Band-gap tunneling states in DNA.

An important issue regarding DNA electrical conductivity is the electron (hole) transfer rate. Experiments have found that this transfer rate involves quantum mechanical tunneling for short distances and thermally activated hopping over large distances. The electron (or hole) tunneling probability through a molecule depends on the length of molecule L, as e(-beta(E)L), where the tunneling betaE factor is strongly energy dependent. We have calculated betaE in DNA for poly(dA)-poly(dT) and poly(dG)-poly(dC) for the first time using a complex band structure approach. Although the DNA band gap is not exceptionally large, we find that the very large beta value near midgap makes DNA a poor tunneling conductor. The tunneling decay in DNA is more rapid than many other organic molecules, including those with a far wider gap.