Dynamics of single-molecule force-ramp experiments : The role of fluctuations

In the force-ramp mode of the atomic force microscope, the force with which a macromolecule is stretched is increased linearly in time by properly controlling the motion of the cantilever through a feedback loop. Using a master equation approach for the coupled cantilever-macromolecule system, we mimic such a feedback loop, to study nonequilibrium effects in the measurements of force-extension curves and fluctuations. In particular, it is shown that the fluctuations are the same for force-ramp experiments and for the more commonly used constant velocity experiments. Thus the exact same statistics suffice for the explanation of either experiment. Specific results are presented for the stretching of Dextran.