COMPARISON OF EXPERIMENTAL AND CALCULATED HYPERFINE COUPLING-CONSTANTS - WHICH RADICALS ARE FORMED IN IRRADIATED GUANINE

The geometries, spin density distributions, and hyperfine coupling con stants (HFCC) in possible radiation products of guanine are studied th rough the use of density functional theory. Numerous hydrogenated, deh ydrogenated, and hydroxylated radicals are examined and the coupling c onstants are compared to those obtained from detailed ESR/ENDOR studie s on single crystals of different guanine derivatives. The calculated couplings for the guanine anion deviate from those observed experiment ally. The possibility that the radical assigned experimentally to the guanine anion may in fact be guanine hydrogenated at the O6 position i s discussed. The anion and its O6 protonated form can be distinguished through the experimental determination of the amino hydrogen coupling s. The HFCCs calculated for the guanine cation radical do not match th e experimental results. In contrast with the results for the guanine a nion and cation, theory and experiment agree well for many of the obse rved hydrogenated and dehydrogenated radical products. Various N7-prot onated 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 desira ble.