Free radical formation in crystals of guanine hydrochloride dihydrate: an ESR and ENDOR study.

Radiation-induced free radical formation in single crystals of guanine hydrochloride dihydrate has been studied at temperatures between 20 and 300 K using ESR and ENDOR spectroscopy. At low temperatures three radical species are trapped. Two of these are the C8 H-addition radical R1 previously analysed by Alexander and Gordy (1967) and the O6-protonated anion radical R2. The third species (R4) remains unidentified. Upon annealing at 280 K for an extended period the protonated anion R2 transforms into a new radical R3 which exhibit a well-defined hyperfine pattern but still could not be identified unambiguously. Also radical R4 probably transforms into a new radical (R5) upon such treatment. One proton coupling due to R5 was detected. A scheme of radical reactions incorporating these five radicals is proposed. This scheme also suggests that differences in radical formation between the monohydrate and dihydrate crystals of guanine hydrochloride depends upon differences in the hydrogen bonding network.