Mechanism of multi‐metal(loid) methylation and hydride generation by methylcobalamin and cob(I)alamin: a side reaction of methanogenesis

Metal(loid)s are subject to many transformation processes in the environment, such as oxidation, reduction, methylation and hydride generation, predominantly accomplished by prokaryotes. Since these widespread processes affect the bioavailability and toxicity of metal(loid)s to a large extent, the investigation of their formation is of high relevance. Methanogenic Archaea are capable of methylating and hydrogenating Group 15 and 16 metal(loid)s arsenic, selenium, antimony, tellurium, and bismuth due to side reactions between central methanogenic cofactors, methylcobalamin (CH₃Cob(III)) and cob(I)alamin (Cob(I)). Here, we present systematic mechanistic studies on methylation and hydride generation of Group 15 and 16 metal(loid)s by CH₃Cob(III) and Cob(I). Pentavalent arsenical species showed neither methylation nor reduction as determined by using a newly developed oxidation state specific hydride generation technique, which allows direct determination of tri‐ and pentavalent arsenic species in a single batch. In contrast, efficient methylation of trivalent species without a change in oxidation state indicated that the methyl transfer does not proceed via a Challenger‐like oxidative methylation, but via a non‐oxidative methylation. Our findings also point towards a similar mechanism for antimony, bismuth, selenium, and tellurium. Overall, we suggest that the transfer of a methyl group does not involve a free reactive species, such as a radical, but instead is transferred either in a concerted nucleophilic substitution or in a caged radical mechanism. For hydride generation, we propose the intermediate formation of hydridocobalamin, transferring a hydride ion to the metal(loid)s. Copyright © 2012 John Wiley & Sons, Ltd.     

Improving charge transport in poly(3‐hexylthiophene) transistors via blending with an alkyl‐substituted phenylene–thiophene–thiophene–phenylene molecule

A prototypical semiconducting bicomponent system consisting of a conjugated polymer, that is, poly(3‐hexylthiophene) (P3HT), blended with a small thiophene containing conjugated molecule, that is, an alkyl‐substituted bisphenyl‐bithiophene [phenylene–thiophene–thiophene–phenylene (PTTP)], has been used as an electroactive active layer in field‐effect transistors (FETs). The self‐assembly of this bicomponent system at surfaces has been studied at different length scales, from the nanoscale to the macroscale, and compared with the behavior of monocomponent films of PTTP and P3HT. The correlation between morphology and electric properties of the semiconducting material is explored by fabricating prototypes of FETs varying the relative concentrations of the two‐component blend. The maximum charge carrier mobility value, achieved with a few percent of PTTP component, is not simply due to a uniform dispersion of the molecules in the polymer matrix, but rather to the generation of very long percolation paths, whose composition and electrical properties can be tuned with the PTTP concentration. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Semiclassical matrix model for quantum chaotic transport with time-reversal symmetry

We show that the semiclassical approach to chaotic quantum transport in the presence of time-reversal symmetry can be described by a matrix model. In other words, we construct a matrix integral whose perturbative expansion satisfies the semiclassical diagrammatic rules for the calculation of transport statistics. One of the virtues of this approach is that it leads very naturally to the semiclassical derivation of universal predictions from random matrix theory.     

TELOMERISATION PAR CATALYSE REDOX IX ADDITION DE TELOGENES PHOSPHORES SUR DES COMPOSES POSSEDANT DEUX DOUBLES LIAISONS NON CONJUGUEES

Abstract

The synthesis of telechelic oligomers with phosphorated end groups was realized by redox catalysis telomerization reaction by addition of phosphorated telogens containing a trichloromethyl group on monomers with two non-conjugated double bonds.

Nous effectuons la synthèse d'oligoméres téléchéliques à extrémités phosphorées en additionnant au moyen de la réaction de télomerisation par catalyse Redox des télogènes phosphorés renfernant le groupement trichlorométhyle, sur des monomères à deux doubles liaisons non conjuguées.     

Crosslinked Networks Based on Polysaccharides and Collagen for Pilocarpine Sustained Release

The paper presents the experimental studies regarding synthesis and characterization of hydrogels based on gellan (Gel)/chitosan (CS) and collagen (Col), obtained by crosslinking with glutaraldehyde (GLA). The influence of the polysaccharide content and GLA ratio on the final composition and swelling characteristics was evaluated. Hydrogels swelling analysis, in distilled water and phosphate buffer (PBS, pH 7.2) has shown higher swelling degrees at increased concentration of polysaccharide into hydrogels. In vitro release of pilocarpine has demonstrated the possibility to use gellan-collagen and chitosan-collagen hydrogels as ophthalmic drug delivery matrix.     

Bioactive Polymers. XXVI. Immobilization of Pepsin on Biozan R

Pepsin was immobilized on BIOZAN R (Hercules) with dicyclohexylcarbodiimide as activator. The reaction product obtained has a protein content of 35–110 mg/g of polymer and a proteolytic activity between 20.85–28.75 μmol tyrosine/L·min·g of polymer). The coupling reaction rate is maximum under the following conditions: pepsin/BIOZAN R ratio = 0.52 g/g, DCCI/BIOZAN R ratio = 0.2 g/g, pH = 3.4, reaction time = 4 h.     

Non‐symmetric bent‐core mesogens with one terminal vinyl group

Two series of non‐symmetric banana‐shaped compounds, both with one alkyl and one alkenyl terminal tail, have been synthesized and studied. Both series were compared with the corresponding series with two saturated terminal alkyl tails. All the compounds have a bent central 1,3‐phenylene bis(4‐benzoyloxy)benzoate core; their mesophases were characterized by polarizing optical microscopy, differential scanning calorimetry, X‐ray diffraction and switching current response experiments. In all four series one of the terminal tails is varied from OC₈H₁₇ to OC₁₆H₃₃. The other terminal tails are OC₁₁H₂₃, O(CH₂)₉CH?=?CH₂, OC₁₀H₂₁ and O(CH₂)₈CH?=?CH₂. The short‐tailed compounds show monotropic or enantiotropic B₁ phases and the long‐tailed compounds the B₂ phase. The introduction of one terminal vinyl group slightly lowers the transition temperatures. The introduction of a second terminal vinyl group further suppresses the liquid crystalline properties. All compounds with B₂ phases have layer spacings that suggest a tilt of ～45° of the bent molecules in the layers, and their switching behaviour is antiferroelectric.     

A Designed 5‐Fluorouracil‐Based Bridged Silsesquioxane as an Autonomous Acid‐Triggered Drug‐Delivery System

Two new prodrugs, bearing two and three 5‐fluorouracil (5‐FU) units, respectively, have been synthesized and were shown to efficiently treat human breast cancer cells. In addition to 5‐FU, they were intended to form complexes through H‐bonds to an organo‐bridged silane prior to hydrolysis‐condensation through sol–gel processes to construct acid‐responsive bridged silsesquioxanes (BS). Whereas 5‐FU itself and the prodrug bearing two 5‐FU units completely leached out from the corresponding materials, the prodrug bearing three 5‐FU units was successfully maintained in the resulting BS. Solid‐state NMR (²⁹Si and ¹³C) spectroscopy show that the organic fragments of the organo‐bridged silane are retained in the hybrid through covalent bonding and the ¹H NMR spectroscopic analysis provides evidence for the hydrogen‐bonding interactions between the prodrug bearing three 5‐FU units and the triazine‐based hybrid matrix. The complex in the BS is not affected under neutral medium and operates under acidic conditions even under pH as high as 5 to deliver the drug as demonstrated by HPLC analysis and confirmed by FTIR and ¹³C NMR spectroscopic studies. Such functional BS are promising materials as carriers to avoid the side effects of the anticancer drug 5‐FU thanks to a controlled and targeted drug delivery.     

Structure and Biosynthesis of Xenoamicins from Entomopathogenic Xenorhabdus

During the search for novel natural products from entomopathogenic Xenorhabdus doucetiae DSM17909 and X. mauleonii DSM17908 novel peptides named xenoamicins were identified in addition to the already known antibiotics xenocoumacin and xenorhabdin. Xenoamicins are acylated tridecadepsipeptides consisting of mainly hydrophobic amino acids. The main derivative xenoamicin A ( 1 ) was isolated from X. mauleonii DSM17908, and its structure elucidated by detailed 1 D and 2 D NMR experiments. Detailed MS experiments, also in combination with labeling experiments, confirmed the determined structure and allowed structure elucidation of additional derivatives. Moreover, the xenoamicin biosynthesis gene cluster was identified and analyzed in X. doucetiae DSM17909, and its participation in xenoamicin biosynthesis was confirmed by mutagenesis. Advanced Marfey’s analysis of 1 showed that the absolute configuration of the amino acids is in agreement with the predicted stereochemistry deduced from the nonribosomal peptide synthetase XabABCD. Biological testing revealed activity of 1 against Plasmodium falciparum and other neglected tropical diseases but no antibacterial activity.