Magnetic Tweezers for the Measurement of Twist and Torque

Abstract

Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques include so-called force spectroscopy approaches such as atomic force microscopy, optical tweezers, flow stretching, and magnetic tweezers. Amongst these approaches, magnetic tweezers have distinguished themselves by their ability to apply torque while maintaining a constant stretching force. Here, it is illustrated how such a "conventional" magnetic tweezers experimental configuration can, through a straightforward modification of its field configuration to minimize the magnitude of the transverse field, be adapted to measure the degree of twist in a biological molecule. The resulting configuration is termed the freely-orbiting magnetic tweezers. Additionally, it is shown how further modification of the field configuration can yield a transverse field with a magnitude intermediate between that of the "conventional" magnetic tweezers and the freely-orbiting magnetic tweezers, which makes it possible to directly measure the torque stored in a biological molecule. This configuration is termed the magnetic torque tweezers. The accompanying video explains in detail how the conversion of conventional magnetic tweezers into freely-orbiting magnetic tweezers and magnetic torque tweezers can be accomplished, and demonstrates the use of these techniques. These adaptations maintain all the strengths of conventional magnetic tweezers while greatly expanding the versatility of this powerful instrument.

Rotating magnetic particles for lab-on-chip applications – a comprehensive review

“…No physical movement Panel A sketches a pMTT system (p = permanent magnet) with two linear magnets. 80 Panel B sketches a cross-section of a pMTT system with a circular magnet. 80 Both pMTT systems generate a large gradient-induced force on the particle.…”

“…80 Panel B sketches a cross-section of a pMTT system with a circular magnet. 80 Both pMTT systems generate a large gradient-induced force on the particle. Panel C sketches an eMTT system (e = electromagnet) with cores of soft magnetic material.…”

“…Rotating MPs are also well-suited for studying the torsional properties of biological systems at the single-molecule level 69,80 (Fig. 1E).…”

Bioconjugation Strategies for Connecting Proteins to DNA-Linkers for Single-Molecule Force-Based Experiments

“…Single molecule force spectroscopy (SMFS) has proven to be a powerful tool to investigate the properties of individual proteins, including mechanical stability [ 1 ], ligand-binding [ 2 ] and protein folding [ 3 , 4 ]. The three most commonly used methods are optical tweezers [ 4 , 5 ] (OT), atomic force microscopy [ 6 , 7 ] (AFM), and magnetic tweezers [ 8 , 9 ] (MT), which are able to measure forces in the piconewton range. While each method has its own features and limitations [ 10 ], it is typically required that the molecule under study is attached to a probe, i.e., an optically trapped bead, a tip of the AFM cantilever or a magnetic bead ( Figure 1 ).…”

Simple Horizontal Magnetic Tweezers for Micromanipulation of Single DNA Molecules and DNA—Protein Complexes

“…Magnetic tweezers have emerged as a powerful tool for studying DNA–protein interactions at the single molecule level (1–8). They are well-suited for experiments applying controlled forces on macromolecular tethers while monitoring their extensions (the latter are subject to thermal fluctuations and, thus, measured extensions are thermally averaged).…”