Rose Bengal–Sensitized Photooxidation of the Dipeptides l-Tryptophyl-l-Phenylalanine, l-Tryptophyl-l-Tyrosine and l-Tryptophyl-l-Tryptophan: Kinetics, Mechanism and Photoproducts¶

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Abstract
The Rose Bengal–sensitized photooxidations of the dipeptides l-tryptophyl-l-phenylalanine (Trp-Phe), l-tryptophyl-l-tyrosine (Trp-Tyr) and l-tryptophyl-l-tryptophan (Trp-Trp) have been studied in pH 7 water solution using static photolysis and time-resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O2(1Δg)) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di- and polypeptides of photooxidizable amino acids. The ratio between chemical (kr) and overall (kt) rate constants for the interaction O2(1Δg)-dipeptide indicates that Trp-Phe and Trp-Trp are good candidates to suffer photodynamic action, with kr/kt values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp-Tyr, a lower kr/kt value (0.18) has been found, likely as a result of the high component of physical deactivation of O2(1Δg) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O2(1Δg) attack is the Trp group and suggests a much lower accumulation of kynurenine-type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a-oxidized hydrogenated pyrrolo[2,3-b]indoles.

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