The radiolysis of uracil in oxygenated aqueous solutions. A study by product analysis and pulse radiolysis

Abstract

Hydroxyl radicals are generated by the radiolysis of N₂O–O₂(4 : 1 v/v)-saturated aqueous solutions of uracil. They add to the 5,6-double bond of the substrate [80% at C(5), 20% at C(6)]. These radicals are converted by oxygen into the corresponding peroxyl radicals (I) and (II), respectively. Peroxyl radical (I) undergoes a base-induced O₂^–˙ elimination (k_obs 8 × 10³ s^–1 at pH 10.5). As an intermediate 5-hydroxy-isopyrimidine is formed which rearranges into isobarbituric acid and adds water forming 5,6-dihydro-5,6-dihydroxyuracil. Competing with this base-induced reaction of radical (I) there is a bimolecular decay of radicals (I) and (II). These processes become predominat at low pH. For this reason a strong pH dependence of G(products) is observed. The major products are (G values at pH 3 and 10 in parentheses) 5,6-dihydroxy-5,6-dihydrouracil (1.1; 2.4), isobarbituric acid (0; 1.2), N-formyl-5-hydroxyhydantoin (1.6; 0.2), 5-hydroxybarbituric acid (0.9; 0.2). 5-Hydroxybarbituric acid is formed in its keto form. Its deprotonation (k 4.4 s^–1) has been followed by pulse conductometry. Details of the reaction mechanism, e.g. the involvement of oxyl radicals in the bimolecular decay of (I) and (II), are discussed.