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.     

Die genaue Bestimmung des Sauerstoffs in Gasgemengen

Ohne Zusammenfassung     

Pseudospark switches for the power supply circuits of pulsed lasers

A family of sealed-off low-pressure spark gap switches developed at the Institute of High Current Electronics (Tomsk) and at the Plasma Scientific Research Institute (Ryazan) with participation of researchers from the Physical Institute of the University of Erlangen (Germany) is described. The devices are of cermet construction, are capable of switching pulsed currents over wide ranges of amplitudes and durations, and have a number of unique characteristics. The principle of operation of the spark gap switches is described and their designs are given. The triggering circuits of the switches are considered. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Plazma Scientific Research Institute of Gas-Discharge Devices. Physical Institute of the University of Erlangen. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 97–105, April, 2000.     

Isoperimetric balls in cones over tori

In the cone over a cubic three-torus T ³, balls about the vertex are isoperimetric if the volume of T ³ is less than π/16 times the volume of the unit three-sphere. The conjectured optimal constant is 1.