High-Force Freely-Orbiting and Torque-Wrench Magnetic Tweezers

Magnetic tweezers have been extensively applied to study the stability and interactions of nucleic acids and protein structures under versatile mechanical constraints. Most current high-resolution magnetic tweezers have a vertical design, with the sample stage between a pair of permanent magnets that generates force to the molecule of interest and an objective lens that records the bead image. This design requires a small opening in the middle of the magnets to allow light to pass through, compromising the flexibility of magnets arrangement and the force range it can generate. Previously we developed magnetic tweezers based on objective illumination, which provides high flexibility on the arrangement of magnets above the sample channel. In this study, we further show that this design enables developing high-force torque-wrench and free-orbiting magnetic tweezers, which is not easy to achieve in traditionally designed transmission-illumination. We also show that by using a reflective coverslip on the top of sample chamber, high-quality imaging of bead can be obtained using air objective lens, making it possible to directly control the sample temperature. The capabilities of the high-force torque-wrench/freely-orbiting magnetic tweezers are demonstrated with several examples.