  It is well-established that many physical properties of DNA at sufficiently
long length scales can be understood by means of simple polymer models. One of
the most widely used elasticity models for DNA is the twistable worm-like chain
(TWLC), which describes the double helix as a continuous elastic rod with
bending and torsional stiffness. An extension of the TWLC, which has recently
received some attention, is the model by Marko and Siggia, who introduced an
additional twist-bend coupling, expected to arise from the groove asymmetry. By
performing computer simulations of two available versions of oxDNA, a
coarse-grained model of nucleic acids, we investigate the microscopic origin of
twist-bend coupling. We show that this interaction is negligible in the oxDNA
version with symmetric grooves, while it appears in the oxDNA version with
asymmetric grooves. Our analysis is based on the calculation of the covariance
matrix of equilibrium deformations, from which the stiffness parameters are
obtained. The estimated twist-bend coupling coefficient from oxDNA simulations
is $G=30\pm1$~nm. The groove asymmetry induces a novel twist length scale and
an associated renormalized twist stiffness $\kappa_{\rm t} \approx 80$~nm,
which is different from the intrinsic torsional stiffness $C \approx 110$~nm.
This naturally explains the large variations on experimental estimates of the
intrinsic stiffness performed in the past.
