[Electronic coupling and charge transfer of DNA: energy control]

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 the superexchange mechanism. The DNA double helix is described by a tight-binding electronic Hamiltonian model, in which all orbitals have the same energy and interact with one another through an exponentially decaying function of distance. Our numerical results concerning the beta(eff) values obtained for two different DNA molecules are analyzed within the theoretical framework of the "continuous-medium approximation," previously developed by Lopez-Castillo et al. (J.-M. Lopez-Castillo, A. Filali-Mouhim, I.L. Plante, and 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 orbitals (energetic gap) and gamma is the electronic band width of the bridge considered as a continuous medium. In the narrow-band regime, our "through-space" coupling model predicts beta(eff) values that are in good order of magnitude agreement with those calculated by other theoretical approaches as well as with those obtained from experiment. Moreover, under equivalent energetic conditions, the DNA-mediated transfers of holes and electrons differ considerably. This difference depends upon the sign of the parameter gamma/E.