Oxyhalogen-Sulfur Chemistry: Kinetics and Mechanism of Oxidation of Tiopronin by Acidified Bromate and Aqueous Bromine

N-2-(Mercaptopropionyl)glycine (MPG) is a free-radical scavenger, a detoxicating synthetic aminothiol which also acts as an antioxidant with a wide range of clinical applications. The oxidation of MPG by aqueous bromine and acidified bromate has been studied by spectrophotometric techniques. The stoichiometry for the reaction of acidic bromate with MPG is 1:1, HS(CH3)CH(=O)N(H)CH2COOH+BrO3-HO3S(CH3)CH(=O)N(H)CH2COOH+Br-, with reaction occurring only at the thiol centre. The involvement of thiyl radicals in the oxidation of MPG competes with a non-radical pathway involving two-electron oxidations of the sulfur centre. A bimolecular rate constant of 5.68 (+/- 0.94)x10(3)M(-1)s(-1) for the direct reaction of MPG with bromine was determined. Electrospray ionization spectral data show that MPG is oxidized through its sulfinic acid, by-passing the unstable sulfenic acid. A simplified reaction network consisting of 19 reactions was simulated and it gave a very good fit to the experimental data.