Hole traps in DNA calculated with exponential electron-lattice coupling

Pairs or triples of guanine, G, on DNA are readily oxidized, forming hole traps. Lewis et al. (J. Am. Chem. Soc. 2000, 122, 12037) have measured the free energy liberated when a hole goes from the radical cation G+ to GG or GGG. We calculated these free energies for the sequences used experimentally using a simple tight-binding model in which the transfer integral was assumed to vary linearly with the spacing between adjacent bases (J. Am. Chem. Soc. 2001, 123, 11441). Later calculations of the transfer integral indicated that it is smaller than the value we used and varies exponentially with the spacing. We have recalculated using this information, again taking into account polaron formation. Good agreement is again obtained with Lewis et al. Because the measured free energies are the differences of two energies, the effects of the environment must essentially cancel; the agreement we have found with the present calculations is thus good evidence for the hole wave function being delocalized over a number of bases.