Reaction Mechanisms of H₂S Oxidation by Naphthoquinones

1,4-naphthoquinones (NQs) catalytically oxidize H₂S to per- and polysufides and sulfoxides, reduce oxygen to superoxide and hydrogen peroxide, and can form NQ-SH adducts through Michael addition. Here, we measured oxygen consumption and used sulfur-specific fluorophores, liquid chromatography tandem mass spectrometry (LC-MS/MS), and UV-Vis spectrometry to examine H₂S oxidation by NQs with various substituent groups. In general, the order of H₂S oxidization was DCNQ ~ juglone > 1,4-NQ > plumbagin >DMNQ ~ 2-MNQ > menadione, although this order varied somewhat depending on the experimental conditions. DMNQ does not form adducts with GSH or cysteine (Cys), yet it readily oxidizes H₂S to polysulfides and sulfoxides. This suggests that H₂S oxidation occurs at the carbonyl moiety and not at the quinoid 2 or 3 carbons, although the latter cannot be ruled out. We found little evidence from oxygen consumption studies or LC-MS/MS that NQs directly oxidize H₂S_2-4, and we propose that apparent reactions of NQs with inorganic polysulfides are due to H₂S impurities in the polysulfides or an equilibrium between H₂S and H₂S_n. Collectively, NQ oxidation of H₂S forms a variety of products that include hydropersulfides, hydropolysulfides, sulfenylpolysulfides, sulfite, and thiosulfate, and some of these reactions may proceed until an insoluble S₈ colloid is formed.