Molecular mechanism of the metal-independent production of hydroxyl radicals by thiourea dioxide and H₂O₂.

Abstract

It is well-known that highly reactive hydroxyl radicals (HO^•) can be produced by the classic Fenton system and our recently discovered haloquinone/H₂O₂ system, but rarely from thiol-derivatives. Here, we found, unexpectedly, that HO^• can be generated from H₂O₂ and thiourea dioxide (TUO₂), a widely used and environmentally friendly bleaching agent. A carbon-centered radical and sulfite were detected and identified as the transient intermediates, and urea and sulfate as the final products, with the complementary application of electron spin-trapping, oxygen-18 isotope labeling coupled with HPLC/MS analysis. Density functional theory calculations were conducted to further elucidate the detailed pathways for HO^• production. Taken together, we proposed that the molecular mechanism for HO^• generation by TUO₂/H₂O₂: TUO₂ tautomerizes from sulfinic acid into ketone isomer (TUO₂-K) through proton transfer, then a nucleophilic addition of H₂O₂ on the S atom of TUO₂-K, forming a S-hydroperoxide intermediate TUO₂-OOH, which dissociates homolytically to produce HO^•. Our findings represent the first experimental and computational study on an unprecedented new molecular mechanism of HO^• production from simple thiol-derived sulfinic acids, which may have broad chemical, environmental, and biomedical significance for future research on the application of the well-known bleaching agent and its analogs.