Radical formation in X-irradiated single crystals of guanine hydrochloride monohydrate. III. Secondary radicals and reaction mechanisms.

This work involves an ESR and ENDOR study of the reactions of the three primary radicals observed in X-irradiated single crystals of guanine hydrochloride monohydrate. Radical I, the O6-protonated anion, decays at 250 K yielding a stable room temperature radical (Radical V). The experimental evidence indicates that Radical V results from H-abstraction at N9 of a neighboring molecule. Radical II, the N7-deprotonated cation, decays at 60 K with no detectable successor. Radical IV, a C8 H-addition radical, is formed when an imidazole ring-opened radical (Radical III) decays at 150 K. The added H-atom was found to be from an easily exchangeable source. It is proposed that Radical III decays by the formation of a diamagnetic formamido molecule and an H-atom. It is important to note that Radical IV, the purine H-addition radical, is the result of basic oxidation events. Previous assumptions have been that purine H-addition radicals result either from reduction, i.e., protonation of a pristine anion, or from the "excitation pathway," by addition of H-atoms dissociated from superexcited purine bases.