ESR of Spin-Trapped Radicals in Aqueous Solutions of Dihydropyrimidine Bases

The reactions of hydroxyl radicals with several dihydropyrimidine base derivatives in aqueous solutions have been investigated by the technique of spin-trapping and ESR spectroscopy. Hydroxyl radicals were generated by the uv photolysis of H/sub 2/O/sub 2/ and tertiary nitrosobutane was used as the spin trapping reagent. The dihydro derivatives of uracil, 6-methyluracil, 1-methyluracil, 1-ethyluracil, 1-methylcytosine, 5-methylcytosine, thymine, and orotic acid were investigated. The predominant reaction in all cases was the abstraction of the H atoms bound to the C(6) position. To a lesser extent H-abstraction took place from the methyl groups attached to the N(1), C(5), and C(6) positions. For dihydro-1-ethyluracil, H atoms were abstracted from both the carbon atoms of the ethyl group. The radicals produced by H-abstraction from the C(5) or C(6) positions of the dihydropyrimidines are equivalent to those formed by H atom addition to the C(5) to C(6) double bonds of the corresponding pyrimidines. Hence the present results are helpful in distinguishing between the spin-trapped radicals formed by H or OH addition to the C(5) to C(6) double bond of pyrimidines of ..gamma..-irradiated aqueous solutions.