  In solution, DNA is a highly charged macromolecule which bears a unit of
negative charge on each phosphate of its sugar-phosphate backbone. Although
partially compensated by counterions adsorbed at or condensed near it, DNA
still produces a substantial electric field in its vicinity, which is screened
by buffer electrolyte at longer distances from the DNA. Such field has been
explored so far predominantly within the scope of a primitive model of the
electrolytic solution, not considering more complicated structural effects of
the water solvent. In this paper we investigate the distribution of electric
field around DNA using linear response nonlocal electrostatic theory, applied
here for helix-specific charge distributions, and compare the predictions of
such theory with specially performed fully atomistic large scale molecular
dynamics simulations. The main finding of this study is that oscillations in
the electrostatic potential distribution are present around DNA, caused by the
overscreening effect of structured water. Surprisingly, electrolyte ions at
physiological concentrations do not strongly disrupt these oscillations, and
rather distribute according to these oscillating patterns, indicating that
water structural effects dominate the short-range electrostatics. We also show
that (i) structured water adsorbed in the grooves of DNA lead to a positive
electrostatic potential core, (ii) the Debye length some 10 {\AA} away from the
DNA is reduced, effectively renormalised by the helical pitch of the DNA, and
(iii) Lorentzian contributions to the nonlocal dielectric function of water,
effectively reducing the dielectric constant close to the DNA, enhances the
overall electric field. The impressive agreement between the atomistic
simulations and the developed theory substantiates the use of nonlocal
electrostatics when considering solvent effects in molecular processes in
biology.
