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 (kobs 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.