Free radical formation in X-irradiated crystals of 2 '-deoxycytidine hydrochloride. Electron magnetic resonance studies at 10 K

Single crystals of deoxycytidine hydrochloride (CdR . HCl) have been X-irra diated at 10 K with doses up to about 150 key and studied using 24 GHz (K-b and) EPR, ENDOR and FSE spectroscopy. In this system, the cytosine base is protonated at the N3 position. Nine different radicals were characterized a nd identified, Three of these are ascribed to three versions of the one-ele ctron reduced species, probably differing in their protonation state. Radic als formed by net hydrogen addition to the cytosine C5 and C6 positions wer e observed at 10 K, The hydrogen-abstraction radical at the deoxyribose C1' position most probably results from initial oxidation of the base, The rem aining radical species are all localized to the sugar moiety, representing products formed by net hydrogen abstraction from three of the five availabl e carbons of the deoxyribose sugar. The lack of base-centered oxidation pro ducts as well as the structures of the one-electron reduced species is rati onalized by considering the specific proton donor-acceptor properties of th is crystalline lattice in comparison with similar systems. (C) 2000 by Radi ation Research Society.