Polymer-protein conjugates from omega-aldehyde endfunctional poly(N-vinylpyrrolidone) synthesised via xanthate-mediated living radical polymerisation

Aldehyde omega-endfunctional poly(N-vinylpyrrolidone) was synthesised via quantitative conversion of a xanthate endfunctional precursor obtained via RAFT-mediated polymerisation.     

Ruphos-mediated Suzuki cross-coupling of secondary alkyl trifluoroborates

A Ruphos-mediated Suzuki cross-coupling between (hetero)aryl bromides and secondary alkyltrifluoroborates is described using palladium catalysis. The Ruphos ligand showed superior properties as compared to S-Phos in this type of reaction. This method constitutes a valuable extension to current methods for the straightforward production of secondary-alkylated (hetero)aryl derivatives.     

Solid-state NMR spectroscopy applied to a chimeric potassium channel in lipid bilayers

We show that solid-state NMR can be used to investigate the structure and dynamics of a chimeric potassium channel, KcsA-Kv1.3, in lipid bilayers. Sequential resonance assignments were obtained using a combination of (15)N- (13)C and (13)C- (13)C correlation experiments conducted on fully labeled and reverse-labeled as well as C-terminally truncated samples. Comparison of our results with those from X-ray crystallography and solution-state NMR in micelles on the closely related KcsA K (+) channel provides insight into the mechanism of ion channel selectivity and underlines the important role of the lipid environment for membrane protein structure and function.     

Moduli spaces of quadratic complexes and their singular surfaces

We construct the coarse moduli space of quadratic line complexes with a fixed Segre symbol σ as well as the moduli space of the corresponding singular surfaces. We show that the map associating to a quadratic line complex its singular surface induces a morphism . Finally we deduce that the varieties of cosingular quadratic line complexes are almost always curves.      

Identification of sulfoglycolipids from the alga Porphyridium purpureum by matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight mass spectrometry

Sulfoglycolipids, isolated from different phototrophic organisms, particularly plants and algae, have already been identified as bioactive compounds. In addition to their antiviral activity their influence on the immune response in mammalian cells is the focus of many studies. For the first time it has been possible to investigate purified sulfoquinovosyldiacylglycerols (SQDGs) from the microalga Porphyridium purpureum by matrix-assisted laser desorption/ionisation (MALDI) in the negative ion reflectron mode. Thereby, different solid and ionic liquid matrices have been tested to improve signal intensity during the laser ionisation. By using the MALDI Trap time-of-flight (ToF) multiple-stage (MS(n)) hybrid mass spectrometer the fatty acid compositions of the SQDGs were analysed by MS, and confirmed by MS(2) and MS(3) experiments. Thereby, hexadecanoic acid (C16:0), octadecadienoic acid (C18:2), eicosatetraenoic acid (C20:4), and eicosapentaenoic acid (C20:5) were detected in the purified fraction of SQDGs. The localisation of hexadecanoic acid (C16:0) at the sn-2 position, and unsaturated fatty acids at the sn-1 position of the SQDGs, determined by specific enzymatic hydrolysis, marks a procaryotic biosynthesis of SQDGs in the eucaryotic alga cells.     

Langmuir and Langmuir-Blodgett monolayers of molecular tweezers and clips

Molecular clips and tweezers are able to selectively bind electron-deficient aromatic and aliphatic substrates. By means of pressure-area isotherms and Brewster angle microscopy (BAM), the self-association process and phase behavior of dimethylene-bridged molecular clips and tetramethylene-bridged molecular tweezers each substituted with two acetoxy groups as polar head groups were investigated. In a series of experiments, we observed that the molecular surface area of the clips and tweezers only depended on the skeletal structure and not on the polar groups. The measured areas agreed with the effective molecular diameters of the molecules if the aromatic side walls of the clips or tweezers were assumed to be aligned perpendicularly to the water surface. We compared the phase behavior of the pure molecular clips and tweezers with that of the host-guest complexes of these molecules, which were formed with 1,2,4,5-tetracyanobenzene (TCNB) as the guest molecule. For the clips with a central benzene (I) and naphthalene spacer unit (II), the complex formation with TCNB had no measurable influence on the phase diagrams of the films. We observed, however, a dramatic difference in the BAM images and pi-A isotherms between the pure molecular tweezers III and its complex with TCNB (TCNB@III). In addition to the pi-A isotherms, we used the surface potential (V)-area (A) isotherms to compare the pure tweezers III with the corresponding complex (TCNB@III). There was a strong difference in the maximum surface potential value for the pure tweezers (450 mV) and that for the complex (300 mV). In additional experiments, we prepared LB layers of such molecules, which were investigated by fluorescence spectroscopy. In comparison to the pure tweezers III, a luminescence emission of charge-transfer (CT) origin was observed for the host-guest complex (TCNB@III) fixed on the solid substrate. It turned out that the spectra were in good agreement with the results observed in chloroform solution.     

A Novel Method for Analysis of Xanthate Group Distribution in Viscoses

Analytical monitoring of xanthation in the viscose process along with xanthate group analysis in the viscose material is a long-debated problem in cellulose chemistry. The task is rendered extremely intricate by the lability of the starting material and the harshness of the reaction medium, which adds to a lack of suitable analytical approaches. In a four-years' endeavor in our lab, a method is being developed which allows to analyze the distribution of xanthate groups in viscoses relative to the anhydroglucose units and along the cellulose chain. In a first step the xanthate groups are stabilized by alkylation, which was optimized towards quantitative conversion. In a second step, the remaining free hydroxyl groups are protected by carbanilation, followed by selective removal of the stabilized xanthate groups. Steps two and three thus generate an inverse image of the initial xanthate pattern. In the forth and fifth step, the liberated hydroxyl groups are methylated, and the carbanilates are removed, so that in the overall process the xanthates were replaced by methyl groups. All reaction steps have been comprehensively tested with regard to completeness of conversion and orthogonality of the protecting groups.     

Numerical simulations of the nonlinear propagation of femtosecond optical pulses in gases

A two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations. Received: 19 June 1998 / Received in final form: 14 January 1999