  B-DNA solutions of suitable concentration form left-handed chiral nematic
phases (cholesterics). Such phases have also been observed in solutions of
other stiff or semiflexible chiral polymers; magnitude and handedness of the
cholesteric pitch are uniquely related to the molecular features. In this work
we present a theoretical method and a numerical procedure which, starting from
the structure of polyelectrolytes, lead to the prediction of the cholesteric
pitch. Molecular expressions for the free energy of the system are obtained on
the basis of steric and electrostatic interactions between polymers; the former
are described in terms of excluded volume, while a mean field approximation is
used for the latter. Calculations have been performed for 130 bp fragments of
B-DNA. The theoretical predictions provide an explanation for the experimental
behavior, by showing the counteracting role played by shape and charge
chirality of the molecule.
