Identification of the products resulting from the direct effects of γ-radiation on thymidine

Abstract

The products formed upon γ-irradiation of thymidine in frozen aqueous solution have been identified. These products may all be explained in terms of the fates of now well documented primary charged radical species, and our results complement well those of previous e.s.r. studies on those primary and secondary radicals. In order to probe the mechanisms of formation of the major products, we have carried out experiments using heavy isotopes, including deuterium oxide and ¹⁸O₂. The radical cation undergoes both hydration, yielding the four isomers of 6-hydroxy-5,6-dihydrothymidine, and deprotonation at the methyl group to yield 5-hydroxymethyl-2′-deoxyuridine. As expected, the radical anion reacts primarily by protonation at C-6 to yield the (5R)- and (5S)- diastereoisomers of 5,6-dihydrothymidine and 5-hydroxy-5,6-dihydrothymidine. Excitation processes were shown to occur by the observation of the thymidine cyclobutane dimers. The release of significant amounts of thymine and 2-deoxy-D-ribono-1,4-lactone indicates the formation of radicals centred within the sugar moiety. The proposed role of the initial charged radicals in the formation of certain products has been supported by quantitative experiments carried out using compounds expected to be electron scavengers and electron donors with respect to thymidine. Finally, subsequent to our earlier paper on the e.s.r. identification of the radicals formed upon irradiation of thymidine bromohydrin in frozen aqueous solution, we have isolated the products formed on annealing and identified them as thymidine and 5-hydroxymethyl-2′-deoxyuridine. We discuss their mechanisms of formation.