Radicals in 5-methylcytosine induced by ionizing radiation. Electron magnetic resonance for structural and mechanistic analyses.

Single crystals of 5-methylcytosine hemihydrate and 5-methylcytosine hydrochloride were X-irradiated and studied at 10 K and at higher temperatures using X- and K-band EPR, ENDOR and EIE spectroscopy. In the hemihydrate crystals, four radicals were identified at 10 K, one of them being the recently reported N1-deprotonated one-electron oxidation product (Krivokapić et al., J. Phys. Chem. A 113, 9633-9640, 2009). The other radicals were the 3alphaH radical and the C6 and C5 H-addition radicals (the 5-yl and 6-yl radicals, respectively). After irradiation at 295 K, only the 3alphaH and the 5-yl radicals were observed. In the hydrochloride crystals, at least seven different radicals were present after irradiation at 10 K. These were the N1-deprotonated one-electron oxidation product, the 3alphaH radical, three different one-electron reduction products, and the 5- and 6-yl radicals. DFT calculations were used to assist in assigning the observed couplings. The 3alphaH and 5-yl radicals were dominant after thermal annealing to room temperature. In neither crystal system did the N1-deprotonated oxidation product transform into the 3alphaH radical upon warming. The radical yield was significantly greater after irradiation at 300 K compared to that after irradiation at 10 K followed by warming to 300 K and was also considerably greater in the hydrochloride crystals than in the hemihydrate crystals.