Comparison of Experimental and Calculated Hyperfine Coupling Constants. Which Radicals Are Formed in Irradiated Guanine?

The geometries, spin density distributions, and hyperfine coupling constants (HFCC) in possible radiation products of guanine are studied through the use of density functional theory. Numerous hydrogenated, dehydrogenated, and hydroxylated radicals are examined and the coupling constants are compared to those obtained from detailed ESR/ENDOR studies on single crystals of different guanine derivatives. The calculated couplings for the guanine anion deviate from those observed experimentally. The possibility that the radical assigned experimentally to the guanine anion may in fact be guanine hydrogenated at the O6 position is discussed. The anion and its O6 protonated form can be distinguished through the experimental determination of the amino hydrogen couplings. The HFCCs calculated for the guanine cation radical do not match the experimental results. In contrast with the results for the guanine anion and cation, theory and experiment agree well for many of the observed hydrogenated and dehydrogenated radical products. Various N7-protonated guanine radicals are also examined. The extent of disagreement between experiment and high-level theoretical calculations for many of the N7-protonated guanine radicals suggests that further experimental and theoretical investigations of these radiation products are desirable.