Using resonance synchronous spectroscopy to characterize the reactivity and electrophilicity of biologically relevant sulfane sulfur

Sulfane sulfur is common inside cells, playing both regulatory and antioxidant roles. However, there are unresolved issues about its chemistry and biochemistry. We report the discovery that reactive sulfane sulfur such as polysulfides and persulfides could be detected by using resonance synchronous spectroscopy (RS2). With RS2, we showed that inorganic polysulfides at low concentrations were unstable with a half-life about 1 min under physiological conditions due to reacting with glutathione. The protonated form of glutathione persulfide (GSSH) was electrophilic and had RS2 signal. GSS− was nucleophilic, prone to oxidation, but had no RS2 signal. Using this phenomenon, pKa of GSSH was determined as 6.9. GSSH/GSS− was 50-fold more reactive than H2S/HS− towards H2O2 at pH 7.4, supporting reactive sulfane sulfur species like GSSH/GSS− may act as antioxidants inside cells. Further, protein persulfides were shown to be in two forms: at pH 7.4 the deprotonated form (R-SS-) without RS2 signal was not reactive toward sulfite, and the protonated form (R-SSH) in the active site of a rhodanese had RS2 signal and readily reacted with sulfite to produce thiosulfate. These data suggest that RS2 of sulfane sulfur is likely associated with its electrophilicity. Sulfane sulfur showed species-specific RS2 spectra and intensities at physiological pH, which may reveal the relative abundance of a reactive sulfane sulfur species inside cells.