  Recent experimental evidence shows that the pi-orbitals along the stacking of
base pairs can facilitate the long-range charge transfer in DNA. Proton motion
in the base pair hydrogen bonds has also been found to affect the transfer
rate. To explain this behavior we propose a model considering interactions of
doped charges with hydrogen bonds and vibrations in DNA. The charge trapped by
either protons or vibrations can cause structural distortions leading to
polaron formation. By further considering polaron diffusion in DNA we find that
the charge transfer rate derived from the diffusion coefficient is in agreement
with the experimental results.
