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

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, pK_a 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.