Applied Physics

Tracking a Trap

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Science  10 Dec 2004:
Vol. 306, Issue 5703, pp. 1862
DOI: 10.1126/science.306.5703.1862a

The movement of molecular motors along nucleic acids can be detected by imaging the fluorescence of single molecules or by following the movement of attached beads in optical traps. Both methods have resolution limits of 1 to 2 nm. For optical trapping, noise from Brownian motion can be decreased by time averaging, but the other source of noise, instrumental drift, cannot; and methods such as interferometry and back-focal plane detection have been used to combat this noise. Nugent-Glandorf and Perkins have developed a differential back-focal plane detection method that reduces instrument noise. They used two diode lasers, with wavelengths of 785 and 850 nm, to follow the motion of two 200-nm polystyrene beads stuck to the same glass coverslip; they also mechanically stabilized each beam to improve pointing stability. Both bead positions drifted several nanometers in 1 min, but the differential position drifted only 0.5 nm, and the resolution was better than 0.1 nm on the millisecond time scale. They could also follow apparent motion of 0.4-nm steps (equivalent to a one-base step along the DNA helix) by stepping one beam while leaving the other in place. — PDS

Opt. Lett. 29, 2611 (2004).

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