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

Abstract

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 (RS 2 ). With RS 2 , 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 RS 2 signal. GSS − was nucleophilic, prone to oxidation, but had no RS 2 signal. Using this phenomenon, p K a of GSSH was determined as 6.9. GSSH/GSS − was 50-fold more reactive than H 2 S/HS − towards H 2 O 2 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 RS 2 signal was not reactive toward sulfite, and the protonated form (R-SSH) in the active site of a rhodanese had RS 2 signal and readily reacted with sulfite to produce thiosulfate. These data suggest that RS 2 of sulfane sulfur is likely associated with its electrophilicity. Sulfane sulfur showed species-specific RS 2 spectra and intensities at physiological pH, which may reveal the relative abundance of a reactive sulfane sulfur species inside cells.

Enzymatic Regulation and Biological Functions of Reactive Cysteine Persulfides and Polysulfides

“…Under the same conditions, parental reduced glutathione (GSH) or H 2 S failed to reduce H 2 O 2 [ 5 ]. Li et al reported that GSSH was 50-fold more reactive than was H 2 S toward H 2 O 2 at physiological pH [ 46 ]. In cell systems, the overexpression of CSE enhanced GSSH levels and, more importantly, protected cells from H 2 O 2 -induced death [ 5 ].…”

“…Persulfides are stronger acids than are thiols, so at physiological pH (at which many thiols occur predominantly in the protonated form), a significant proportion of hydropersulfide species exist as a deprotonated persulfide anion (RSS – ) [ 19 ]. In fact, Li et al recently reported that the p K a of GSSH was 6.9, that is, two orders of magnitude smaller than that of GSH (p K a = 8.9) [ 46 ]. RSS – , thus formed, readily reacted with radical species to form persulfide radicals (RSS•) [ 47 ].…”

A Red Fluorescent Protein-Based Probe for Detection of Intracellular Reactive Sulfane Sulfur

“…We used the linear equation to fit the data obtained from the psRFP-GSSH reaction. Parameters showed that the slope is 0.003 and the intercept is 0.4. psRFP showed higher sensitivity toward HS n H than GSSH, probably because HS n H is more active than GSSH [ 7 ].…”

“…Cellular sulfane sulfur exists as hydrogen polysulfide (HS n H, n ≥ 2), organopolysulfide (RS n H, n ≥ 2), and dialkyl polysulfide (RS n R, n ≥ 3). They come in different reactivities with a rough order of HS n H > RS n H > RS n R [ 7 , 8 ]. HS n H and RS n H have both electrophilic and nucleophilic properties, and vividly react with reactive oxygen species (ROS), reactive nitrogen species (RNS), and signaling molecules at physiological pH (7.0~7.6).…”

Sulfane sulfur – new findings on an old topic

“…Also very recently, it has been reported that sulfane sulfur, including that in polysulfides and persulfides, could be detected by using resonance synchronous spectroscopy (RS 2 ) (Li et al, 2019). Sulfane sulfur displayed species-specific RS 2 spectra.…”