Effect of Compressive Force on Unbinding Specific Protein–Ligand Complexes with Force Spectroscopy

Atomic force microscopy (AFM) is used extensively for the investigation of noncovalent molecular association. Although the technique is used to derive various types of information, in almost all instances the frequency of complex formation, the magnitude of rupture forces, and the shape of the force–distance curve are used to determine the behavior of the system. We have used AFM to consider the effect of contact force on the unbinding profiles of lactose–galectin-3, as well as the control pairs lactose–KDPG aldolase, and mannose–galectin-3, where the interacting species show negligible solution-phase affinity. Increased contact forces (>250 pN) resulted in increased probabilitites of binding and decreased blocking efficiencies for the cognate ligand–receptor pair lactose–G3. Increased contact force applied to two control systems with no known affinity, mannose–G3 and lactose–KDPG aldolase, resulted in nonspecific ruptures that were indistinguishable from those of specific lactose–G3 interactions. These results demonstrate that careful experimental design is vital to the production of interpretable data, and suggest that contact force minimization is an effective technique for probing the unbinding forces and rupture lengths of only specific ligand–receptor interactions.