A catalysis-driven artificial molecular pump

All biological pumps are autonomous catalysts; they maintain the out-of-equilibrium conditions of the cell by harnessing the energy released from their catalytic decomposition of a chemical fuel(1-3). A number of artificial molecular pumps have been reported to date(4), but they are all either fuelled by light(5-10) or require repetitive sequential additions of reagents or varying of an electric potential during each cycle to operate(11-16). Here we describe an autonomous chemically fuelled information ratchet(17-20) that in the presence of fuel continuously pumps crown ether macrocycles from bulk solution onto a molecular axle without the need for further intervention. The mechanism uses the position of a crown ether on an axle both to promote barrier attachment behind it upon threading and to suppress subsequent barrier removal until the ring has migrated to a catchment region. Tuning the dynamics of both processes(20,21) enables the molecular machine(22-25) to pump macrocycles continuously from their lowest energy state in bulk solution to a higher energy state on the axle. The ratchet action is experimentally demonstrated by the progressive pumping of up to three macrocycles onto the axle from bulk solution under conditions where barrier formation and removal occur continuously. The out-of-equilibrium [n]rotaxanes (characterized with n up to 4) are maintained for as long as unreacted fuel is present, after which the rings slowly de-thread. The use of catalysis to drive artificial molecular pumps opens up new opportunities, insights and research directions at the interface of catalysis and molecular machinery.

Automated summary

This study introduces an autonomous chemically fueled information ratchet that continuously pumps crown ether macrocycles from bulk solution onto a molecular axle without the need for further intervention. By tuning the dynamics of the processes, the molecular machine is able to move macrocycles from their lowest energy state in the solution to a higher energy state on the axle, maintaining out-of-equilibrium [n]rotaxanes as long as unreacted fuel is present.