  We studied the structure and mechanical properties of DNA i-motif nanowires
by means of molecular dynamics computer simulations. We built up to 230 nm long
nanowires, based on a repeated TC5 sequence from crystallographic data, fully
relaxed and equilibrated in water. The unusual stacked C*C+ stacked structure,
formed by four ssDNA strands arranged in an intercalated tetramer, is here
fully characterized both statically and dynamically. By applying stretching,
compression and bending deformation with the steered molecular dynamics and
umbrella sampling methods, we extract the apparent Young's and bending moduli
of the nanowire, as wel as estimates for the tensile strength and persistence
length. According to our results, the i-motif nanowire shares similarities with
structural proteins, as far as its tensile stiffness, but is closer to nucleic
acids and flexible proteins, as far as its bending rigidity is concerned.
Furthermore, thanks to its very thin cross section, the apparent tensile
toughness is close to that of a metal. Besides their yet to be clarified
biological significance, i-motif nanowires may qualify as interesting
candidates for nanotechnology templates, due to such outstanding mechanical
properties.
