More
than 50 years later, synthetic molecular switches are a dime a dozen,
but synthetically designed molecular machines are few and far between.
Northwestern
University chemists recently teamed up with a University of Maine
physicist to explore the question, “Can artificial molecular machines
deliver on their promise?” Their provocative analysis provides a roadmap
outlining future challenges that must be met before full realization of
the extraordinary promise of synthetic molecular machines can be
achieved.
The tutorial review will be published Nov. 25 by the journal Chemical Society Reviews.
The
senior authors are Sir Fraser Stoddart, Board of Trustees Professor of
Chemistry, and Bartosz A. Grzybowski, the K. Burgess Professor of
Physical Chemistry, both in Northwestern’s Weinberg College of Arts and
Sciences, and Dean Astumian, professor of physics at the University of
Maine. (Grzybowski is also professor of chemical and biological
engineering in the McCormick School of Engineering and Applied Science.)
One
might ask, what is the difference between a switch and a machine at the
level of a molecule? It all comes down to the molecule doing work.
“A
simplistic analogy of an artificial molecular switch is the piston in a
car engine while idling,” explains Ali Coskun, lead author of the paper
and a postdoctoral fellow in Stoddart’s laboratory. “The piston
continually switches between up and down, but the car doesn’t go
anywhere. Until the pistons are connected to a crankshaft that, in turn,
makes the car’s wheels turn, the switching of the pistons only wastes
energy without doing useful work.”
Astumian
points out that this analogy only takes us part of the way to
understanding molecular machines. “All nanometer-scale machines are
subject to continual bombardment by the molecules in their environment
giving rise to what is called ‘thermal noise,'” he cautions. “Attempts
to mimic macroscopic approaches to achieve precisely controlled machines
by minimizing the effects of thermal noise have not been notably
successful.”
Scientists
currently are focused on a chemical approach where thermal noise is
exploited for constructive purposes. Thermal “activation” is almost
certainly at the heart of the mechanisms by which biomolecular machines
in our cells carry out the essential tasks of metabolism. “At the
nanometer scale of single molecules, harnessing energy is as much about
preventing unwanted, backward motion as it is about causing forward
motion,” Astumian says.
In
order to fulfill their great promise, artificial molecular machines
need to operate at all scales. A single molecular switch interfaced to
its environment can do useful work only on its own tiny scale, perhaps
by assembling small molecules into chemical products of great
complexity. But what about performing tasks in the macroscopic world?
To
achieve this goal, “there is a need to organize the molecular switches
spatially and temporally, just as in nature,” Stoddart explains. He
suggests that “metal-organic frameworks may hold the key to this
particular challenge on account of their robust yet highly integrated
architectures.”
What
is really encouraging is the remarkable energy-conversion efficiency of
artificial molecular machines to perform useful work that can be
greater than 75%. This efficiency is quite spectacular when
compared to the efficiency of typical car engines, which convert only 20
to 30% of the chemical energy of gasoline into mechanical work,
or even of the most efficient diesel engines with efficiencies of 50%.
“The
reason for this high efficiency is that chemical energy can be
converted directly into mechanical work, without having to be first
converted into heat,” Grzybowski says. “The possible uses of artificial
molecular machines raise expectations expressed in the fact that the
first person to create a nanoscale robotic arm, which shows precise
positional control of matter at the nanoscale, can claim Feynman’s Grand
Prize of $250,000.”
Great Expectations: Can Artificial Molecular Machines Deliver on Their Promise?