Following single molecules by force spectroscopy

Dynamic force spectroscopy of single molecules, in which an adhesion bond is driven away from equilibrium by a spring pulled with velocity V, is described by a model that predicts the distribution of rupture forces (mean and variance), all amenable to experimental tests. The distribution has a pronounced asymmetry, which has recently been observed experimentally. The mean rupture force follows a (lnV)^2/3 dependence on the pulling velocity and differs from earlier predictions. Interestingly, at low pulling velocities a rebinding process is observed whose signature is an intermittent behavior of the spring force that delays the rupture. Based on the rupture mechanism, we propose a new "pick-up-and-put-down" method to manipulate individual molecules with scanning probes. We demonstrate that the number of molecules picked up by the tip and deposited at a different location can be controlled by adjusting the pulling velocity of the tip and the distance of closest approach of the tip to the surface.