High-resolution dual-trap optical tweezers with differential detection: an introduction.
Single-molecule observation has come of age. Parallel developments of sensitive mechanical probes and single fluorophore detection now fuse into unique combinations, allowing investigators to examine shape and chemical transitions of single molecules with ever increasing precision and finesse. Optical trapping, using focused laser beams to constrain dielectric particles in solution (for reviews see Ref. 1), has emerged as a widely used and versatile tool to examine mechanically interesting proteins and DNA. The associated forces of light on matter can be rendered sufficiently weak that single molecules compete with them. In most applications to date, molecules of interest are attached to uniform dielectric beads, which are trapped and used as handles to configure an appropriate experimental geometry. One can detect bead position with high precision, monitoring biological activity by tracking probe displacement. Such methods allow accurate, quantitative characterization of force and displacement transients driven or experienced by single molecules, providing a unique edge in deciphering the underlying mechanisms and reaction schemes. Several biomolecules have met variants on this theme. Here, we focus attention on three classes: processive motors, nonprocessive motors, and proteins experiencing significant strain.