  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.

Pattern Formation and Solitons;Soft Condensed Matter;Adaptation and Self-Organizing Systems