Introducing the Electromagnetic Torque Tweezers

Abstract

Most single-molecule manipulation methods, including atomic force microscopy, optical tweezers, and magnetic tweezers, report on parameters such as force and extension. Recently several techniques have been developed that permit one to additionally monitor rotational motion and torque. Magnetic torque tweezers (MTT) (Lipfert et al., Nature Methods 2010) are one such technique. Current MTT schemes however have the drawback that the force and torque degrees of freedom are intrinsically coupled.Here, we present the electromagnetic torque tweezers (eMTT), a novel instrument that decouples the force and torque degrees of freedom. The eMTT combines a vertically-oriented, cylindrically-shaped permanent magnet that provides a strong field gradient and enables the application of a stretching force, with two pairs of Helmholtz geometry that produce nearly homogenous magnetic fields in the horizontal plane. We use these in-plane fields to rotate the bead about the vertical axis, to accurately set the stiffness of the magnetic torsional trap, and to vary its stiffness at will. We fully calibrate and characterize the eMTT, showing how it allows one to span the range between conventional and freely orbiting magnetic tweezers with the turn of a knob, and demonstrate its implementation on DNA and other biopolymers.View Large Image | View Hi-Res Image | Download PowerPoint Slide