Reactive oxygen species in aerobic decomposition of thiourea dioxides

Abstract

Thiourea dioxides decompose in air-saturated alkaline solutions to give dithionite, S₂O₄²⁻. Kinetics of decomposition of aminoiminomethanesulfinic acid (AIMSA), methylaminoiminomethanesulfinic acid (MAIMSA) and dimethylaminoiminomethanesulfinic acid (DMAIMSA) were studied in alkaline solutions under aerobic and anaerobic conditions. No dithionite was formed in strictly anaerobic conditions. Dithionite, however, was formed in the presence of KO₂ and H₂O₂ under anaerobic conditions. The rate of decomposition was fastest for DMAIMSA and slowest for MAIMSA. The proposed mechanism involves the initial formation of the dioxosulfate(2−) ion, SO₂²⁻, through the heterolytic cleavage of the C–S bond. The dioxosulfate(2−) ion then reacts with dioxygen to give a series of reactive oxygen species: superoxide, peroxide and the hydroxyl radical. The expected dismutation of superoxide is important only in weakly alkaline solutions of pH less than 10. It is suggested, for the first time, that the reactive oxygen species and the sulfur leaving groups may be responsible for the toxicity observed in most thioureas.