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 pl ots 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 b iotin-streptavidin bond strength in solution was extended from loading rate s of similar to 10(4) to similar to 10(7) pN/s with the atomic force micros cope (AFM). A Poisson statistical analysis method was applied to extract th e magnitude of individual bond-rupture forces and nonspecific interactions from the AFM force-distance curve measurements. The bond strengths were fou nd to scale linearly with the logarithm of the loading rate. The nonspecifi c interactions also exhibited a linear dependence on the logarithm of loadi ng rate, although not increasing as rapidly as the specific interactions. T he dynamic force spectra acquired here with the AFM combined well with BFP measurements by Merkel et al. The combined spectrum exhibited two linear re gimes, consistent with the view that multiple energy barriers are present a long the unbinding coordinate of the biotin-streptavidin complex. This stud y demonstrated that unbinding forces measured by different techniques are i n agreement and can be used together to obtain a dynamic force spectrum cov ering 9 orders of magnitude in loading rate.