THIOL PEROXIDASE-ACTIVITY OF DIARYL DITELLURIDES AS DETERMINED BY A H-1-NMR METHOD

A H-1 NMR method was developed for the assessment of the glutathione peroxidase-like activity of synthetic compounds. In this assay, thiols (N-acetylcysteine, tert-butyl mercaptan and 1-octyl mercaptan) were oxidized to the corresponding disulfides in CD3OD or CD3OD/D2O in the presence of hydrogen peroxide and the catalyst to be evaluated. The time required to reduce the thiol concentration with 50%, t50, was determined as a measure of the thiol peroxidase activity of the catalyst. Several diaryl ditellurides were efficient catalysts when present in low concentrations (0.3 mol %), whereas compounds with well-documented glutathione peroxidase-like activity in other assays were inactive (Ebselen, diaryl diselenides). The glutathione peroxidase-like activity of diaryl ditellurides was also assessed by using the classical coupled reductase assay. A mechanistic study showed that diaryl ditellurides, in the presence of hydrogen peroxide and a thiol, were rapidly converted to tellurosulfides. These species were stable enough to be isolated in some cases. The tellurosulfides reacted very slowly with added thiol, but in the presence of thiol/hydrogen peroxide the thiol was rapidly converted to its corresponding disulfide. On the basis of these observations, a mechanism involving a tellurinic acid thiol ester was proposed for the thiol peroxidase reaction of ditellurides. In contrast to tellurosulfides, selenosulfides, obtained either from diphenyl diselenide/hydrogen peroxide/1-octyl mercaptan or from Ebselen and 1-octyl mercaptan, were found to react very slowly with thiols in the presence of hydrogen peroxide.