Phenol Coupling Initiated by One-electron Oxidation of Tyrosine Units in Peptides and Histone

Summary

Phenoxyl radicals generated pulse radiolytically by the reaction of N·₃ with Gly-Tyr decay bimolecularly (2k = 4·7 × 10⁸ M⁻¹ s⁻¹) with efficient formation of 2,2′-dimers, which enolize rapidly (k = 2·7 × 10⁴ s⁻¹) to produce the 2,2′-biphenolic product. The build-up of the characteristic 2,2′-biphenol fluorescence (400 nm) and absorption also indicated a delayed (k = 80 s⁻¹) process, probably involving the phenoxyl ↔ phenoxy-quinol equilibrium. About 60 per cent of the Gly-Tyr phenoxyls were found to dimerize to the 2,2′-biphenol, and a similarly efficient 2,2′-coupling seems to occur with other tyrosyls, such as Lys-Tyr-Lys and histone. γ-Radiolysis was applied to estimate relative yields of formation of 2,2′-biphenols under various conditions. Dimerization is almost completely inhibited by cysteine or oxygen, consistent with phenoxyl ‘repair’ by cysteine or ; disproportionation of with SOD prevents repair. The phenol 2,2′-coupling is less efficient for ·OH- and inefficient for e⁻_aq-initiation.

• Tyrosine phenoxyl radicals
• phenol coupling
• peptides
• histone
• pulse radiolysis