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

It is well-known that highly reactive hydroxyl radicals (HO center dot) 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 center dot 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 center dot production. Taken together, we proposed that the molecular mechanism for HO center dot 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 center dot. Our findings represent the first experimental and computational study on an unprecedented new molecular mechanism of HO center dot 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.