Single and multiple bonds in (strept)avidin-biotin interactions

Thanks to Dynamic Force Spectroscopy (DFS) and developments of massive data analysis tools, such as YieldFinder, Atomic Force Microscopy (AFM) becomes a powerful method for analyzing long lifetime ligand-receptor interactions. We have chosen the well-known system, (strept)avidin-biotin complex, as an experimental model due to the lack of consensus on interpretations of the rupture force spectrum (Walton et al., 2008). We present new measurements of force-displacement curves for the (strept)avidin-biotin complex. These data were analyzed using the YieldFinder software based on the Bell-Evans formalism. In addition, the Williams model was adopted to interpret the bonding state of the system. Our results indicate the presence of at least two energy barriers in two loading rate regimes. Combining with structural analysis, the energy barriers can be interpreted in a novel physico-chemical context as one inner barrier for H-bond ruptures (?<1 ?), and one outer barrier for escaping from the binding pocket which is blocked by the side chain of a symmetry-related Trp120 in the streptavidin tetramer. In each loading rate regime, the presence of multiple parallel bonds was implied by the Williams model. Interestingly, we found that in literature different terms created for addressing the apparent discrepancies in the results of avidin-biotin interactions can be reconciled by taking into account multiple parallel bonds.