a 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 (CH 3 Cob(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 CH 3 Cob(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.
The sugar replacement compound xylitol has gained increasing attention because of its use in many commercial food products, dental-hygiene articles, and pharmaceuticals. It can be classified by the origin of the raw material used for its production. The traditional "birch xylitol" is considered a premium product, in contrast to xylitol produced from agriculture byproducts such as corn husks or sugar-cane straw. Bulk stable-isotope analysis (BSIA) and compound-specific stable-isotope analysis (CSIA) by liquid-chromatography isotope-ratio mass spectrometry (LC-IRMS) of chewing-gum extracts were used to determine the δC isotope signatures for xylitol. These were applied to elucidate the original plant type the xylitol was produced from on the basis of differences in isotope-fractionation processes of photosynthetic CO fixation. For the LC-IRMS analysis, an organic-solvent-free extraction protocol and HPLC method for the separation of xylitol from different artificial sweeteners and sugar-replacement compounds was successfully developed and applied to the analysis of 21 samples of chewing gum, from which 18 could be clearly related to the raw-material plant class.
In order to simulate methane oxidation within landfill covers in changing environmental conditions, a computational approach via the extended Theory of Porous Media (eTPM) was created. Its purpose is to gain an overall view on the ongoing processes such as concentration progression of methane, oxygen, carbon dioxide and nitrogen. For that, advective and diffusive transport mechanisms as well as energy production from the exothermic oxidation reaction are considered. Regarding the diffusive gas transport different approaches to model the diffusion coefficient are made use of.
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