Molecular Mechanism of Unexpected Metal-Independent Hydroxyl Radical Production by Mercaptotriazole and H₂O₂

It is well known that hydroxyl radical (^·OH) can be largely produced either through the classic iron-mediated inorganic-Fenton system or our recently discovered haloquinones/H₂O₂ organic-Fenton-like system, but rarely produced via thiol compounds. Here, unexpectedly, we found that ^·OH can be unequivocally generated by incubation of H₂O₂ and mercaptotriazole (MTZ), a typical heterocyclic thiol which has been used as an environmentally friendly corrosion inhibitor for mild steel. By the complementary applications of HPLC-MS and oxygen-18 isotope-labeling method, MTZ-derived sulfenic (MTZ-SOH) and sulfinic acids were detected and identified as transient intermediates, and sulfonic acid as final products. More interestingly, among all the products, MTZ-SOH was found to be the critical one directly responsible for the ^·OH formation. Not only MTZ, but also its derivatives can activate H₂O₂ to produce ^·OH. Taken together, we found an unexpected sulfenic acid-dependent ^·OH production from activation of H₂O₂ by heterocyclic thiol compounds, which may provide a new free radical perspective to further explore the environmental and biological behaviors of these widely used thiol compounds.