Molecular machines: knowledge is power!

T he molecular ratchet is a seductive concept for nanoscientists. If particles can be made to travel past a barrier in just one direction, pressure will build at the receiving end, which can, in principle, be used to perform useful work. A springloaded one-way gate that fast-moving molecules can push open and pass through would be especially attractive. Th e energy needed to create the pressure diff erential could simply be drawn from random thermal motion. Sadly, such a system — an archetypal perpetual motion machine — is impossible. Experiment1 and theory2,3 both show that molecular ratchets move with equal ease in either direction, however asymmetric they may seem. Th is is hardly surprising since the overall process (conversion of ambient thermal energy to work) contravenes the Second Law of Th ermodynamics4. Suppose, however, that the gate can be opened deliberately and selectively for molecules approaching from one side. In James Clerk Maxwell’s classic thought experiment, a miniature demon acts as a gatekeeper between two compartments containing molecules in random motion. Th e demon can perceive molecules heading for the gate, and can open it to let them through. He chooses to do this only for molecules in one of the compartments. As a result, molecules accumulate in the other compartment, creating unequal pressures and shift ing the system from equilibrium. Although Maxwell originally thought otherwise, it turns out that the demon does not break the Second Law, because in performing his task he must dissipate energy5. Maxwell’s demon is thus a theoretically viable agent for transforming energy into work on the molecular scale. Th e problem, of course, is that demons of the right type are hard to fi nd. Now, writing in Nature, David Leigh and co-workers from the University of Edinburgh show that a molecular system with some similarity to Maxwell’s can be put into practice6. Popular with researchers who make synthetic molecular machines are rotaxanes — assemblies consisting of ‘rings’ threaded onto ‘axles’ and kept in place by large blocking groups. In Leigh’s case, his rotaxane is an organic molecule, with a single ring trapped on an axle (Fig. 1). Th e axle possesses three additional features, a ‘gate’ (depicted as a door in Fig. 1) — which, when closed, prevents the ring from passing from one side of the axle to the other — and two binding sites around which the ring preferentially sits. Th e affi nity of the ring for the left -hand binding site is slightly greater than for the one on the right and so in a large ensemble of these molecules at thermodynamic equilibrium, 65% of them will have rings sitting on the left and 35% will have them on the right. Th e special feature of this system is that when it is exposed to ultraviolet light, the distribution of the rings changes — a greater proportion of them become trapped on the right-hand side of the molecule and the system is pushed away from its equilibrium position. Th e key to this is the operation of the gate. In molecular terms, the gate is an alkene (a carbon–carbon double bond) that A molecular motor inspired by Maxwell’s demon can be driven away from equilibrium using the information provided by the location of one of its interlocked components. MOLECULAR MACHINES