Multiple barriers in forced rupture of protein complexes

Curvatures in the most probable rupture force ($f^{\ast }$) 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 $\nu$, from $(1- f^*/f_c)^{1/\nu}\sim(\log r_f)$with$f_c$ being a critical force in the absence of force, is restricted to $0.5\le \nu \le 1$. 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 $\nu <0.5$, suggesting that ligand unbinding is associated with multiple-barrier crossing.