  The DNA double helix is a polyanionic macromolecule that in water solutions
is neutralized by metal ions (counterions). The property of the counterions to
stabilize the water network (positive hydration) or to make it friable
(negative hydration) is important in terms of the physical mechanisms of
stabilization of the DNA double helix. In the present research, the effects of
positive hydration of Na$^{+}$ counterions and negative hydration of K$^{+}$
and Cs$^{+}$ counterions, incorporated into the hydration shell of the DNA
double helix have been studied using molecular dynamics simulations. The
results have shown that the dynamics of the hydration shell of counterions
depends on region of the double helix: minor groove, major groove, and outside
the macromolecule. The longest average residence time has been observed for
water molecules contacting with the counterions, localized in the minor groove
of the double helix (about 50 ps for Na$^{+}$, and lower than 10 ps for K$^{+}$
and Cs$^{+}$). The estimated potentials of mean force for the hydration shells
of the counterions show that the water molecules are constrained too strong,
and, consequently, the effect of negative hydration for K$^{+}$ and Cs$^{+}$
counterions has not been observed in the simulations. The analysis has shown
that the effects of counterion hydration can be described more accurately with
water models having lower dipole moments.
