Reversible Hydrogen Transfer Reactions in Thiyl Radicals From Cysteine and Related Molecules: Absolute Kinetics and Equilibrium Constants Determined by Pulse Radiolysis

The mercapto group of cysteine (Cys) is a predominant target for oxidative modification, where one-electron oxidation leads to the formation of Cys thiyl radicals, CysS•. These Cys thiyl radicals enter 1,2- and 1,3-hydrogen transfer reactions, for which rate constants are reported in this paper. The products of these 1,2- and 1,3-hydrogen transfer reactions are carbon-centered radicals at position C3 (α-mercaptoalkyl radicals) and C2 (•Cα radicals) of Cys, respectively. Both processes can be monitored separately in Cys analogues such as cysteamine (CyaSH) and penicillamine (PenSH). At acidic pH, thiyl radicals from CyaSH permit only the 1,2-hydrogen transfer according to equilibrium 12, +H3NCH2CH2S• ⇌ +H3NCH2 •CH–SH, where rate constants for forward and reverse reaction are k 12 ≈ 105 s–1 and k–12 ≈ 1.5 × 105s–1, respectively. In contrast, only the 1,3-hydrogen transfer is possible for thiyl radicals from PenSH according to equilibrium 14, (+H3N/CO2H)­Cα–C­(CH3)2–S• ⇌ (+H3N/CO2H)•Cα–C­(CH3)2–SH, where rate constants for the forward and the reverse reaction are k 14 = 8 × 104 s–1 and k–14 = 1.4 × 106 s–1. The •Cα radicals from PenSH and Cys have the additional opportunity for β-elimination of HS•/S•–, which proceeds with k 39 ≈ (3 ± 1) × 104 s–1 from •Cα radicals from PenSH and k–34 ≈ 5 × 103 s–1 from •Cα radicals from Cys. The rate constants quantified for the 1,2- and 1,3-hydrogen transfer reactions can be used as a basis to calculate similar processes for Cys thiyl radicals in proteins, where hydrogen transfer reactions, followed by the addition of oxygen, may lead to the irreversible modification of target proteins.