Molecular mechanism of the metal-independent production of hydroxyl radicals by thiourea dioxide and H 2 O 2

It is well-known that highly reactive hydroxyl radicals (HO • ) can be produced by the classic Fenton system and our recently discovered haloquinone/H 2 O 2 system, but rarely from thiol-derivatives. Here, we found, unexpectedly, that HO • can be generated from H 2 O 2 and thiourea dioxide (TUO 2 ), 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 TUO 2 /H 2 O 2 : TUO 2 tautomerizes from sulfinic acid into ketone isomer (TUO 2 -K) through proton transfer, then a nucleophilic addition of H 2 O 2 on the S atom of TUO 2 -K, forming a S-hydroperoxide intermediate TUO 2 -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.