Effect of the peptide bond on the singlet-molecular-oxygen-mediated sensitized photo-oxidation of tyrosine and tryptophan dipeptides. A kinetic study

Abstract

The dye-sensitized photo-oxidation of tyrosine (tyr), tryptophan (trp) and their simplest dipeptides with glycine (gly) (tyr-gly, gly-tyr, trp-gly and gly-trp), mediated by singlet molecular oxygen (O 2( 1Δ g)), was studied. The overall rate constants ( k t) of O 2( 1Δ g) quenching were measured by time-resolved IR detection. Reactive rate constants ( k r) were determined in mixtures of polar organic solvents and in water to discriminate between the overall and physical contributions to the quenching. For trp and its two gly peptides, the peptide bond has practically no effect on the kinetics of photo-oxidation; k t is of the order of 10 7 M −1 s −1 in C 2H 5OHCH 3CN (80:20) and the k q/ k r ratio is in the range 11–13. In water (pH 7), k r is in the (3–5) × 10 7 M −1 s −1 range, the highest value corresponding to gly-trp. Loss of −NH 2 in water suggests the formation of pyrroloindole-like photo-oxidation products in the case of trp and gly-trp. For tyr derivatives, photo-oxidation is hindered when the amino group of tyr is masked: k t and k r are significantly lower for gly-tyr. The k q/ k r ratio in organic alkaline medium increases from 1 and 5 for tyr-gly and tyr respectively to 34 for gly-tyr. In buffered alkaline solutions, the k r values are in the range (1.4–5) × 10 7 M −1 s −1 with a small increase on going from pH 10 to 11.5. Only physical quenching of O 2( 1Δ g) by tyr derivatives is observed in the acidic range. The results, together with the loss of −NH 2, indicate a similar product distribution for tyr and tyr-gly on sensitized photo-oxidation.