Reaction Mechanisms of H 2 S Oxidation by Naphthoquinones.

1,4-naphthoquinones (NQs) catalytically oxidize H 2 S 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 H 2 S oxidation by NQs with various substituent groups. In general, the order of H 2 S 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 H 2 S to polysulfides and sulfoxides. This suggests that H 2 S 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 H 2 S 2-4 , and we propose that apparent reactions of NQs with inorganic polysulfides are due to H 2 S impurities in the polysulfides or an equilibrium between H 2 S and H 2 S n . Collectively, NQ oxidation of H 2 S forms a variety of products that include hydropersulfides, hydropolysulfides, sulfenylpolysulfides, sulfite, and thiosulfate, and some of these reactions may proceed until an insoluble S 8 colloid is formed.