Optical traps have been used in a multitude of applications in biology, such as stretching DNA molecules and bacterial tails, due to their ability to study molecules in solution. In addition to being capable of applying and sensing forces, optical traps also have the ability to accurately apply and sense torques. Birefringent particles experience a torque when trapped in elliptically polarized light which has resulted in rotation rates over 400 Hz. By measuring the frequency content of the exiting beam, the rotation rates can be tracked and controlled. Here we describe an optical trap with feedback torque control that maintains rotational rates in the presence of increasing drag. As a result, this research has the potential to advance the understanding of rotary motor proteins such as F1ATPase.

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