Loading-rate dependence of individual ligand-receptor bond-rupture forces studied by atomic force microscopy

It is known that bond strength is a dynamic property that is dependent upon the force loading rate applied during the rupturing of a bond. For biotin-avidin and biotin-streptavidin systems, dynamic force spectra, which are plots of bond strength vs log,(loading rate), have been acquired in a recent biomembrane force probe (BFP) study(1) at force loading rates in the range 0.05-60 000 pN/s. In the present study, the dynamic force spectrum of the biotin-streptavidin bond strength in solution was extended from loading rates of similar to 10(4) to similar to 10(7) pN/s with the atomic force microscope (AFM). A Poisson statistical analysis method was applied to extract the magnitude of individual bond-rupture forces and nonspecific interactions from the AFM force-distance curve measurements. The bond strengths were found to scale linearly with the logarithm of the loading rate. The nonspecific interactions also exhibited a linear dependence on the logarithm of loading rate, although not increasing as rapidly as the specific interactions. The dynamic force spectra acquired here with the AFM combined well with BFP measurements by Merkel et al. The combined spectrum exhibited two linear regimes, consistent with the view that multiple energy barriers are present along the unbinding coordinate of the biotin-streptavidin complex. This study demonstrated that unbinding forces measured by different techniques are in agreement and can be used together to obtain a dynamic force spectrum covering 9 orders of magnitude in loading rate.