The structure of the guanine cation: ESR/ENDOR of cyclic guanosine monophosphate single crystals after X irradiation at 10 K.

Following X irradiation of 3',5'-cyclic guanosine monophosphate single crystals at 10 K, several free radicals were trapped and detected by ESR/ENDOR/FSE spectroscopy. The two dominant species both have unpaired spin located on the guanine base. One is the product of net hydrogen atom loss from the exocyclic amino group. The spectroscopic characteristics of this resonance leave this assignment unambiguous. The experimental conditions make it likely that this species was formed by deprotonation of the guanine base cation. The nature of the other species is more uncertain. However, the evidence is consistent with the assignment that it is a net OH adduct to the C4 position of the base. Several species in which the unpaired spin was located on the sugar-phosphate region of the molecule were also observed. The mechanisms for the decay of the primary radicals, also leading to the well-known C8 hydrogen addition radical of the guanine base, are described and discussed.