The products formed upon y-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 lSO,. The radical cation undergoes both hydration, yielding the four isomers of 6-hydroxy-5,6-dihydrothymidine, and deprotonation at the methyl group to yield 5hydroxymethyl-2'-deoxyuridine. As expected, the radical anion reacts primarily by protonation at C-6 to yield the ( 5 R )and (5s)-diastereoisomers of 5,6-dihydrothymidine and 5hydroxy-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-~-ribono-
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