Effects of base π-stacking on damage to DNA by low-energy electrons

In this work, we extend our earlier studies on single-strand break (SSB) formation in DNA to include the effects of base pi-stacking. In thsese studies, we consider SSBs induced by low-energy electrons that attach to DNA bases' pi* orbitals. Here, we conclude that the inclusion of pi-stacking effects causes an increase of the energy barriers (corresponding to accessing the stretched C-O bond that break in the SSB formation) that govern the rates of SSB formation. As a result, the rates of SSB formation are predicted (in the CCC codon considered here) to lie below 0.8 x 10(5) s(-1) for electrons having kinetic energies E less than or equal to 2.0 eV and thus to be not very competitive with electron autodetachment whose rate is ca. 10(14) s(-1). However, in the presence of even modest solvation, autodetachment is rendered inoperative, so SSBs can occur with considerable yield via the electron-attament pathway. In addition to these studies of sugar-phosphate C-O bond cleavage, we find that the barrier height for sugar-cytosine N-C bond breaking is 43 kcal/mol, which is much higher than the corresponding value estimated for the sugar-phosphate C-O bond breaking, which makes the N-C route not likely to be operative in such electron-induced SSBs.