The conversion of dihydrouracil to uracil, dihydrothymine to thymine, and dihydro-6-methyl uracil to 6-methyl uracil by gamma irradiation both in aqueous solution and in the solid state has been studied. In aqueous media, G values for the conversion process in dihydropyrimidine solutions (1 mg/ml) bubbled with oxygen, nitrous oxide, and nitrogen were 2.55, 2.27, and 0.73, respectively, for uracil; 2.17, 1.78, and 0.41 for thymine; and 2.40, 2.25, and 1.43 for 6-methyl uracil. In the solid state, G values for uracil, thymine, and 6-methyl uracil were 1.0, 0.9, and 0.9, respectively. The effect of dihydrouracil concentration on the G value for uracil production in oxygenated aqueous solution was measured. Free radicals formed in polycrystalline dihydrothymine and dihydro-6-methyl uracil were studied by electron spin resonance spectroscopy. Dose-response relationships for radical production and destruction by the radiation were measured. The G values for radical production in dihydrothymine and dihydro-6-methyl uracil were found to be 0.85 and 0.45, respectively. The close correspondence between the G values for radical production in these dihydropyrimidines and the G values for their conversion to unsaturated compounds in the solid state suggests the possibility that the radicals are intermediates in this process.

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