  Motivated by the wide ranging experimental results on the conductivity of
DNA, we have investigated extraordinary configurations and chemical
environments in which DNA might become a true molecular wire, perticularly from
enhanced electronic overlaps or from small activation energies. In particular,
we examine A- vs B-DNA, the ribbon-like structures proposed to arise from
molecular stretching, the potential role of counterions in hole doping the DNA
orbitals, the possibility of backbone conduction, and the effects of water. We
find that small activation gaps observed in conductivity experiments may arise
in the presence of water and counter ions. We further discuss the role of
harmonic vibration and twisting motion on electron tight binding matrix
elements using ab initio density functional theory and model Koster-Slater
theory calculations. We find that partial cancellation between pp-sigma and
pp-pi interaction of Pz orbitals on adjacent base pairs, along with destructive
interference of phase factors are needed to explain the weak conductance of
A-DNA. Our results lead also to a physical interpretation of the angular
dependence of inter-base pair tight binding matrix elements. Furthermore, we
estimate Franck-Condon factors, reorganization energies and nuclear frequencies
essential for charge transfer rates, and find our estimated hole transfer rates
between base pairs to be in excellent agreement with recent picosecond dynamics
data.
