  To study the elastic properties of rod-like DNA nanostructures, we perform
long simulations of these structure using the oxDNA coarse-grained model. By
analysing the fluctuations in these trajectories we obtain estimates of the
bend and twist persistence lengths, and the underlying bend and twist elastic
moduli and couplings between them. Only on length scales beyond those
associated with the spacings between the interhelix crossovers do the bending
fluctuations behave like those of a worm-like chain. The obtained bending
persistence lengths are much larger than that for double-stranded DNA and
increase non-linearly with the number of helices, whereas the twist moduli
increase approximately linearly. To within the numerical error in our data, the
twist-bend coupling constants are of order zero. That the bending persistence
lengths we obtain are generally somewhat higher than in experiment probably
reflects both that the simulated origami have no assembly defects and that the
oxDNA extensional modulus for double-stranded DNA is too large.
