Electronic coupling and charge transfers in DNA - Energy control analysis

The influence of the energetic gap on the effective distance-decay rate of electronic coupling (beta (eff)) in DNA is investigated in the context of t he superexchange mechanism. The DNA double helix is described by a tight-bi nding electronic Hamiltonian model, in which all orbitals have the same ene rgy and interact with one another through an exponentially decaying functio n of distance. Our numerical results concerning the beta (eff) values obtai ned for two different DNA molecules are analyzed within the theoretical fra mework of the "continuous-medium approximation," previously developed by Lo pez-Castillo et al. (J.-M. Lopez-Castillo, A. Filali-Mouhim, I.L. Plante, a nd J.-P. Jay-Gerin. J. Phys. Chem. 99 : 6864-6875, 1995). We find that the intervening DNA bridge between the donor and acceptor sites is defined by a unique dimensionless control parameter Gamma /E, where E is the energy of the orbitals of this medium with respect to those of the redox site orbital s (energetic gap) and Gamma is the electronic band width of the bridge cons idered as a continuous medium. In the narrow-band regime, our "through-spac e" coupling model predicts beta (eff) values that are in good order of magn itude agreement with those calculated by other theoretical approaches as we ll as with those obtained from experiment. Moreover, under equivalent energ etic conditions, the DNA-mediated transfers of holes and electrons differ c onsiderably. This difference depends upon the sign of the parameter Gamma / E.