On the Reaction of Molecular Oxygen with Thiyl Radicals: A Re-examination

Abstract

Absolute rate constants for the addition of oxygen to thiyl radicals, i.e. RS + O₂ → RSOO^·, have been determined by applying a new competition method based on RS^· formation via one-electron reduction of the corresponding disulphides, and the competition between RS^· 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 × 10⁷ mol⁻¹ dm³ s⁻¹), penicillamine (2·5 × 10⁷ mol⁻¹ dm³ s⁻¹), homocysteine (8·0 × 10⁷ mol⁻¹ dm³ s⁻¹), cysteamine (2·8 × 10⁷ mol⁻¹ dm³ s⁻¹), 3-thiopropionic acid (2·2 × 10⁸ mol⁻¹ dm³ s⁻¹) and glutathione (3·0 × 10⁷ mol⁻¹ dm³ s⁻¹), respectively. The values obtained for the O₂ addition to the thiyl radicals from glutathione and cysteine are considerable lower (by about two orders of magnitude) than those previously published. This indicates that the RS^· + O₂ reaction may be of complex nature and is generally a process which is not solely controlled by the diffusion of the reactants.