Intermolecular Charge Transfer Involving Tryptophan, Tyrosine and Three Electron-bonded Intermediates Derived from Methionine

Summary

Oxidation processes of radiation-generated three-electron-bonded intermediates derived from methionine Met₂ and Met[S∴X] (X = Cl, Br) were investigated through reaction with tryptophan and tyrosine, using the optical pulse radiolysis method. Bimolecular rate constants have been measured for the reactions Met₂ with TrpH(k = 3·8 × 10⁸ dm³ mol⁻¹ s⁻¹ and k = 4·9 × 10⁸ dm³ mol⁻¹ s⁻¹ at pH 1 and 4·3, respectively) and Met₂ with tyrosine, k = 3·8 × 10⁷ dm³ mol⁻¹ s⁻¹. Rate constants for intermolecular transformation of Met[S∴Br] and Met[S∴Cl] into TrpH^+· or Trp· were also estimated. They varied from 4·7 × 10⁸ dm³ mol⁻¹ s⁻¹ (bromide species) to 1·0 × 10⁹ dm³ mol⁻¹ s⁻¹ (chloride species). It has also been established that azide radicals , N·₃ in contrast to dihalide radicals do not form transients of Met[S∴X] (X = N₃)-type. However, oxidation of N⁻₃ by Met₂ occurs with a bimolecular rate constant of 2·8 × 10⁸ dm³ mol⁻¹ s⁻¹. These results are discussed in the light of some equilibria which have been proposed earlier for methionine-halide systems.

• three-electron-bonded radicals
• methionine
• tryptophan
• tyrosine
• intermolecular electron transfer
• pulse radiolysis