Aqueous solutions of thymidine-5′-monophosphate and cytidine-5′-monophosphate were γ-irradiated with 2-methyl-2-nitrosopropane as a spin-trapping reagent. Liquid chromatography was applied to separate the stable spin-trapped radicals in the irradiated solutions. The eluate was passed through flow cells of ESR and uv detectors and collected by a fraction collector. The amount of residual sugar in the each fraction was determined by the <tex-math>$\text{phenol-}{\rm H}_{2}{\rm SO}_{4}$</tex-math> method. From the combined results of the ESR, uv, and sugar determinations, the structures of most of the trapped radicals were identified. The short-lived radicals which were trapped by 2-methyl-2-nitrosopropane were radicals on N-1, C-5, and N-3 positions of the base in the thymidine-5′-monophosphate and C-5 and C-6 base radicals in the cytidine-5′-monophosphate.

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