Radicals produced in irradiated aqueous solutions of 5-nitrouracil, 5-nitroorotic acid, and 5-nitrobarbituric acid have been studied by the in situ radiolysis steady state ESR method, and the kinetics of their formation and disappearance were followed by pulse radiolysis. The reactions of hydroxyl radicals with 5-nitrouracil and its derivatives are nearly diffusion controlled and involve addition of OH to the 5,6 double bond. Addition of OH to the carbon bearing the nitro group leads to oxidative denitration by subsequent rapid elimination of HNO2. This reaction is analogous to dehalogenation following OH addition to 5-halouracils, and the radicals produced from both the nitro and the halo compounds are identical. Addition of OH to position 6 is also important, but the radicals formed were not observed by ESR. The distribution of OH addition between position 5 and 6 was determined by the pulse radiolysis experiments and found to involve 25 and 30% addition to the 5 position of 5-nitrouracil and 5-nitrobarbituric acid, respectively, i.e., in these two cases the efficiency of oxidative denitration is 25-30%. The transient optical absorption spectra recorded immediately after the reaction of OH with 5-nitrouracil, 5-bromouracil, and iso-barbituric acid are very similar and are attributed to the same radical. The rate of reaction of OH has been monitored both by the buildup of the transient absorption and by the destruction of the parent compound absorption, and the two measurements give rate constants of <tex-math>$7\pm 2\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$</tex-math> for the four nitrouracil derivatives examined. These findings indicate that denitration, like dehalogenation, occurs rapidly following a rate limiting OH addition. Hydrated electrons were found to react with the 5-nitrouracils with rate constants of <tex-math>$1.9\pm 0.2\times 10^{10}\ M^{-1}\ {\rm sec}^{-1}$</tex-math> to produce the corresponding nitro anion radicals, which have been identified by their ESR spectra.

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