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 center dot/RSS- couple is +0.68 V, 0.28 V less than that of the RS center dot, H+/RSH couple. S-2(center dot-) is unstable with respect to HSS- and S-2. Generally, polysulfur compounds, with the exception of RSSR, are thermodynamically unstable with respect to disproportionation and ultimately lead to formation of kinetically inert Sg(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. (C) 2016 Elsevier Inc. All rights reserved.