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

• Published in: Applied organometallic chemistry Vol. 26; no. 2; pp. 94 - 101
• Main Authors: Wuerfel, Oliver, Thomas, Frank, Schulte, Marcel Sven, Hensel, Reinhard, Diaz-Bone, Roland Arturo
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.02.2012
• Subjects: cob(I)alamin; hydride generation; metalloids; methylation mechanism; methylcobalamin
• ISSN: 0268-2605; 1099-0739
• DOI: 10.1002/aoc.2821

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. Systematic mechanistic studies on methylation and hydride generation of Group 15 and 16 metal(loid)s by CH3Cob(III) and Cob(I) are presented. Efficient methylation of trivalent arsenic species without a change in oxidation state indicated that the methyl transfer does proceed via a non‐oxidative methylation. Our findings also point towards a similar mechanism for antimony, bismuth, selenium, and tellurium and indicate that the methyl group is transferred either in a concerted nucleophilic substitution or in caged radical mechanism