Radiolysis of Uracil in the Absence of Oxygen

Abstract

The stable products resulting from the radiolysis of uracil in deoxygenated solutions have been characterized, and their yields have been measured. The G-value for the destruction of uracil varies from 4.6 at pH 5 to 1.5 at pH 9 in the presence of N20, and from 3.3 at pH 5 to 0.7 at pH 9 in the presence of N2. In both cases, a plot of the G-value against pH follows a titration curve with a pK, of approximately 7. The G-value does not depend on pH if the solutions contain 0.2 M t-butanol or oxygen. We have isolated two classes of stable products: monomeric products and dimers. The first class of products includes uracil glycols, uracil hydrates, and dihydrouracil. The dimeric products, which have not been observed before, have a variety of structures, but they all consist of two saturated pyrimidine rings joined by a single bond. The yield of dimers is more pH dependent than the yields of the monomeric products; however, the dimers are the major products under all conditions studied. In addition, the properties of the dimers depend on the pH of the solution and on the gas present. We present a mechanism which explains (i) how the different types of products arise by reactions of the transient uracil adducts with themselves and each other, and (ii) how the state of dissociation of the transient uracil adducts causes the observed pH dependence of the G-values.