Using atomic force microscopy (AFM) we performed dynamic force measurements of the adhesive forces in two model systems: avidin-biotin and streptavidin-biotin. In our experiments we used glutaraldehyde for immobilization of (strept)avidin on the tip and biotin on the sample surface. Such interface layers are more rigid than those usually reported in the literature for AFM studies, when (strept)avidin is coupled with biotinylated bovine albumin and biotin with agarose polymers. We determined the dependence of the rupture forces of avidin-biotin and streptavidin-biotin bonds in the range 300-9600 pN/s. The slope of a semilogarithmic plot of this relation changes at about 1700 pN/s. The existence of two different regimes indicates the presence of two activation barriers of these complexes during the dissociation process. The dissociation rates and activation energy barriers, calculated from the Bell model, for the avidin-biotin and streptavidin-biotin interactions are similar to each other for loading rates > 1700 pN/s but they are different from each other for loading rates < 1700 pN/s. In the latter case, the dissociation rates show a higher stability of the avidin-biotin complex than the streptavidin-biotin complex due to a larger outer activation barrier of 0.8 kBT. The bond-rupture force is about 20 pN higher for the avidin-biotin pair than for the streptavidin-biotin pair for loading rates < 1700 pN/s. These two experimental observations are in agreement with the known structural differences between the biotin binding pocket of avidin and of streptavidin.
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