  Crossover motifs are integral components for designing DNA based
nanostructures and nanomechanical devices due to their enhanced rigidity
compared to the normal B-DNA. Although the structural rigidity of the double
helix B-DNA has been investigated extensively using both experimental and
theoretical tools, to date there is no quantitative information about
structural rigidity and the mechanical strength of parallel crossover DNA
motifs. We have used fully atomistic molecular dynamics simulations in explicit
solvent to get the force-extension curve of parallel DNA nanostructures to
characterize their mechanical rigidity. In the presence of mono-valent Na+
ions, we find that the stretch modulus (\gamma_1) of the paranemic crossover
(PX) and its topo-isomer JX DNA structure is significantly higher (~ 30%)
compared to normal B-DNA of the same sequence and length. However, this is in
contrast to the original expectation that these motifs are almost twice rigid
compared to the double-stranded B-DNA. When the DNA motif is surrounded by a
solvent with Mg2+ counterions, we find an enhanced rigidity compared to Na+
environment due to the electrostatic screening effects arising from the
divalent nature of Mg2+ ions. This is the first direct determination of the
mechanical strength of these crossover motifs which can be useful for the
design of suitable DNA for DNA based nanostructures and nanomechanical devices
with improved structural rigidity.
