Figure 2.

Configurations for the measurement of force and displacement with optical tweezers. (a) Single bead or single trap geometry. The trap is static and the bead displacement x_bead measures protein displacement. (b) Three-bead assay. Both traps are static and the trapped beads displacement (x_bead) measures protein displacement. (c) Two-bead or double trap assay. The left trap is stationary and measures the force applied to the polymer. The right bead moves in steps or ramps and, for each displacement, the forces applied to the polymer and its extension are measured. (d) Force-clamp or isotonic clamp. A feedback system moves the trap to keep force on the bead constant. Trap displacements (x_trap) measure protein displacements. (e) Position-clamp or isometric clamp. The left bead detects movements of the dumbbell (x_bead), whereas the right bead moves using an AOD to oppose the detected movements. The right bead measures the force applied by the motor protein (F_motor). (f) Dynamic force spectroscopy. The molecular bond is subjected to constant loading rates and rupture forces and bond lifetimes are measured.