Reaction of SO4–˙ with methylated uracils. An electron spin resonance study in aqueous solution

Radicals obtained by reaction of photolytically generated SO4–˙ with 1-methyluracil (1), 1,3-dimethyluracil (2), 1-methylthymine (3), 1,3-dimethylthymine (4), and 1,3,6-trimethyluracil (5) were studied by e.s.r. spectroscopy in aqueous solutions. In situ photolysis of neutral and acidic solutions containing (1) and persulphate in low concentrations (3mM) resulted in the e.s.r. spectrum of the C(5)-OH adduct radical. E.s.r. spectra obtained from (1) and (2) with high persulphate concentrations (30mM) were assigned to the C(6)-OH adduct radicals. It is proposed that this change in radical population is due to a persulphate-induced chain reaction which results in selective oxidation of the C(5)-OH radicals and simultaneous accumulation of the C(6)-OH adducts generated in side reactions. When the persulphate concentration was raised to 60mM, 5-oxo-6-yl radicals were formed in secondary processes from (1) and (2) besides the OH adducts. In contrast to these results the thymines (3) and (4) yielded only C(6)-OH adduct radicals. Addition of phosphate dianions to the photolysis solutions containing persulphate and the pyrimidine bases (1)–(4) resulted in the e.s.r. spectra of the C(6)-phosphate adduct radicals (pH 6.5–9.5). Identical spectra were obtained by reaction of (1)–(4) with HPO4–˙ radical anions generated by photolysis of Li4P2O8. The results of the experiments with 1,3,6-trimethyluracil (5) were completely different. First, reaction of SO4–˙ with (5), even at low persulphate concentration (3mM), did not lead to an OH adduct but to a 5-oxo-6-yl secondary radical. Secondly it was not possible to generate phosphate adduct radicals from (5) either with a mixture of persulphate and phosphate or with peroxodiphosphate. The spectral parameters of the radicals derived from (1)–(5) are given and possible pathways for the SO4–˙-induced radical formation are discussed.