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 m M )), but rather to the elimination of CO 2 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 CS 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 CO 2 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.