Radiolysis of dihydrouracil and dihydrothymine in aqueous solutions containing oxygen; first- and second-order reactions of the organic peroxyl radicals; the role of isopyrimidines as intermediates

Abstract

The nature and yields of the products of radiolysis of aqueous solutions of dihydrouracil containing both N2O and O2 strongly depend on pH and dose rate. At a dose rate of 0.3 Gy s–1 and at pH 3, the major products are barbituric acid (G 2.4), labile material (G 2.8), and uracil (G 0.1). The labile material is converted into uracil upon treatment with acid and is largely composed of 5,6-dihydro-6-hydroxyuracil. At pH 7 barbituric acid is absent and uracil becomes the major product (G 4.5); some labile material is also formed (G 0.7). At pH 5 the pattern of the product distribution changes as a function of dose rate. With increasing dose rate G(uracil) decreases whereas G(barbituric acid) increases. The predominant radical formed in the dihydrouracil system is the 6-peroxyl radical, and it has been found, using pulse radiolysis with both optical and conductometric detection, that this peroxyl radical can eliminate O2–˙ to give uracil via an unstable isomeric form of the pyrimidine (isouracil). The elimination is base-catalysed so that, under alkaline conditions, uracil is the major radiolysis product. At lower pH, and also at higher dose rates, bimolecular decay of the peroxyl radical competes with the O2–˙ elimination process, barbituric acid being a specific product of the bimolecular decay route. The mechanisms of these processes are discussed.Irradiation of dihydrothymine–N2O–O2 solutions gave similar results.