Reduction Potentials and Exchange Reactions of Thiyl Radicals and Disulfide Anion Radicals

Article abstract of DOI:10.1021/j100310a022

Redox equilibria between RS^. and -S-S-^- radicals, and between these types of radical and phenoxyl and chlorpromazine (ClPz^.2+) radicals, have been investigated in aqueous solutions at pHs over the range 6-10 to obtain a self-consistent set of redox potentials for the reactions PhO^. + H⁺ + e^- = PhOH (4), RSS^.R + 2H⁺ + e^- = 2RSH (11), and RS^. + H⁺ + e^- = RSH (18), in sulfur systems with alkyl R groups.Absolute standard potentials were calculated on the of E⁰ = 0.83 V for the chlorpromazine couple.The results for E⁰₄ (=1.35 +/- 0.02 V) and E⁰₁₈ (=1.33 +/- 0.02 V) were in agreement with values calculated from thermodynamic data within the known uncertainties.E⁰₁₈ was found to exhibit a falloff when electron-rich groups, such as the two methyls of penicillamine or the CO2^- of β-mercaptoacetic acid, were present on the carbon adjacent to the S atom.However, the effect was relatively small (ca. 10-14 mV).E⁰₁₁ was 1.72 +/- 0.02 V for β-mercaptoethanol.The corresponding potentials for the cyclic anions of dithiothreitol, dithioerythritol, and lipoamide were the same within experimental error, but the uncertainties were larger (+/- 0.04 V).For the reaction e^- + -S-S- = -S-S-^- (22), the magnitude of E⁰₂₂ was calculated to be -1.60 V, showing that only strongly reducing species could donate electrons to disulfide.Rate constants for several of the forward and backward reactions in the equilibria were also determined.