A new twist on magnetic tweezers

Like their optical-tweezer cousins, magnetic tweezers have become a standard tool for stretching individual biological molecules to gain insight into their physical properties and behavior. Magnetic tweezers (and optical ones, too) can also apply torques to rotate or twist specimens. Last year, Sean Sun of the Johns Hopkins University and colleagues developed a technique to control and measure the applied torque with magnetic tweezers. In their approach a biomolecule, such as DNA, was connected to the middle of a 2-µm-long nanorod that was magnetically latched at one end to a superparamagnetic bead. In a dipole magnetic field, the bead held the nanorod and the top of the molecule in place while the experimenters introduced a controlled amount of twist by rotating the substrate bound to the molecule’s other end. Nynke Dekker and her coworkers at the Delft University of Technology have now presented a new take on magnetic torque tweezers, this time using standard spherical beads. A nonmagnetic bead 1 µm in diameter served as a landmark on the 2.8-µm-diameter superparamagnetic bead to which it was attached (a third, nonmagnetic reference bead corrected for mechanical drift). The larger bead was tethered to a substrate by a single DNA molecule, and the beads were placed in a slightly asymmetric dipole magnetic field. Rotating the magnet twisted the DNA, and by monitoring the beads’ orientation the team could extract the DNA’s torsional stiffness and the torque. With their tweezer setup, Dekker and colleagues could document torque-induced twisting, buckling, and denaturing of DNA under a wide range of torques and stretching forces and could study protein-DNA interactions. (J. Lipfert et al., Nat. Meth. , in press, doi:10.1038/nmeth.1520.)