Molecular machines are ubiquitous in nature and have evolved over billions of years to exploit energy from sunlight or complex chemical reactions in the body. They are made up of complicated assemblies of proteins responsible for a host of processes in living organisms, such as ion transport, ATP synthesis and cell division. In fact, our muscles are controlled by the co-ordinated movement of thousands of these machines.

“Our new molecular pump is in a sense reminiscent of the pump proteins in our cells, vital components of life involved in transferring energy from food to a form that is compatible with our cells,” explains Paul McGonigal, a researcher in Fraser Stoddart’s team at Northwestern. “We have designed a relatively simple small molecule that can also drive a system away from equilibrium with chemical energy from redox (oxidation-reduction) reactions.”

One-way valves and rings

The new pump is based on a molecule called a rotaxane, which contains a linear axle capable of restricting the motion of a ring-shaped component threaded onto it. Rotaxanes have been used to make molecular machines before now and in this work, the chemical structure of the axle is such that the rings can move in one direction via a complex mechanism that involves two one-way valves (see figure).

The machine contains several components. The first is a positively charged pyridinium unit (red) that acts as the first one-way valve. The second is a viologen unit (orange) that acts as the pump. The third is a bulky isopropylphenyl chemical group that acts as the second one-way valve (purple). Finally, the fourth component is an alkyl chain (green) that acts as the collection unit. This chain contains a chemical group at its end that is big enough to stop the rings from de-threading.

Pumping process transfers and stores energy

The machine works thanks to reduction-oxidation cycles and precisely organised non-covalent bonding interactions," explains team member Chuyang Cheng. "It pumps positively charged rings from solution and ensnares them around an oligomethylene chain. The redox-active viologen unit at the heart of this dumb-bell shaped molecular pump plays a dual role in first of all attracting and then secondly repelling the rings during redox cycling.

“The pumping process is actually way of transferring and storing energy at the molecular level,” he continues. “Part of the energy released during a reaction is siphoned off and stored in the high-energy molecules produced. In the long term, we could imagine that the energy stored by such an artificial molecular pump might be used to power another molecular machine – perhaps one that is part of an artificial muscle, for example.”

The team, reporting its work in Nature Nanotechnology doi:10.1038/nnano.2015.96, says that it would now like to be able to anchor its molecular pump in a membrane so that it pumps molecules from one side to another during operation. “Such a pump would be directly inspired by nature’s molecular machines, and especially carrier proteins,” adds McGonigal.