Nature inspires first artificial molecular pump

Simple design mimics pumping mechanism of life-sustaining proteins found in living cells
May 20, 2015

A blueprint for an artificial molecular pump that acts to organize rings in a high-energy state on a polymethylene chain. The flashing energy ratchet mechanism shows how redox chemistry can be used to prime the pump with rings (top) under reducing conditions and then have it pump the rings (bottom) under oxidative conditions into a high-energy state. Hypothetical distributions of the components of the pump on a simplified potential energy surface diagram are indicated by purple dots in the reduced state and blue dots in the oxidized state. A solid green arrow indicates a surmountable energy barrier. (credit: Chuyang Cheng et al./Nature Nanotechnology)

Northwestern University scientists have developed the first entirely artificial molecular pump, in which molecules pump other molecules. The pump might one day be used to power other molecular machines, such as artificial muscles.

The new machine mimics the pumping mechanism of proteins that move small molecules around living cells to metabolize and store energy from food. The artificial pump draws power from chemical reactions, driving molecules step-by-step from a low-energy state to a high-energy state — far away from equilibrium.

While nature has had billions of years to perfect its complex molecular machinery, modern science is now beginning to scratch the surface of what might be possible in tomorrow’s world.

Imitating how nature transfers energy

“Our molecular pump is radical chemistry — an ingenious way of transferring energy from molecule to molecule, the way nature does,” said Sir Fraser Stoddart, the senior author of the study. Stoddart is the Board of Trustees Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences.

“All living organisms, including humans, must continuously transport and redistribute molecules around their cells, using vital carrier proteins,” he said. “We are trying to recreate the actions of these proteins using relatively simple small molecules we make in the laboratory.”

“In some respects, we are asking the molecules to behave in a way that they would not do normally,” Cheng said. “It is much like trying to push two magnets together. The ring-shaped molecules we work with repel one another under normal circumstances. The artificial pump is able to syphon off some of the energy that changes hands during a chemical reaction and uses it to push the rings together.”

The tiny molecular machine threads the rings around a nanoscopic chain — a sort of axle — and squeezes the rings together, with only a few nanometers separating them. At present, the artificial molecular pump is able to force only two rings together, but the researchers believe it won’t be long before they can extend its operation to tens of rings and store more energy.

Compared to nature’s system, the artificial pump is very simple, but it is a start, the researchers say. They have designed a novel system, using kinetic barriers, that allows molecules to flow “uphill” energetically.

Powering artificial muscles

“This is non-equilibrium chemistry, moving molecules far away from their minimum energy state, which is essential to life,” said Paul R. McGonigal, an author of the study. “Conducting non-equilibrium chemistry in this way, with simple artificial molecules, is one of the major challenges for science in the 21st century.”

Ultimately, they intend to use the energy stored in their pump to power artificial muscles and other molecular machines. The researchers also hope their design will inspire other chemists working in non-equilibrium chemistry.

“This is completely unlike the process of designing the machinery we are used to seeing in everyday life,” Stoddart said. “In a way, one must learn to see things from the molecules’ point of view, considering forces such as random thermal motion that one would never consider when building an agricultural water pump or any other mechanical device.”

The National Science Foundation supported the research, published May 18 in the journal Nature Nanotechnology.

Northwestern University | Artificial Molecular Pump Animation

Animation shows the steps of the pumping mechanism, which operates in response to redox cycling, with simplified illustrations of the corresponding energy profiles. The dumbbell and the ring repel each other initially, then reduction favors complexation both thermodynamically and kinetically. Oxidation restores the repulsion between the components and causes the ring to be trapped around the dumbbell during thermal relaxation. When another reduction step is performed, attraction of a second ring from the bulk solution is kinetically favored. After oxidation and thermal relaxation, the second ring falls into the same kinetic trap as the first, resulting in the mutually repulsive rings being held in close proximity to one another.

Abstract of An artificial molecular pump

Carrier proteins consume fuel in order to pump ions or molecules across cell membranes, creating concentration gradients. Their control over diffusion pathways, effected entirely through noncovalent bonding interactions, has inspired chemists to devise artificial systems that mimic their function. Here, we report a wholly artificial compound that acts on small molecules to create a gradient in their local concentration. It does so by using redox energy and precisely organized noncovalent bonding interactions to pump positively charged rings from solution and ensnare them around an oligomethylene chain, as part of a kinetically trapped entanglement. A redox-active viologen unit at the heart of a dumbbell-shaped molecular pump plays a dual role, first attracting and then repelling the rings during redox cycling, thereby enacting a flashing energy ratchet mechanism with a minimalistic design. Our artificial molecular pump performs work repetitively for two cycles of operation and drives rings away from equilibrium toward a higher local concentration.