Effect of tetrapyrrole-based photosensitizers on spin trapping by 5,5-dymethyl-1-pyrroline N-oxide

Kinetics of the effect of three different tetrapyrrole-based photosenzitizers, pheophorbide a, meso-tetrahydroxyphenyl chlorin, and 3,7-bis(2-carboxyethyl)-2,8,12,17-tetramethyl-13,18-bis[(5-propoxycarbonyl-3-4-dihidroxy-phenyloxy) ethyl] porphyrin, on spin trapping with of 5,5-dymethyl-1-pyrroline N-oxide (DMPO)-a method widely applied in photodynamic therapy research-was investigated in the chemical model system of initiated oxidation of ethylbenzene. Spin adducts were measured by ESR spectroscopy, and kinetics of the consumption of the spin trap and accumulation of main products of oxidation (acetophenon, 1-phenyl-ethanol and 1-phenyl-ethyl-hydroperoxide) were followed using high performance liquid chromatography. The ESR spectrum of the non-illuminated system corresponded to an adduct formed between the spin trap and alpha-phenylethyl peroxyl radical, while under illumination, the spectrum indicated competitive formation of another adduct. Based on analysis of the corresponding g factor and hyperfine coupling constant values of the latter adduct (g=2.00587, alpha(N)=13.57 G, alpha(H)=2.27 G), the nitrone ring has presumably been split up and an adduct was formed between a-phenylethyl radical and 4-methyl-4-nitrosopentanoic acid-a product of the oxidation of the parent spin trap by singlet oxygen. Computer modeling based on detailed reaction mechanism and fitted to the measured data confirmed this assumption. The rate constants gave values of 2.3x10(5) M-1.s(-1) for the formation of the adduct and 1.0x10(4) M-1.s(-1) for its decay by interaction with free radicals offering a more detailed quantitative understanding of spin trapping in the presence of tetrapyrrole photosensitizers under illumination. Copyright (C) 2005 Society of Porphyrins & Phthalocyanines.