Multiple barriers in forced rupture of protein complexes

Curvatures in the most probable rupture force (f(?)) versus log-loading rate (log?r(f)) observed in dynamic force spectroscopy (DFS) on biomolecular complexes are interpreted using a one-dimensional free energy profile with multiple barriers or a single barrier with force-dependent transition state. Here, we provide a criterion to select one scenario over another. If the rupture dynamics occurs by crossing a single barrier in a physical free energy profile describing unbinding, the exponent ν, from (1 - f(?)∕f(c))(1∕ν) ～ (log?r(f)) with f(c) being a critical force in the absence of force, is restricted to 0.5 ≤ ν ≤ 1. For biotin-ligand complexes and leukocyte-associated antigen-1 bound to intercellular adhesion molecules, which display large curvature in the DFS data, fits to experimental data yield ν < 0.5, suggesting that if ligand unbinding is assumed to proceed along one-dimensional pulling coordinate, the dynamics should occur in a energy landscape with multiple-barriers.