Irradiation at 77 K of single crystals of the 1:1 complex of cytidine and salicylic acid produces a phenoxyl radical formed by oxidation of the salicylic acid. Anisotropic hyperfine coupling tensors have been determined for this radical which are associated with theparaandorthohydrogens. No cytidine oxidation products (alkoxy or hydroxyalkyl radicals) were observed at 77 K. Following the decay of the phenoxyl radical at room temperature, four radicals were detected. These include the cytosine 5ndash;yl and 6ndash;yl radicals, formed by H addition to the cytosine ring, and an anisotropic doublet. By UV irradiation at room temperature, it is possible to convert a significant fraction of 6hyphen;yl radicals into 5hyphen;yl radicals. Hyperfine coupling andgtensors determined for the anisotropic doublet indicate that this radical is formed in theC1'hyphen;C2'region of the sugar moiety. These results indicate a shift in radiation damage away from the salicylic acid upon warming, and show that the radiation chemistry of the cocrystalline complex is different from that of the isolated bases.
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