Active Force Clamp Control of Optical Tweezers

Abstract

In a typical high-resolution optical tweezers (OT) experiment a molecular motor changes the contour length of a trapped dumbbell-construct. Unless the inter-trap distance is actively controlled the OT increases the load on the molecular motor as it steps along the template. To counter this phenomenon we implement a real-time controller for the OT to be used in constant force measurements.We trap a dumbbell construct (bead-DNA-bead) in an inverted microscope by dividing a CW laser beam into a stationary trap and a steerable trap. Separate low power detection lasers and position sensitive detectors in the back-focal plane measure the position of both beads. The position of the bead in the stationary trap is used for constant-force feedback control. The feedback algorithm runs a Proportional-Integral-Derivative-controller on a field programmable gate array, and acousto-optical deflectors update the steerable trap position at a rate of 200 kHz.We test the force clamp control with a 10kb dsDNA molecule and present a theory explaining the power spectrum of the force clamped bead's position. We study the effect of controller bandwidth by digitally filtering the signal used for feedback control, and test the response time of our real-time controlled optical tweezers with a RNA hairpin opening/closing reaction.