The "azido gauche effect"-implications for the conformation of azidoprolines

The "azido gauche effect" was examined both experimentally and theoretically and was found to determine the conformation of, for example, (4R)- and (4S)-azidoproline (Azp) derivatives. For (4R)Azp derivatives, the azido gauche effect induces a preferred C(4)-exo conformation of the pyrrolidine ring, which leads to stabilization of the s-trans amide conformer of, e.g., Ac-(4R)Azp-OCH(3) (5R) via an n-->pi interaction between the nonbonding electrons of the oxygen of the acetyl group and the carbonyl group of the ester. For (4S)Azp derivatives, the azido gauche effect results in a C(4)-endo conformation of the pyrrolidine ring that does not allow for this stabilizing n-->pi interaction of the s-trans conformer. Consequently, a significantly higher s-trans:s-cis amide conformer ratio is observed for (4R)Azp compared to (4S)Azp derivatives (e.g., 6.1:1 versus 2.6:1 in D(2)O for Ac-(4R)Azp-OCH(3) (5R) compared to Ac-(4S)Azp-OCH(3) (5S)). These conformational preferences are reflected in the higher tendency of (4S)Azp-containing peptides to form cyclic peptides with all-cis amide bonds compared to (4R)Azp derivatives. Ab initio calculations demonstrate that the strength of the azido gauche effect is comparable to that of the well-known "fluorine gauche effect". For azidoethane derivatives N(3)-CH(2)CH(2)-X (X = N(3), NHCOH, NHAc, or N(CH(3))Ac), the ab initio calculations revealed energy differences of 5-13 kJ mol(-)(1) between the anti and gauche conformations in favor of the gauche conformer. Calculations were also performed for the (4R)Azp and (4S)Azp derivatives 5R and 5S, supporting the experimentally observed data.     

The interaction of cationic polymers with human hair

We present a combination of electrochemical methods, i.e. polyelectrolyte titration and streaming potential measurement, and AFM to characterise the adsorption and desorption behaviour and the morphology of a set of polyquaternium polymers on human hair. Specific charge density and molar mass are correlated to the amount of adsorbed polymer and the ease of desorption. Results are in line with a simple model of coulombic interaction between hair and polymer and are interpreted on this basis.     

Hydrophobin can prevent secondary protein adsorption on hydrophobic substrates without exchange

By combining several surface analytical tools, we show that an adsorbed layer of the protein H*Protein B prevents the adsorption of secondary proteins bovine serum albumin, casein, or collagen at low-salinity conditions and at pH 8. H*Protein B is an industrially producible fusion protein of the hydrophobin family, known for its high interfacial activity. While applications of hydrophobin have been reported to facilitate adhesion of proteins under different pH conditions, careful analysis by quartz-crystal microbalance and ellipsometry prove that no additional adsorption can be found on top of the H*Protein B layer in this study. Surface analysis by X-ray photoelectron spectroscopy and secondary ion mass spectrometry proves that the hydrophobin layer stays intact even after hours of exposure to solutions of the secondary proteins and that no exchange of proteins can be detected.     

Synthesis and Thin Film Phase Behaviour of Functional Rod‐Coil Block Copolymers Based on Poly(para‐phenylenevinylene) and Poly(lactic acid)

We report the synthesis of a series of block copolymers consisting of a rod‐like semiconducting poly(2,5‐di(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (DEH‐PPV) block and a flexible poly(lactic acid) (PLA) block that can be selectively degraded under mild conditions. Such selectively degradable block copolymers are designed as self‐assembling templates for bulk heterojunction donor–acceptor layers in organic solar cells. A lamellar microphase‐separated domain structure was identified for block copolymers with PLA volume fractions between 29 and 79% in bulk and thin films using SAXS, TEM, and AFM. Depending on the ratio of the two blocks we find either lamellae oriented parallel or perpendicular to the substrate in thin films.

     

Metal-free direct arylations of indoles and pyrroles with diaryliodonium salts

Direct arylations of indoles and pyrroles with differently substituted diaryliodonium salts were shown to efficiently proceed in the absence of metal catalysts.     

Adsorption behavior of 4-methoxypyridine on gold nanoparticles

We demonstrate a phase transfer method to create stable colloidal solutions of Au nanoparticles with 4-methoxypyridine ligands. We then investigate the adsorption behavior of 4-methoxypyridine onto gold surfaces by Raman spectroscopy, DFT calculations, and (1)H NMR. In contrast to unsubstituted pyridine and the frequently used (N,N-dimethylamino)pyridine (DMAP), a flat adsorption of 4-methoxypyridine on gold was found.     

Solid‐state NMR as a tool to describe and quantify the morphology of photoactive layers used in plastic solar cells

The optimization and control of the nanomorphology of thin films used as active layer in bulk heterojunction (BHJ) plastic solar cells is of key importance for a better understanding of the photovoltaic mechanisms and for increasing the device performances. Hereto, solid‐state NMR relaxation experiments have been evaluated to describe the film morphology of one of the “work‐horse” systems poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene‐vinylene)/[6, 6]‐phenyl‐C₆₁butyric acid methyl ester (MDMO‐PPV/PCBM) in a quantitative way. Attention is focused on the influence of the processing solvent (toluene vs. chlorobenzene), the blend composition, and the casting technique, that is, spin coating versus doctor blading. It is demonstrated that independently of the solvent and casting technique, part of the PCBM becomes phase separated from the mixed phase. Whereas casting from toluene results in the development of well‐defined PCBM crystallites, casting from chlorobenzene leads to the formation of PCBM‐rich domains that contain substructures of weakly organized PCBM nanoclusters. The amount and physico‐chemical state of the phase separated PCBM is quantified by solid‐state NMR relaxation times experiments. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

THE PECULIAR α-AMIDOALKYLATION OF VINYLTRIMETHYLSILANE

Abstract

α-Amidoalkylation of allyttrimethylsilane with methyl-2-chloro-2-(p-chlorobenzoylamino)-ethanoate 1 gave in the presence of Lewis acid racemic methyl-2- (p-chlorobenzoylamino)-4-penteno ate 2. Under the same conditions, vinlytrimethylsilane afforded (±)-trans-2-(p-chlorophenyl)-5,6-dihydro-4-methoxycarbonyl-6-trimethylsilyl-4H-1, 3-oxazine 4 as the major, by n.m.r. data and x-ray crystallography established product.     

Two Years of Pesticides Monitoring in a Belgian Watershed

A monitoring study was carried out in 1998-1999 on a medium-sized (c. 4580 km²) watershed of the Dyle (or Dijle) river in central Belgium, composed of both rural and urbanized areas. This watershed may be considered as representative of a large part of the country. Samples were taken each month from seven sites along the course of the river, plus one from its major affluent the Demer. The eight molecules monitored were the main herbicides used on the major crops in the area, plus some used in nonagricultural sectors. The concentrations found were mostly in the 0-3 μg/L range, with some peaks between 3 and 14 μg/L. The averages over all spring and summer samples analyzed ranged from 0.14 (bentazone) to 1.54 μg/L (diuron). Time and space specific patterns could be observed.     

Synthesis of an unsymmetrically doubly bridged calix[4]arene in the 1,3-alternate conformation

We report the synthesis of the first calix[4]arene constrained to a 1,3-alternate conformation by one crown ether and one di-aza-benzo crown ether bridgings. Preliminary binding properties are also given.