  The electrostatic contribution to twist rigidity of DNA is studied, and it is
shown that the Coulomb self-energy of the double-helical sugar-phosphate
backbone contributes considerably to twist rigidity of DNA--the electrostatic
twist rigidity of DNA is found as $C_{\rm elec}\approx 5$ nm, which makes up
about 7% of its total twist rigidity ($C_{\rm DNA}\approx 75$ nm). The
electrostatic twist rigidity is found, however, to only weakly depend on the
salt concentration, because of a competition between two different screening
mechanisms: (1) Debye screening by the salt ions in the bulk, and (2)
structural screening by the periodic charge distribution along the backbone of
the helical polyelectrolyte. It is found that depending on the parameters, the
electrostatic contribution could stabilize or destabilize the structure of a
helical polyelectrolyte.

Soft Condensed Matter;Biomolecules