Signaling by sulfur-containing molecules. Quantitative aspects.

There is currently interest in sulfur-containing molecules that may or may not play a role in signaling. We have collected relevant thermodynamic data, namely standard Gibbs energies of formation and electrode potentials at pH 7, and used these to construct a Frost diagram. Thermodynamic data not available in the literature could be estimated with reasonable confidence. At pH 7, the electrode potential of the RSS/RSS− couple is +0.68 V, 0.28 V less than that of the RS, H+/RSH couple. S2− is unstable with respect to HSS− and S2. Generally, polysulfur compounds, with the exception of RSSR, are thermodynamically unstable with respect to disproportionation and ultimately lead to formation of kinetically inert S8(s). About thermoneutral is the formation of RSS− from RSSR and HS−, but formation of HNO from HS− and SNO−, and from HS− and RSNO, is unfavorable. The formation of SSNO− in vivo is kinetically unlikely.