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

Equilibria between disulfide anion radicals (RS <(therefore)over bar> SR) a nd thiolate anions and thiyl radicals, namely: RS. + RS- reversible arrow RS <(therefore)over bar> SR have been studied as a function of pH with alkyl R substituents of differen t structure and net charge, for the purpose of obtaining thermochemical dat a. The thermodynamic stability of the RS <(therefore)over bar> SR species w as examined in terms of: (a) the reduction potential for its formation from (RSSR)-R-., and: (b) the magnitude of the equilibrium constant for its for mation from RS. and RS-. It was found that the RS <(therefore)over bar> SR stability increased when protonated amino groups were present and rose with their proximity to the S <(therefore)over bar> S group. For each protonate d amino group beta to the S atoms E degrees(RSSR/RS <(therefore)over bar> S R) typically rose by about 0.1 V. In parallel with this the equilibrium con stant for formation of RS <(therefore)over bar> SR from RS. and RS- increas ed in the doubly protonated systems by a typical factor of ten. This equili brium constant is strongly depressed by methyl groups on the C atoms adjace nt to the sulfur, and is reduced in structures with ionised carboxylate gro ups beta to the S atom. The magnitude of the latter effect is diminished wh en the distance between the sulfur centers and the carboxylate groups incre ased. The changes in RS <(therefore)over bar> SR stability can be understoo d in terms of inductive effects and Coulombic interactions. The value of E degrees(RSSR/RS <(therefore)over bar> SR) = - 1.41 V found for glutathione disulfide is an indication of the reduction potential for the cystine resid ue in proteins. Estimates of the effects of nearby -NH3+ groups on the stab ility of disulfide anions have been made.