Influence of the nuclear and extranuclear substitution on the singlet molecular oxygen [O 2(1Δg)]-mediated photooxidation of tyrosine derivatives: A kinetic study

Summary The effect of the substitution pattern on the kinetics of the Type II (O2(1Δg)-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(1Δg) 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.