Nonlinear charge transport in DNA mediated by twist modes

Recent works on localized charge transport along DNA, based on a three--dimensional, tight--binding model (Eur. Phys. J. B 30:211, 2002; Phys. D 180:256, 2003), suggest that charge transport is mediated by the coupling of the radial and electron variables. However, these works are based on a linear approximation of the distances among nucleotides, which forces for consistency the assumption that the parameter $\alpha$, that describes the coupling between the transfer integral and the distance between nucleotides is fairly small. In the present letter we perform an improvement of the model which allows larger values of $\alpha$, showing that there exist two very different regimes. Particulary, for large $\alpha$, the conclusions of the previous works are reversed and charge transport can be produced only by the coupling with the twisting modes.