Counting and Breaking Individual Biological Bonds: Force Spectroscopy of Tethered Ligand-Receptor Pairs

Force spectroscopy provides a direct approach for probing biological interactions at the single-molecule level. Tethered systems, in which flexible polymer linkers connect the interacting molecules to the surfaces of the atomic force microscope probe and sample, provide a particularly attractive platform for studying such interactions. We will review the basic physical principles of force spectroscopy measurements in these systems, and show that mechanical properties of the tether linkages allow independent determination of the bond rupture forces and the number of ruptured bonds. Forces measured in these systems obey the predictions of a Markovian model for the strength of multiple parallel bonds. Finally, we discuss the use of the dynamic force spectra of single and multiple protein-ligand bonds for determination of kinetic parameters for multivalent interactions.