Oxidative Degradation of Thiaproline Derivatives in Aqueous Solutions Induced by ?OH Radicals

The participation of neighboring amino and carboxyl groups in thiazolidyne-4-carboxylic acid (thiaproline) and its two derivatives, thiazolidyne-2-carboxylic acid and thiazolidyne-2,4-dicarboxylic acid, is demonstrated during ^.OH-radical-induced oxidation in aqueous solutions. The reaction of ^.OH radicals with these compounds does not lead to the expected one-electron oxidized sulfur-centered radical cations and subsequently to the intermolecularly (S∴S)-bonded dimeric radical cations (even at very high concentration of thiaprolines (>50 mM )), but rather to the elimination of CO₂ with the parallel formation of αN radicals. The latter radicals formed in thiaproline and thiazolidyne-2,4-dicarboxylic acid subsequently undergo β-fragmentation of the C? S bond, leading to the thiyl radicals (RS^.). The αN and RS^. radicals were probed and quantified by one-electron reduction of p-nitroacetophenone (PNAP) and by one-electron oxidation of the monoanion of ascorbic acid (AH^?), respectively. The calculated stabilization enthalpies of the αN radicals show that the loss of CO₂ is thermodynamically a favorable process. The Gibbs free energies of β-fragmentation processes in thiaproline and thiazolidyne-2,4-dicarboxylic acid are equal to ?3.7 and ?10.2 kJ mol^?1, respectively, showing that at room temperature both are exergonic. A general ^.OH-radical-induced oxidation mechanism of thiaproline derivatives in aqueous solutions is proposed.