Mechanism of tryptophan oxidation by some inorganic radical-anions: a pulse radiolysis study

Abstract

Oxidation of halide, thiocyanate, carbonate and selenite ions by hydroxyl radicals yields radicalanions which themselves oxidise amino-acids such as tryptophan and tyrosine. The free-radical product(s) of oxidation of tryptophan (TrpH) by Br^–₂, (SCN)^–₂, CO^–₃ and a selenite free radical have similar spectral and kinetic properties in neutral and alkaline solutions. The spectral shifts and decay kinetics of the tryptophan free-radical observed at pH ∼ 3 show that oxidation of TrpH by Br^–₂ and (SCN)^–₂ gives a radical which protonates (pK_a= 4.3) and which is singly charged at pH ∼ 3. The different prototropic behaviour of the absorption of the product of oxidation of TrpH by the selenite radical suggests that in this case, more than one species is produced. The protonated free radical TrpH⁺ reacts with SCN^– so that there is an apparent loss of reactivity of (SCN)^–₂ towards TrpH, especially at low pH, high [SCN^–] and low [TrpH]. A kinetic model for the (SCN)^–₂/TrpH reaction predicts a variation of the equilibrium concentration of (SCN)^–₂ with [H⁺], [TrpH] and [SCN^–]. The experimental data are in accord with the model and enable the equilibrium constants of the reactions to be obtained.