On the reaction of molecular oxygen with thiyl radicals: a re-examination

Absolute rate constants for the addition of oxygen to thiyl radicals, i.e. RS radicals + O2 → RSOO radicals, have been determined by applying a new competition method based on RS radicals formation via one-electron reduction of the corresponding disulphides, and the competition between RS radicals reacting with O2 and an electron donor such as ascorbate. Bimolecular rate constants have been obtained for the thiyl radicals derived from cysteine (6.1 x 107 mol-1 dm3 s-1), penicillamine (2.5 x 107 mol-1 dm3 s-1), homocysteine (8.0 x 107 mol-1 dm3 s-1), cysteamine (2.8 x 107 mol-1 dm3 s-1), 3-thiopropionic acid (2.2 x 108 mol-1 dm3 s-1) and glutathione (3.0 x 107 mol-1 dm3 s-1), respectively. The values obtained for the O2 addition to the thiyl radicals from glutathione and cysteine are considerably lower (by about two orders of magnitude) than those previously published. This indicates that the RS radicals + O2 reaction may be of complex nature and is generally a process which is not solely controlled by the diffusion of the reactants. (author)