Hydroxyl-Radical-Induced Decomposition of 2‘-Deoxycytidine in Aerated Aqueous Solutions

Abstract

The stable products resulting from γ-irradiation of 2‘-deoxycytidine in aerated aqueous solutions have been purified to homogeneity and characterized by MS and NMR analyses. The main group of products, which are similar in structure to those that arise from the •OH-mediated decomposition of thymidine, included four diastereomers of 5,6-dihydroxy-5,6-dihydro-2‘-deoxyuridine (2−5), 5-hydroxy-2‘-deoxycytidine (6), two diastereomers of N1-(2-deoxy-β-d-erythro-pentofuranosyl)-5-hydroxyhydantoin (7 and 8), and N1-(2-deoxy-β-d-erythro-pentofuranosyl)formamide (9). These products are likely formed by way of either intermediate 5(6)-hydroxy-6(5)-peroxyl radicals or the corresponding hydroperoxides. In labeling experiments, 18O2 was incorporated into products 2−9 with about 30% label at C5 and 60% at C6. Several other products were observed in the γ-irradiation of 2‘-deoxycytidine in aerated aqueous solutions. They included four isomeric nucleosides of biuret (10−13), two diastereomers of N1-(2-deoxy-β-d-erythro-pentofuranosyl)-1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine (14, 15), and aminocarbonyl[2-(2-deoxy-β-d-erythro-pentofuranosyl)amino]-2-oxoethylcarbamic acid (16). The first step in the formation of these products is probably intramolecular addition of the 5-hydroxy-6-hydroperoxide to C4 of the pyrimidine ring, resulting in a 4,6-endoperoxide which decomposes into an open-chain α-hydroxyaldehyde by C4−C5 cleavage. The formation of products 10−13 can be explained by subsequent fragmentation of the intermediate α-hydroxyaldehyde, whereas cyclization gives rise to products 14 and 15. In the case of product 16, a series of reactions have been proposed, starting with the rearrangement of the α-hydroxyaldehyde. The above pathways are supported by 18O2-labeling experiments. Finally, we characterized two diastereomers of 5‘,6-cyclo-5-hydroxy-5,6-dihydro-2‘-deoxyuridine (17, 18) in the γ-irradiation of 2‘-deoxycytidine in aerated aqueous solutions. These products likely arise from initial •OH abstraction of a proton from C5‘ of the sugar moiety, followed by intramolecular addition of the resulting radical to C6 of the cytosine moiety. The •OH-mediated decomposition of 2‘-deoxycytidine in DNA is discussed.