Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Studies of X-Irradiated Crystals of Cytosine Hydrochloride. Part I: Free Radical Formation at 10 K After High Radiation Doses

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Anhydrous single crystals of cytosine hydrochloride (protonated at N3) have been X-irradiated at 10 K and studied using K-band EPR, ENDOR and FSE spectroscopy. At least seven radicals were present at 10 K after X irradiation with a dose of about 150 kGy. Two different protonation states of the one-electron reduced cytosine cation were observed: an amino-protonated species (RI) and the pristine one-electron reduced species (R2) with zero net charge. Apparently three deprotonated versions of the one-electron oxidized cytosine cation were formed: the amino-deprotonated cation (R3), an N3- deprotonated cation (R4) and an N1-deprotonated cation (R5). Finally, two products formed by net hydrogen addition to the cytosine base were observed: a C5 hydrogen-addition radical (R6) and a C6 hydrogen-addition radical (R7). The crystalline lattice of cytosine hydrochloride is characterized in part by a cytosine base initially protonated at the N3-position, thus forming a cytosine base cation, and in part by an extended network of hydrogen bonding involving the chlorine anions. Proton transfer properties of pristine one- electron oxidation and reduction base products in this lattice are discussed and are suggested as explanations of the unusual multitude of positions for deprotonation of the one-electron oxidized species as well as for the two protonation states of the reduction product observed. The magnetic parameters for the amino-protonated species R1 agree well with those extracted from previous studies of cytosine derivatives in frozen solutions and in various glasses.