Journal of Physical Chemistry p. 6532 - 6537 (1987)
Update date:2022-08-17
Topics:
Surdhar, Parminder S.
Armstrong, David A.
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.
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