Disulfide anion radical equilibria: effects of -NH3+, -CO2–, -NHC(O)- and -CH3 groups

Abstract

Equilibria between disulfide anion radicals (RS⇌SR) and thiolate anions and thiyl radicals, namely:

have been studied as a function of pH with alkyl R substituents of different structure and net charge, for the purpose of obtaining thermochemical data. The thermodynamic stability of the RS⇌SR species was examined in terms of: (a) the reduction potential for its formation from RSSR, and: (b) the magnitude of the equilibrium constant for its formation from RS˙ and RS^–. It was found that the RS⇌SR stability increased when protonated amino groups were present and rose with their proximity to the S⇌S group. For each protonated amino group beta to the S atoms E ^o(RSSR/RS⇌SR) typically rose by about 0.1 V. In parallel with this the equilibrium constant for formation of RS⇌SR from RS˙ and RS^– increased in the doubly protonated systems by a typical factor of ten. This equilibrium constant is strongly depressed by methyl groups on the C atoms adjacent to the sulfur, and is reduced in structures with ionised carboxylate groups beta to the S atom. The magnitude of the latter effect is diminished when the distance between the sulfur centers and the carboxylate groups increased. The changes in RS⇌SR stability can be understood in terms of inductive effects and Coulombic interactions. The value of E ^o(RSSR/RS⇌SR) = –1.41 V found for glutathione disulfide is an indication of the reduction potential for the cystine residue in proteins. Estimates of the effects of nearby -NH₃⁺ groups on the stability of disulfide anions have been made.