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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.

The role of quinones and hydroxyquinones as sensitizers and as quenchers in Type II photooxygenations has been examined. The second aspect is discussed here, through a systematic study, for the first time in the open literature. Quinonic compounds are excellent generators of O2(1Δg) in aprotic solvents (excluding those quinones possessing substituents in positions adjacent to the carbonyl groups, in the case of anthraquinone derivatives). Benzoquinones, anthraquinones, and hydroxy derivatives are good O2(1Δg) quenchers upon dye-sensitized photoirradiation. The excited oxygen species is deactivated with rate constants in the range 106–107 M−1 s−1 depending on the solvent employed. The quenching process deactivates O2(1Δg) without further destruction of the quinone. The main interaction with O2(1Δg) is driven by the quinone moiety, in spite of the presence of potentially active nuclear substituents. The quenching mechanism could involve a reversible charge transfer intermediate, with the quinonic compound acting as an electron donor. Keywords: photooxidation, quenching, quinones, rose bengal, singlet oxygen.

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Abstract:A kinetic study on the aerobic (02('Ag)-mediated) photooxidation of a series of dihydroxynaphthalenes was carried out. Members of this family of compounds include important environmental contaminants. The interaction with 0 2 ( l~g ) , determined by both time-resolved phosphorescence and static methods, yields a quenching rate constant in the range of 10'-lo9 M-I s-l, depending on the solvent polarity, pH, and substitution pattern of the dihydroxynaphthalene. According to experimental evidence, the mechanism of the interaction seems to be mediated by an encounter complex with a considerable charge transfer component. From a mechanistic point of view this characteristic of DHN indicates a behaviour very similar to that of simple substituted phenols and dihydroxybenzenes. Quantum yields for DHN photooxidation (@,) indicate that these substrates are excellent candidates for Oz(l~g)-mediated degradation under environment conditions. Furthermore, these compounds proved to be good self-sensitizers upon irradiation at 337 nm. Quantum yields (QA) for the production of Oz('Ag) were determined to be on the order of 0.25 in benzene.Key words: dihydroxynaphthalenes, photooxidation, quenching, sensitization.RksumC : On a rCalisi une Ctude cinetique de la photooxydation aCrobique (avec midiation par le OZ('Ag)) d'une sCrie de dihydroxynaphtalttnes. Les membres de cette famille sont des contaminants environnementaux importants. L'interaction avec le O2('Ag) a CtC diterminie par des mCthodes statiques ainsi que par phosphorescence rCsolue en fonction du temps et elle conduit h une constante de vitesse de desactivation de I'ordre de lo5 a lo9 M-I s-I, suivant la polarit6 du solvant, le pH et le patron de substitution du dihydroxynaphtalkne. Sur la base des donnCes expCrimentales obtenues, il semble que le mecanisme de I'interaction soit I'objet d'une mediation de la part d'un complexe de rencontre qui implique une composante importante de transfert de charge. D'un point de vue mecanistique, cette caractbristique du DHN indique un comportement trks semblable a celui des phCnols et des dihydroxybenzenes simples, substituks. Les rendements quantiques pour la photooxydation du DHN (@, ) indiquent que ces substrats sont d'excellents candidats pour des digradations avec midiation du Ol(lAg) dans des conditions environnementales naturelles. De plus, lorsqu'on les a irradiCs i 337 nm, ces composts ont montrk d'excellentes propriCtCs comme autosensibilisateurs. On a ditermint que les rendements quantiques (OA) pour la production du Oz('Ag) sont de l'ordre de 0,25, dans le benzene.

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The effect of the substitution pattern on the kinetics of the Type II (O2((1)Δg)-mediated) dye-sensitized photooxidation of a series of nine tyrosine derivatives was investigated. Overall (kt) and reactive (kr) rate constants for the interaction of the excited oxygen species with the amino acid derivatives were determined. A parallel study on solvent and pH effects was carried out.The presence of different substituents in nuclear positions or in the amino acid side chain greatly affect the photooxidation rates.An upper limit for photooxidation quantum yield, calculated from the kinetic data, varies from 0.03 to 0.25, being the higher for halogenated tyrosines and the lower for esterified tyrosines and for the nitro-derivative.The variation of solvent polarity and pH of the reaction medium confirm that the presence of the ionized phenolate group in tyrosine, clearly dominates the quenching process. As already postulated for generic phenolic derivatives, it proceeds through a polar intermediate complex which posses some component of charge-transfer character.Esterification of the carboxilic acid of tyrosine selectively decreases the contribution of the reactive step to the overall process of O2((1)Δg) quenching. An amide group in the same position does not produce noticiable changes in this sense. The presence of a highly deactivating nitro group in nuclear positions greatly diminishes the magnitude of both overall and reactive interactions.For all three, o-, m- and p-tyrosine the values of photooxidation quantum yields show an excellent parallelism with the rates of consumption of the - NH2 group of the amino acid chain, upon sensitized irradiation. It could react, in the cases of 0- and m-tyrosine in a secondary, non photochemical, step.