Single Molecular Bonds

■ Abstract On laboratory time scales, the energy landscape of a weak bond along a dissociation pathway is fully explored through Brownian-thermal excitations, and energy barriers become encoded in a dissociation time that varies with applied force. Probed with ramps of force over an enormous range of rates (force/time), this kinetic profile is transformed into a dynamic spectrum of bond rupture force as a function of loading rate. On a logarithmic scale in loading rate, the force spectrum provides an easy-to-read map of the prominent energy barriers traversed along the force-driven pathway and exposes the differences in energy between barriers. In this way, the method of dynamic force spectroscopy (DFS) is being used to probe the complex relation between force—lifetime—and chemistry in single molecular bonds. Most important, DFS probes the inner world of molecular interactions to reveal barriers that are difficult or impossible to detect in assays of near equilibrium dissociation but that determine bond lifetime and strength under rapid detachment. To use an ultrasensitive force probe as a spectroscopic tool, we need to understand the physics of bond dissociation under force, the impact of experimental technique on the measurement of detachment force (bond strength), the consequences of complex interactions in macromolecular bonds, and effects of multiply-bonded attachments.