Unzipping DNA with optical tweezers: high sequence sensitivity and force flips.

Force measurements are performed on single DNA molecules with an optical trapping interferometer that combines subpiconewton force resolution and millisecond time resolution. A molecular construction is prepared for mechanically unzipping several thousand-basepair DNA sequences in an in vitro configuration. The force signals corresponding to opening and closing the double helix at low velocity are studied experimentally and are compared to calculations assuming thermal equilibrium. We address the effect of the stiffness on the basepair sensitivity and consider fluctuations in the force signal. With respect to earlier work performed with soft microneedles, we obtain a very significant increase in basepair sensitivity: presently, sequence features appearing at a scale of 10 basepairs are observed. When measured with the optical trap the unzipping force exhibits characteristic flips between different values at specific positions that are determined by the base sequence. This behavior is attributed to bistabilities in the position of the opening fork; the force flips directly reflect transitions between different states involved in the time-averaging of the molecular system.