Molecular motor turns both ways
By constructing the first artificial molecular motor that can be made to turn in different directions, UK chemists have gained an insight into how the energy is used in such systems, and why they don't violate the second law of thermodynamics.
The molecular motor that David Leigh and colleagues at the University of Edinburgh constructed is essentially a circular train set with two stations separated by two signals (2). The track is a large macrocycle, catenated with a smaller cycle (the train) that can stop at either of the two stations. To drive the train around the track, the researchers have a set of four operations at their disposal: shifting the train's affinity towards the stations in favour of one or the other, and switching the two signals such that only one side or the other of the circle is open for traffic.
For example, if the train starts from north station, the chemists can move it clockwise by opening the eastern signal (and closing the western one), then increasing the attractiveness of the south Station, waiting for the train to arrive there, then switching the signals again, and finally decreasing the southern affinity such that the train returns to the north. The reverse operations would make the train run anticlockwise.
Like the molecular motors in the cell, this chemical train set is driven forwards only by Brownian motion and the exclusion of unwanted alternatives. Still, the authors insist that it is not a perpetuum mobile. If it were made to do work against an opposing force, the energy would have to come from the affinity-changing reactions.
1 R D Astumian, Sci. Am., July 2001, 56
2 J V Hernández et al, Science, 2004, 306, 1532