  Recent magnetic tweezers experiments have reported systematic deviations of
the twist response of double-stranded DNA from the predictions of the twistable
worm-like chain model. Here we show, by means of analytical results and
computer simulations, that these discrepancies can be resolved if a coupling
between twist and bend is introduced. We obtain an estimate of 40 $\pm$ 10 nm
for the twist-bend coupling constant. Our simulations are in good agreement
with high-resolution, magnetic-tweezers torque data. Although the existence of
twist-bend coupling was predicted long ago (Marko and Siggia, Macromolecules
27, 981 (1994)), its effects on the mechanical properties of DNA have been so
far largely unexplored. We expect that this coupling plays an important role in
several aspects of DNA statics and dynamics.
