Hydroxyl radicals and purine and pyrimidine radical ions were observed by ESR in γ-irradiated deaerated DNA, containing about an equal weight of water, at -178°C. During thermal annealing, the disappearance of the hydroxyl radicals and the growth of a doublet signal with a splitting of 133 G occurred in the same temperature range. This doublet signal could not be observed during thermal annealing of dry DNA. The doublet signal, which could be assigned either to a phosphate-associated radical or to a formyl radical, is likely to be related to damage in the deoxyribose moiety.

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