Distributions of Parameters and Features of Multiple Bond Ruptures in Force Spectroscopy by Atomic Force Microscopy

Force spectroscopy measurement of rupture forces of bound molecules becomes an important physicochemical tool in characterizing intermolecular interactions. Atomic force microscopy (AFM) measurements are among the most common approaches in implementation of this technique. Kinetic information about the molecular bond under study is usually extracted assuming that the detected rupture force comes from rupturing of a single bond. However, multiple bond ruptures might occur in experiments. In this article, we consider how the presence of multiple bonds is manifested in the distribution of parameters that are typically extracted in force spectroscopy experiments. Of particular interest here are the distributions of rupture forces and Kuhn lengths of polymeric tethers. We show that multiple bond ruptures might contribute to the measured distributions even when these distributions have a well-defined single peak. Also, we consider how the probability to form multiple bonds depends on probe velocity. The developed analytical models are applied to experimental data of biotin-streptavidin ruptures. The velocity dependence of the amplitude of high force tail supports the hypothesis of multiple bond nature of the measured high forces.