Nano machine pumps molecules ‘uphill’
A new molecular pump
capable of pumping other small molecules up an energy gradient has been
developed by researchers at Northwestern University in the US. The new pump is
very much like the protein pumps in living cells, and might be used to design
artificial molecular machines similar to those found in nature. Such machines
could be important for a range of applications, including synthetic muscles,
tiny robots and advanced mechanical motors.
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 that are 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, which are
vital components of life involved in transferring energy from food to a form
that is compatible with our cells," explains Paul McGonigal, who is part of 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 has already been used to create other
molecular machines. The molecule contains a linear axle capable of restricting
the motion of a ring-shaped component threaded onto it. 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 above).
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 organized 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," he says.
"The pumping
process is actually a 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, 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.
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