ABSTRACT
Mechanical force is an important component of life implicit in molecular, cellular, and organismal function. A wide variety of force-probing techniques, including atomic force microscopy (AFM) and optical and magnetic tweezers have been developed. This chapter presents the principles underlying magnetic tweezers for probing single molecules, along with advantages and limitations of the method. The major feature of magnetic tweezers is that the force measurements are done directly without any calibration of the sensor. This property is the inherent feature of the instrument in which force is measured from analysis of the Brownian motion of the tethered bead. The instrument-dependent resolution limits are determined by the charge-coupled device (CCD) and bead-illumination characteristics. The systematic error limitation can come from the sample drift or the finite integration time of the CCD camera, and the effects of these errors arise when the applied force increases.
