Certain ESR spectra observed following the reaction of OH radicals with uracils had previously been assigned to the radicals produced by addition to the 5 position followed by keto-enol tautomerism and proton dissociation. Very similar spectra, however, have recently been observed in the radiolysis of the 5-halouracils and have been assigned to the radicals produced by dehydrohalogenation of the OH adducts. The latter assignment differs from the former by two electrons in the oxidation state. The ESR parameters determined in the two cases are identical within experimental error. This fact suggests that the radicals are also identical so that the previous assignment of the spectra and the interpretation of the mechanism must be reconsidered. Studies of the effect of solute concentration and flow rate have shown that the radical produced from orotic acid results from secondary reactions and it seems likely that in the case of the other uracils the related radicals result from similar mechanisms. The secondary reaction is most probably that of OH with product 5-hydroxyuracils.

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