Synthesis and structure characterization of selenium metabolites†

Abstract

The difficulty in determining trace-level organoseleno metabolites and the lack of commercially available standards have been major barriers to a molecular-level understanding of Se biogeochemistry, ecotoxicology and nutrition, particularly in aquatic ecosystems. To overcome the problem, three important precursors of volatile alkyl selenides were synthesized, including dimethylselenonium propionate (DMSeP), which has only been postulated to exist in nature. A combination of 2-D multinuclear NMR, electrospray MS and GC–MS methods was employed to identify DMSeP, methylselenomethionine and methylselenocysteine in synthetic preparations without extensive clean-up. An alkaline hydroelimination test coupled with GC–MS analysis for the release pattern of dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe) was developed for a diagnostic determination of the three products. The DMSe release pattern of DMSeP confirmed the presence of a DMSeP-like compound in the biomass of 100 mg l^–1 Se-treated Chlorella investigated previously. Silylation–GC–MS was tested for the determination of selenomethionine, selenocysteine and methylselenocysteine in a standard mixture with a detection limit of better than 1 pmol per 0.5 µl injection volume for selenomethionine. This method was applied to the analysis of the acid digest of the proteinaceous fraction of the Chlorella culture. Selenomethionine was found to contain >70% of the protein-bound Se, although this constituted only a minor fraction of the total Se in the Chlorella biomass. These findings revealed the metabolic relationship between Se volatilization and selenomethionine incorporation into proteins. This knowledge is critical to advancement in Se biogeochemistry, ecotoxicology and the development of in situ bioremediation schemes.