Sensitized Photooxidation of Di‐ and Tripeptides of Tyrosine*

Abstract

This paper studies the dye-sensitized photooxidation of tyrosine (tyr) and tyr di- and tripeptides (tyr-tyr and tyr-tyr-tyr) mediated by singlet molecular oxygen (O2[1 delta g]) in alkaline media. Photooxidation quantum efficiencies (phi r) were obtained by determining the overall and reactive rate constants of interaction with the oxidative species, employing the time-resolved O2(1 delta g) phosphorescence detection method and static-photolysis actinometric method, respectively. The interaction of O2(1 delta g)-tyr derivatives occurs through an intermediate encounter complex with polar character. Ionization of the phenolic OH group of tyr derivatives and the polarity of the solvent favors the overall interaction. Nevertheless, phi r values decrease when changing from water to MeCN-water medium. This indicates that the reactive deactivation of the encounter complex, probably an entropy-controlled step, may be affected by solvent polarity in the same way as those processes in which charges are neutralized along the reaction pathway. Photooxidation quantum efficiencies indicate that the contribution to O2(1 delta g) physical quenching (a second alternative deactivation route for the encountered complex [O2(1 delta g)-tyr derivatives]) increases with the complexity of the peptide. As a result, the selfprotection of the peptidic entity against physical quenching also increases. The information obtained from the fractional consumption mol O2/mol tyr derivative (in tyr, the di- and tripeptides and the respective methyl ester of tyr and the tripeptide), together with the evolution (either consumption and/or generation) of primary amino groups upon photosensitized irradiation of the same compounds clearly indicates that the photooxidation of di- and tri-tyr peptides proceeds with the breakage of peptidic bonds. As a consequence, in the final balance each tyr unity behaves as an independent photooxidizable target.

Influence of pH and micellar systems on the sensitized photo‐oxidation of bovine serum albumin

“…It is known that Tyr is a moderate O 2 ( 1 △ g ) physical deactivator when the phenolic group is undissociated, at physiological pH conditions for example. The presence of the phenolate form at alkaline pH confer to Tyr characteristics of a relatively strong O 2 ( 1 △ g ) quencher with chemical and physical components …”

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

“…The rate for the dipeptide Trp-Trp is approximately twice as much as that of free Trp, whereas the slowest rate is exhibited by Trp-Tyr. It is known that free Tyr behaves as a typical phenol with regard to photooxidation kinetics, being practically an exclusive physical quencher of O 2 ( 1 D g ) at pH 7 (5,6,19). Figure 5 shows the fluorimetric analysis of the number of NH 2 groups upon sensitized photooxidation in pH 7 water, after up to 60-80 min irradiation.…”

“…Nevertheless, apparently these findings cannot be extrapolated to other families of dipeptides and polypeptides. The photooxidation efficiencies of the dipeptides Tyr-Gly, Gly-Tyr, Tyr-valine, valine-Tyr, Gly-Met, Gly-His and Tyr-Tyr, of the tripeptide Tyr-Tyr-Tyr and of other tripeptides with Tyr and valine, with Tyr bonded to nonreactive AA, are highly influenced by the presence of the peptide bond (5,6,19,20). A similar result has been reported for the reactivity of some larger peptides with O 2 ( 1 D g ) (21).…”

“…The chemical reactivity of Tyr with O 2 ( 1 D g ) is greatly increased in alkaline media because of the ionization of the OH group (1,(3)(4)(5)(6). The reaction products are known: Tyr gives rise to a lactam through oxidative decarboxylation and subsequent oxygenation (7) and Trp yields N-formylkynurenine and a 3a-hydroperoxipyrroloindole (3,8,9).…”

“…Nevertheless, one must be cautious in the interpretation of these results. Previous studies (4)(5)(6)10) have demonstrated that even the kinetics and mechanism of small peptides cannot be straightforwardly predicted Química, Universidad Nacional de Río Cuarto, Río Cuarto-5800, Argentina. Fax: þ54-358-4676233; e-mail: ngarcia@exa.unrc.edu.ar Abbreviations: AA, amino acid; 9,10-Adpa, 9,10-anthracenedipropionic acid; FFA, furfuryl alcohol; Gly, glycine; His, histidine; HPLC, high performance liquid chromatography; Met, methionine; MS, mass spectrometry; O 2 ( 1 D g ), singlet molecular oxygen; O 2 ( 3 R g À ), ground state molecular oxygen; Phe, phenylalanine; RB, Rose Bengal; Trp, tryptophan; TRPD, time-resolved phosphorescence detection; TrpP, dipeptide with Trp; Trp-Phe, L-tryptophyl-L-phenylalanine; Trp-Trp, Ltryptophyl-L-tryptophan; Trp-Tyr, L-tryptophyl-L-tyrosine; Tyr, tyrosine; UV, ultraviolet.…”