Pulse radiolysis studies of uracil and its derivatives. Primary species attack

The intermediates formed by ·OH, ·H and e–eq attack on the pyrimidine uracil have been observed and identified. Each shows a broad, weak absorption peaking at 385, 420 and 315 nm respectively. As predicted by stationary-state radiolysis, ·OH and ·H addition occur at the 5,6-double bond, the bleaching of this chromophore coinciding with the appearance of the adducts. Three methods for determining the rates of ·OH attack on uracil and its derivatives were compared. In uracil, rates of 6.0 × 109, 6.5 × 109 and 5.0 × 109 M–1 s–1 were obtained for 5,6-double bond bleaching, build-up of ·OH-adduct and CNS– competition, respectively. The ·OH-adduct disappears by second-order kinetics with a rate constant ∼2 × 109 M–1 s–1. The rate of ·OH attack per base decreased slightly in oligo and polyuridylic acids. This contrasts with the large decrease in the rate of e–aq attack and suggests a change in the site of ·OH attack in polynucleotides to the sugar phosphate backbone.