Torque determination on DNA with magnetic tweezers

We deduced the torque applied on a single stretched and twisted DNA by integrating with respect to force the change in the molecule's extension as it is coiled. While consistent with previous direct measurements of the torque at high forces (F>1 pN) this method, which is simple and does not require a sophisticated set-up, allows for lower force estimates. We used this approach to deduce the effective torsional modulus of DNA, which decreases with force and to estimate the buckling torque of DNA as a function of force in various salt conditions.