Fig. 1.

(A) Double optical tweezer force clamp setup for the study of equilibrium protein dynamics, with soft traps approximating a constant tension F. (B) To define linear response functions, consider an individual component at tension F. A small additional oscillatory force f_L exp(-iωt) applied at the left end leads to endpoint oscillations with amplitudes z_L = J_self,L(ω)f_L and z_R = J_cross(ω)f_L, which defines the self- and cross-response functions. (C) Two objects X and Y connected in series behave as a composite object XY whose response functions can be derived through simple rules (Eq. 2) from the individual X and Y response functions. (D) Schematic representation of the optical tweezer setup consisting of beads (B), double-stranded DNA handles (H), and protein (P), with connecting springs.