Figure 3.

Unfolding energy profile of a structural segment in a membrane protein. DFS measurements give an estimate of the distance, x_u, between the energy minimum of the folded state and the transition state (^‡) as well as the height of the transition barrier, ΔG_u^‡. The rigidity of structural segments depends on the curvature of the potential well (shown in red) in the energy profile. Using x_u and ΔG_u^‡ and assuming a parabolic shape of the potential well and a sharp transition barrier, we calculated the spring constant, κ, of each structural segment of rhodopsin and bacteriorhodopsin (Tables 1 and 2). The black dashed trace shows that an increase in x_u decreases the rigidity of the structural segment in rhodopsin and bacteriorhodopsin. In general, the height of the energy barrier can increase or decrease with an increase in x_u as shown by the black and gray dashed traces.