The stable products resulting from gamma-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'-deoxyuridin (2-5), 5-hydroxy-2'-deoxycytidine (6), two diastereomers of N-1-(2-deoxy-beta-D-erythro-pentofuranosyl)-5-hydroxyhydantoin- (7 and 8), and N-1-(2-deoxyP-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,O-18(2) was incorporated into products 2-9 with about 30% label at C-5 and 60% at C-6. Several other products were observed in the gamma-irradiation of 2'-deoxycytidine in aerated aqueous solutions. They included four isomeric nucleosides of biuret (10-13), two diastereomers of N-1-(2-deoxy-beta-D-erythro-pentofuranosyl)-1 -carbamoyl-2-oxo-4,5-dihydroxyimidazolidine (14, 15), and aminocarbonyl[2-(2-deoxy-beta-D-erythro-pentofuranosy) acid (16). The first step in the formation of these products is probably intramolecular addition of the 5-hydroxy-6-hydroperoxide to C-4 Of the pyrimidine ring, resulting in a 4,6-endoperoxide which decomposes into an open-chain alpha-hydroxyaldehyde by C-4-C-5 cleavage. The formation of products 10-13 can be explained by subsequent fragmentation of the intermediate a-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 a-hydroxyaldehyde, The above pathways are supported by O-18(2)-labeling experiments. Finally, we characterized two diastereomers of 5', 6-cyclo-5-hydroxy-5,6-dihydro- 2'-deoxyuridine (17, 18) in the gamma-irradiation of 2'-deoxycytidine in aerated aqueous solutions. These products likely arise from initial . OH abstraction of a proton from C-5' of the sugar moiety, followed by intramolecular addition of the resulting radical to C-6 of the cytosine moiety. The . OH-mediated decomposition of 2'-deoxycytidine in DNA is discussed.