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.     

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Onset of quark-hadron duality in pion electroproduction

A large data set of charged-pion (pi+/-) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark-->pion production mechanisms.     

Investigation of gravity lanthanide separation chemistry

Lanthanides are common fission products and the ability to separate and quantify these elements is critical to rapid radiochemistry applications. Published lanthanide separations using Eichrom Ln Spec resin utilize an HCl gradient. Here it is shown that the efficacy and resolution of the separation is improved when a nitric acid gradient is used instead. The described method allows parallel processing of many samples in 1.5 h followed by 60 min counting for quantification of 9 radioisotopes of 7 lanthanide elements.     

Stepwise formation of a molecular square with bridging NH,O-substituted dicarbene building blocks

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