Lora Nugent-Glandorf and Tom Perkins have come up with an optical trap motion detector that can "see" protein motors moving one base at a time along a DNA helix. For some time scientists have been able to make optical traps that can track the movement of attached beads, but the method had a resolution of 1-2 nanometers, which was not sensitive enough to resolve .338 nm DNA base steps. The lack of resolution was mostly due to instrument drift.
To address this problem, Nugent-Glandorf and Perkins developed a differential, back-focal plane detection method. They used two different diode lasers with wavelengths of 785 and 850 nm to track the movements of two 200 nm polystyrene beads stuck to a glass cover slip. They mechanically stabilized each laser. They were then able to monitor a drift in the bead positions of ~6 nm in 1 minute; however, the differential position of the beads drifted only .5 nm. On a time scale of one millisecond, the resolution was better than .1 nm. When they left one bead in place, they were able to follow the apparent motion of the other bead when it was moved in .4 nm steps. The experiment had a signal-to-noise ratio of 25.
The Perkins's group plans to use their new technique, which removes instrumental noise from their system, to study the movement of a helicase (one type of protein motor) as it travels along a strand of DNA, unzipping the double helix. Their work was published in Optics Letters in November 2004 and highlighted as an Editor's Choice in Science the following month. - Julie Phillips