Reaction Mechanisms of HS Oxidation by Naphthoquinones.

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