EPR and ENDOR studies of X-irradiated single crystals of deoxycytidine 5′-phosphate monohydrate at 10 and 77 K

The EPR spectra of single crystals of deoxycytidine 5'-phosphate monohydrate (5'dCMP), X-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6-H-alpha proton, and an additional small exchangeable hyperfine coupling from the N3-H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5-H-alpha and C1'-H-beta, and two small exchangeable couplings from the C4-NH2 protons. Since no hyperfine coupling to N3-H was observed, the oxidized species is believed to be the N3-H deprotonated cation. At high X-ray dose there is also evidence for both C5 and C6 H-addition radicals at 10 K. The fate of these radicals has been studied under controlled warming conditions. Attempts have been made to relate the fate of the low-temperature radicals with several radicals that have been detected previously at 77 K and at room temperature.