Structural transitions in DNA driven by external force and torque - art. no. 051903

Experiments on single DNA molecules have shown that abrupt transitions betw een states of different extensions can be driven by stretching and twisting . Here we show how a simple statistical-mechanical model can be used to glo bally fit experimental force-extension data of Leger et al. [Phys. Rev. Let t. 83, 1066 (1999)], over a wide range of DNA molecule twisting. We obtain the mean twists, extensions, and free energies of the five DNA states found experimentally. We also predict global force-torque and force-linking numb er phase diagrams for DNA. At zero force, the unwinding torque for zero-for ce structural transition from the double helix to an unwound structure is f ound to be approximate to -2k(B)T, while the right-handed torque needed to drive DNA to a highly overwound state approximate to 7k(B)T.