Hole traps in DNA

Sequences of guanines, GG and GGG, are known to be readily oxidized, formin g radical cations, i.e., hole traps, on DNA. The trapping probability of GG is less than that of GGG. Lewis et al. (J. Am. Chem. Soc. 2000, 122 12037) have used measurements on synthetic hairpins to determine the free energy liberated when a hole goes from the radical cation G(+) to GG or to GGG. Th ey find these free energies to be of the order of thermal energy at room te mperature, in contradiction to the expectation by many of much greater trap depths. We have calculated the wave function of a hole on G, on GG. and on GGG surrounded by adenines, as in the Lewis et al. experiments, using a si mple tight-binding model. We find that to account for the shallow traps fou nd by them it is necessary that the difference in ionization potentials of contiguous guanine and adenine be smaller by about 0.2 eV than the 0.4 eV f ound for isolated bases. Using this value and taking into account polaron f ormation. we find the wave functions of holes trapped on G. GG, or GGG to e xtend over similar to6 sites (bases) and with energy level differences in g ood agreement with the values found by Lewis et al.