Magnetic tweezers

Mechanical force is an important component of life implicit in molecular, cellular, and organismal function. Recent advanced biophysics research has demonstrated the ability to measure these forces at the single molecule level (reviewed in Tanase et al. 2007; Lionnet et al. 2012). In order to probe the interactions between molecules and thereby provide insight into biomechanical mechanism, we need to have sensitive tools. A wide variety of force-probing techniques, including atomic force microscopy (AFM) and optical and magnetic tweezers, have been developed. The first two methods are presented in this book. Briefly, in AFM, the cantilever is used to apply force to a molecule by either pulling or pushing. In contrast, the tweezers-based approaches use either tightly focused laser beams or magnetic fields to “tweeze or trap” objects. The history of single molecule magnetic tweezers methodology is presented in box 4.1 Displacement from the initial point in the trap as a function of opposing or assisting force is then used to provide distance measurements and additional insight, such as the spacing of mechanical steps. This chapter focuses on magnetic tweezers, where a “magnetic trap” is used to exert force on superparamagnetic beads that are coupled to single molecules.