New guaianolides from Berlandiera Pumila and B. Texana,and the X-ray crystal structure of pumilin

The isolation and structure determination of two new guaianolides, pumilin and its 2,3-epoxide, from Berlandiera pumila and B. texana are reported. The structure of pumilin was determined from nmr and mass spectral data, and from an X-ray single crystal study; the structure of the epoxide was found by comparison of nmr spectra. Pumilin is shown to be a Δ¹⁽¹⁰⁾-cis-guaianolide with a cyclopentenone ring attached to C1 and C5, and an α-methylene-γ-lactone ring trans-fused to C6 and C7. The absolute configuration was inferred from spectral considerations and knowledge of previous structural determinations. The cd spectrum of pumilin exhibits a negative Cotton effect, and the lactone chromophore has left-handed chirality. The cyclo-heptene ring is in the chair configuration, with the lactone a half-chair and the cyclopentenone essentially planar. Crystals are orthorhombic, a = 7.065, b = 13.652, c = 19.586 Å, space group P2₁2₁2₁, Z = 4, and the final R value is 3.9%. The crystal structure is strengthened by an O···O hydrogen bond linking the molecules into infinite chains.     

Saturation of resonant frequency conversion processes in cadmium vapour

We have investigated two-photon resonant up conversion of 375 to 159 nm coherent radiation using the four-frequency process ₁₅₉=2₃₇₅+₁₀₄₀ in cadmium vapour, where the ir component (1040 nm) is internally generated by the 375 nm pump tuned to the Cd two-photon transition 5s ^{2 1} S ₀5s6s ¹ S ₀. Scaling laws and tuning behaviour of both the 159 and 1040 nm output power were measured up to 1×10⁸ W cm^–2 pump intensity and 2×10¹⁷ cm^–3 Cd number density. The results are compared to numerical calculations based on semiclassical theory in a stationary perturbation approximation up to third order. Here, in accordance with experimental results, the ir component was assumed to originate from a superposition of stimulated emission and parametric generation. The observed saturation effects turned out to be mainly due to population changes of the atomic levels involved and are qualitatively reproduced by calculations. Discrepancies with respect to absolute output levels are discussed in terms of the approximations in the theoretical formalism.     

A photochemical route for efficient cyclopeptide formation with a minimum of protection and activation chemistry

An elaborated protocol is described which allows the efficient transformation of di-, tri-, and tetrapeptides into cyclopeptides with a minimum of protection and activation chemistry using the photoinduced electron transfer initiated decarboxylation of N-phthaloyl peptides resulting in C-C coupling between the initially formed carbon radicals.     

Improvements of a combined size exclusion chromatography and solid phase extraction approach for the clean-up of marine sediment samples for trace analysis of pesticides

 Two methods based on gel permeation chromatography (GPC) [size exclusion chromatography] for the analysis of traces (ng/kg) of nitrogen and phosphorus containing pesticides (like triazines or phosphothionates) from marine sediment samples are compared: A macro GPC with Biobeads SX-3 and a chromatography on a high-performance (HP-GPC) column with UV-detection. Results for eight triazine herbicides, two triazine metabolites, the phenylurea derivative linuron, two acetanilides and two organophosphorus compounds are given. Concentrations obtained with the HP-GPC are compared with those obtained with a macro GPC approach in an earlier study. Received: 4 October 1996/Revised: 20 December 1996/Accepted: 24 December 1996     

Current-induced coupling between conductance channels in membranes

The driven system of conducting channels in a nerve membrane is investigated. A current flow generates a coupling between the channels: the current through a channel is influenced by the presence of other conducting channels via the deformation of the equipotential surfaces within the media adjacent to the membrane. We derive an integral equation for the membrane voltageV(s) (s in the membrane plane) and solve it for different membrane conductance distributions(s) including models for stochastic distributions of conducting channels.V(s) is a nonlinear functional of(s). The system of coupled channels is compared with an Ising model. The system exhibits a multi-channel interaction which can be characterized by two different rangesd _int andD ₁. For a mean channel distanced ₀d _int interaction effects are negligible, and ford ₀D ₁ all channel-voltages are equal and thus represent a mean-field for the channels. Increasing conductivity of the medium decreasesd _int and increasesD ₁. With experimental data on sodium channels in nerve membranes we find:d _intd ₀, i.e. a 50% decrease of the channel-voltages by the interaction, andD ₁10³10⁴ d ₀, which indicates mean-field behaviour of the channels. In a subsequent paper we shall treat the statistics of channels which open and close stochastically under the influence of the local membrane voltage.     

Zur Charakterisierung einer Klasse von Hyperbelstrukturen

A characterisation of those (B^*G)-Geometries (in the sense of Benz) is given, which can be constructed with the help of nearfields F, char F 2, in which the multiplicative group F^*(·) has a subgroup A with index [F^*:A]=2.

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Werner Burau zum 70. Geburtstag gewidmet     

UV/Vis Spectroscopic Properties of N -(2′-Hydroxy-4′- N , N -dimethyl-aminobenzylidene)-4-nitroaniline in Various Solvents and Solid Environments

 N-(2′-Hydroxy-4′-N,N-dimethylaminobenzylidene)-4-nitroaniline [HDBN] has been used as a model for investigating intra- and intermolecular D–A (donor–acceptor) interactions in various environments by means of UV/Vis spectroscopy. UV/Vis spectra of HDBN have been measured in various solvents, ethanolic solutions of different pH, adsorbed on silica, and in the solid state. A bathochromic shift of ν_max is observed with increasing the dipolarity/polarizability and HBD (hydrogen bond donor) capacity of the solvent, which is described by means of a multiple LSE (linear solvation energy) relationship in terms of the empirical Kamlet-Taft solvent polarity parameters. The adsorption of HDBN on Aerosil^? 300-silica particles in non-HBA (hydrogen bond acceptor) solvents is explained in the same sense. Mobile protons and sol–gel entrapping cause a hypsochromic shift due to protonation of the lone electron pair of the 4′-N,N-dimethylamino group. Hydroxide ions attack the 2′-hydroxy group which causes a bathochromic shift. A strong intramolecular hydrogen bond between the 2′-hydroxyl hydrogen and the imine nitrogen atom is present in the solid-state structure causing an unprecedented bathochromic shift.