Free Radical Formation in X-Irradiated Crystals of 2'-Deoxycytidine Hydrochloride. Electron Magnetic Resonance Studies at 10 k

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Single crystals of deoxycytidine hydrochloride (CdR·HCl) have been X- irradiated at 10 K with doses up to about 150 kGy and studied using 24 GHz (K-band) EPR, ENDOR and FSE spectroscopy. In this system, the cytosine base is protonated at the N3 position. Nine different radicals were characterized and identified. Three of these are ascribed to three versions of the one- electron reduced species, probably differing in their protonation state. Radicals formed by net hydrogen addition to the cytosine C5 and C6 positions were observed at 10 K. The hydrogen-abstraction radical at the deoxyribose C1' position most probably results from initial oxidation of the base. The remaining radical species are all localized to the sugar moiety, representing products formed by net hydrogen abstraction from three of the five available carbons of the deoxyribose sugar. The lack of base-centered oxidation products as well as the structures of the one-electron reduced species is rationalized by considering the specific proton donor-acceptor properties of this crystalline lattice in comparison with similar systems.