  The interaction of heavy charged particles with DNA is of interest for
several areas, from hadrontherapy to aero-space industry. In this paper, a
TD-DFT study on the interaction of a 4 keV proton with an isolated DNA base
pair was carried out. Ehrenfest dynamics was used to study the evolution of the
system during and after the proton impact up to about 193 fs. This time was
long enough to observe the dissociation of the target, which occurs between
80-100 fs. The effect of base pair linking to the DNA double helix was emulated
by fixing the four O3' atoms responsible for the attachment. The base pair
tends to dissociate into its main components, namely the phosphate groups,
sugars and nitrogenous bases. A central impact with energy transfer of 17.9 eV
only produces base damage while keeping the backbone intact. An impact on a
phosphate group with energy transfer of about 60 eV leads to backbone break at
that site together with base damage, while the opposite backbone site integrity
is kept is this situation. As the whole system is perturbed during such a
collision, no atom remains passive. These results suggest that base damage
accompanies all backbone breaks since hydrogen bonds that keep bases together
are much weaker that those between the other components of the DNA.
