Gamma-radiolysis of poly(U) in aqueous solution. The role of primary sugar and base radicals in the release of undamaged uracil.

p6e release of undamaged uracil has been measured after gamma-irradiation of aqueous solutions of poly(U). By varying the experimental conditions, the respective efficiencies with which OH, H and e-aq release uracil were found to be 51, 28 and about 3 per cent. From the determination of G(H2) in acid solutions it is concluded that greater than or equal to 95 per cent of the H atoms add to the base moiety. It is proposed that the H-adduct radicals (and similarly the OH-adduct radicals) react with the sugar moiety of a different nucleotide. Subsequent reactions of the sugar radicals so formed lead to the release of undamaged uracil. In N2O-saturated solutions the addition of tetranitromethane (TNM) causes a sharp drop in released uracil from G = 2.9 to G = 0.19. The strong suppression of uracil release by TNM is presumably due to its scavenging of the major base radical with the remaining G(uracil release) being due to a direct attack of OH at the sugar moiety. In contrast, G(uracil release) from 5'-UMP gamma-irradiated under N2O is only 0.39, dropping to 0.31 in the presence of TNM. This indicates the important role of the polymeric structure in the mechanism leading to efficient base release. These results on base release are compared with reported data on strand breakage.