  DNA is strong polyelectrolyte macromolecule making metal ions (counterions)
condense to a cloud around the double helix. The counterions may be localized
outside the macromolecule and inside the minor and major grooves of the double
helix. In the present work, the distribution of condensed counterions between
inner and outer regions of DNA has been studied using the approaches of
counterion condensation theory. The results have shown that the number of
counterions trapped inside the macromolecule should be greater than 0.16 per
one phosphate group. The maximal number of counterions that may be localized
inside the DNA double helix is limited to about 0.4 per one phosphate group and
it is much lower than the total number of condensed counterions. To analyze the
structure of counterion cloud the molecular dynamics simulations of
\emph{B}-DNA with K$^{+}$ counterions have been performed. The obtained number
of the counterions trapped inside the grooves of the double helix is about
0.22$\pm$0.06 per one phosphate group that agree with the model estimations.
The developed model describes general features of the structure of counterion
cloud around DNA and is able to predict the number of counterions inside the
grooves of the double helix.
