenzimidazoles

2-aminopyridine and 2-aminobenzimidazole were chosen as structural analogues to substitute guanidinium groups in receptor molecules designed as phosphoryl transfer catalysts. Shifting the pKa of the guanidinium analogues toward 7 was expected to raise catalytic activities in aqueous buffer. Although the pKa's of both heterocycles are similar (6.2 and 7.0), only 2-aminobenzimidazole led to active RNA cleavers. All cleavage assays were run with fluorescently labeled substrates and a DNA sequencer. RNase contaminations would degrade RNA enantioselectively. In contrast, achiral catalysts such as 9b and 10b necessarily induce identical cleavage patterns in RNA and its mirror image. This principle allowed us to safely rule out contamination effects in this study. The most active catalysts, tris(2-aminobenzimidazoles) 9b and 10b, were shown by fluorescence correlation spectroscopy (FCS) to aggregate with oligonucleotides. However, at very low concentrations the compounds are still active in the nonaggregated state. Conjugates of 10b with antisense oligonucleotides or RNA binding peptides, therefore, will be promising candidates as site specific artificial ribonucleases.     

Mechanistic studies on lanthanum fluoride crystal electrodes

A mechanistic study of the fluoride electrode involving the impedance measurement technique was performed. The influence of the reference electrode, the fluoride ion concentration and the convection on the indicatrix were examined. From the results it followed that the linear equivalent model of the fluoride electrode may be represented by three components namely, a parallel R-C network representing the bulk of the membrane, two double layer capacities one on each side of the sensor, each shunted by a charge transfer resistor and finally, a diffusion impedance.     

Performance and characteristics of the fluoride-selective electrode in a flow injection system

A simple, inexpensive home-made timer for coupling cam-controlled samplers to injectors for flow injection analysis is presented. Experimental conditions and a flow diagram are described for the determination of fluoride by means of a fluoride-selective electrode in the concetration range 0.3–10 ppm, with a sample rate up to 120 h^?1.     

Post-synthesis deposition of V-zeolitic nanoparticles in SBA-15

A post-synthesis deposition of vanadium silicalite-1 zeolite nanoparticles in the pores of SBA-15 results in a highly ordered hexagonal templated silica material with V-silicalite zeolitic plugs, giving rise to an increased crystallinity of the amorphous mesoporous walls.