The application of optical tweezers methods to the study of biological systems has been one of the outstanding triumphs of the technique. Optical tweezers are partic- ularly suited to the study of the physical properties of single biological molecules due to the overlap of the characteristic length (from a fraction of a nanometre to a fraction of a micrometre) and force (from several femtonewtons to hundreds of piconewtons) scales between biomolecules and optical traps. In many experiments biomolecules are not trapped directly, but are attached to the surface of a micro- sphere that acts as a handle for the optical tweezers. Several powerful variations on the standard optical tweezers technique have been devised to enable the study of single biological molecules, including the two-bead assay [Fig. 13.1], active position- clamping and angular optical trapping (the optical torque wrench). In this Chapter, we illustrate the use of optical tweezers for single molecule biophysical studies focusing on two particular examples: the mechanics of DNA and the dynamics of motor proteins.
13.1 DNA mechanics: Stretching
13.2 DNA mechanics: Thermal fluctuations
13.3 DNA mechanics: Torsional properties
13.4 Motor proteins
13.5 Further reading