Light Drives Molecular Piston

Hank Hogan

Advancing the development of micromachines, researchers from the Netherlands, Italy and the UK have constructed the world's smallest piston. Powered by laser light, the molecule-size piston can move objects a few nanometers at a time and may find applications as an interface with biological systems.

Researchers have developed a laser-driven molecular motor. The application of a 2-ns pulse of 355-nm light causes the ring to shuttle from one end of the molecule to the other in 1 µs. Relaxation to the starting point takes 100 µs.

The new motor is a variant of rotaxane, a molecule that looks like a dumbbell threaded through a ring. The rotaxane piston starts with the ring at one end of the dumbbell, and the application of 355-nm light causes the ring to shuttle to the opposite hydrogen-bonding station. The researchers started the piston moving by illuminating it with an Infinity Q-switched, frequency-tripled Nd:YAG laser from Coherent Inc. in a solution of acetonitrile at room temperature.

A 2-ns laser pulse induced the ring to shuttle from one end to the other in a microsecond, analogous to the power stroke of a piston. After approximately 100 µs, the ring had returned to its original position, similar to a piston's recovery stroke. They found that the process is cyclable and produces 10^-15 W per stroke at 10,000 Hz.

Scientists have constructed other molecular machines and motors, but these systems have been larger, more complicated and more difficult to produce. Albert M. "Fred" Brouwer, one of the researchers on the team and a professor at the University of Amsterdam's Institute of Molecular Chemistry, cited other differences.

"Our rotaxane is unique in the rapidity of the photoinduced shuttling motion," he said. According to the research team, which reported its work in the March 16 issue of Science, other light-powered shuttles take minutes to hours to complete a cycle. Moreover, he said, the researchers have thoroughly dissected the mechanics of the rotaxane motion with spectroscopy, so they have a clear idea of all of the molecular piston's motions.

More tests planned

Don't expect to see miniature motors purring along soon, however. To date, the work has been done with freely moving molecules in solution. In theory, the pistons generate much more power than other biological molecules, but this power has yet to be put to work. That will require attaching the rotaxane to a surface or embedding it in a membrane.

It is unclear what impact this will have on the shuttling motion. Further research is under way to answer such questions, including a test to see if the shuttling phenomenon will occur in a more viscous medium to simulate the effect of a load on the motor.