Structural Transitions of a Twisted and Stretched DNA Molecule

We report results of a micromanipulation study of single double-helical DNA molecules at forces up to 150 pN. Depending on whether the DNA winding is allowed to relax, or held fixed, qualitatively different structural transitions are observed. By studying the transitions as a function of winding the different DNA structures underlying them are characterized; this allows us to report the first estimate of $S-\mathrm{DNA}$ helicity. A model is introduced to describe these transitions; in addition to $B-\mathrm{DNA}$, we find that four DNA states are needed to describe the experiments.