Adduct Formation and Absolute Rate Constants in the Displacement Reaction of Thiyl Radicals with Disulfides

Article abstract of DOI:10.1021/j150669a046

The displacement reaction of thiyl radicals with disulfides is shown to proceed via a transient adduct radical by using time-resolved pulse radiolysis techniques.The relatively long-lived adduct (t_1/2 > 100 μs) formed in the forward reaction of the equilibrum RS^. + RSSR <*> ^. is suggested to be a sulfuranyl radical with the unpaired electron located in an antibonding ?^* orbital within a trisulfide bridge.These species exhibit optical absorptions in the UV, e.g., λ_max = 375 +/- 10 nm and ε = (3.4 +/- 0.4)*10³ M^-1 cm^-1 for the all-methylated radical, and have been identified in aqueous and methanolic solutions.Equilibrum constants of K = 180 +/- 30 and 60 +/- 20 M^-1 have been evaluated for the systems with R = CH3 and cysteine residue, respectively, via two different methods.The corresponding forward reactions occur with k(RS^. + RSSR) = 3.8*10⁶ and 7.7*10⁵ M^-1 s^-1, respectively.