Protonation State of Radiation-Produced Cytosine Anions and Cations in the Solid State. EPR/ENDOR of Cytosine Monohydrate Single Crystals X-Irradiated at 10 K

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Single crystals of cytosine monohydrate were X-irradiated at 10 K and examined using K-band EPR, ENDOR, and FSE spectroscopy at temperatures between 10 K and room temperature. Three radicals dominated the low-temperature spectra. Radical RI is the cytosine anion protonated at the N3-position. A large coupling to HC6 and smaller couplings to HN3 and to the two amino protons were detected and their characteristics unambiguously established the structure of this radical. Radical RII exhibits major couplings to HC5, N1, and to the nitrogen and the protons of the amino group. This radical is shown to be the N1-deprotonated cation of cytosine. No evidence for the trapping of the charged cytosine cation was obtained. Radical RIII was assigned to the cytosine C5 H-addition (cytosin-6-yl) radical formed by net H-addition to the C5-position of the cytosine base. Evidence for several minority species, of which one probably is the C6 H-addition (cytosin-5-yl) radical, was obtained. Radicals RI and RII decayed at elevated temperatures without any observable successors. These results are discussed in comparison with previous results on cytosine derivatives in crystals, frozen solutions, and the behavior of the cytosine base in irradiated DNA.