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.