Comprehensive analysis of DNA strand breaks at the guanosine site induced by low-energy electron attachment.

To elucidate the role of guanosine in DNA strand breaks caused by low-energy electrons (LEEs), theoretical investigations of the LEE attachment-induced C-O sigma-bonds and N-glycosidic bond breaking of 2'-deoxyguanosine-3',5'-diphosphate (3',5'-dGMP) were performed using the B3LYP/DZP++ approach. The results reveal possible reaction pathways in the gas phase and in aqueous solutions. In the gas phase LEEs could attach to the phosphate group adjacent to the guanosine to form a radical anion. However, the small vertical detachment energy (VDE) of the radical anion of guanosine 3',5'-diphosphate in the gas phase excludes either C-O bond cleavage or N-glycosidic bond breaking. In the presence of the polarizable surroundings, the solvent effects dramatically increase the electron affinities of the 3',5'-dGDP and the VDE of 3',5'-dGDP(-). Furthermore, the solvent-solute interactions greatly reduce the activation barriers of the C-O bond cleavage to 1.06-3.56 kcal mol(-1). These low-energy barriers ensure that either C(5')-O(5') or C(3')-O(3') bond rupture takes place at the guanosine site in DNA single strands. On the other hand, the comparatively high energy barrier of the N-glycosidic bond rupture implies that this reaction pathway is inferior to C-O bond cleavage. Qualitative agreement was found between the theoretical sequence of the bond breaking reaction pathways in the PCM model and the ratio for the corresponding bond breaks observed in the experiment of LEE-induced damage in oligonucleotide tetramer CGTA. This concord suggests that the influence of the surroundings in the thin solid film on the LEE-induced DNA damage resembles that of the solvent.