Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals
ABSTRACT Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr–Val, Val–Tyr and Val–Tyr–Val) mediated by singlet molecular oxygen [O2(1Δg)], phosphate (HPO4⋅− and PO4⋅2−) and sulfate (SO4⋅−) radicals was studied, employing time-resolved O2(1Δg) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O2(1Δg)-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O2(1Δg) by Tyr-derivatives (kt/kr values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr–Val, 2.8 for Val–Tyr and 1.5 for Val–Tyr–Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O2(1Δg) quenching process. Products analysis by LC–MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO4⋅− and PO4⋅2−, generated by photolysis of the P2O84− at different pH) and sulfate radicals (SO4⋅−, produced by photolysis of the S2O82−) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO4⋅−≥HPO4⋅−≥PO4⋅2−. Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO4⋅−.