Rotational Drag on DNA: A Single Molecule Experiment

Within a single-molecule configuration, we have studied rotational drag on double stranded linear DNA by measuring the force during mechanical opening and closing of the double helix at different rates. The molecule is cranked at one end by the effect of unzipping and is free to rotate at the other end. In this configuration the rotational friction torque $\tau$ on double-stranded DNA leads to an additional contribution to the opening force. It is shown that the effect of rotational drag increases with the length of the molecule, is approximately proportional to the angular velocity of cranking, and we estimate that the torque $\tau$ is of the order of ${1k}_{B}T$ for 10 000 base pairs of DNA cranked at 2000 turns per second.