Fig. 4.

An unfolding energy landscape for a stable structural segment. The folded state of a stable structure segment is separated from the unfolded state by a free energy barrier. In the absence of an externally applied force (i.e., at equilibrium) the folded structure unfolds with a certain rate (k_u) by overcoming the transition state (‡). The distance from the folded state to the transition state is characterized by x_u and the height of the unfolding free energy barrier is characterized by ΔG. According to the Bell-Evans model, an externally applied force (F) tilts the energy landscape due to the mechanical energy (-F(cosθ)x), thereby lowering the free energy barrier (ΔG_F) that separates the folded state from the unfolded state and increasing the rate of unfolding (k_uF). Lowering of the free energy barrier is dependent on the pulling direction (x) and the angle (θ) of the externally applied force (F). This figure is reproduced from (13) with permission from the American Society for Biochemistry and Molecular Biology.