A molecular information ratchet

In his classic thought experiment of 1867, James Clerk Maxwell imagined a tiny demon guarding a trapdoor separating two gas-filled compartments. By allowing only fast-moving molecules to pass from left to right, and slowmoving molecules right to left, the demon induces heating in the right compartment and cooling in the left. Such shifts from equilibrium violate the second law of thermodynamics. A team from the School of Chemistry at the University of Edinburgh — a cab ride from Maxwell's birthplace — has now developed a molecular 'machine' that mimics the battle of Maxwell's demon against equilibrium. The new demon is a specially designed rotaxane — a molecular ring threaded onto a central axle with binding sites where the ring can attach. Previous rotaxane machines were activated by disturbing the ring binding pattern; the ring shuttled between binding sites and moved the system back towards equilibrium. In the new rotaxane, information about ring location is used to move the system away from equilibrium. But the second law survives the assault, as it costs energy (provided as light) to gather and transfer molecular information.