http://www.sciencemag.org/cgi/content/full/290/5496/1528
Powering the Nanoworld
Robert F. Service
Whereas most scientists who long to produce nanoscale machines might
look to Henry Ford for inspiration, Devens Gust seems to have his
sights set on John D. Rockefeller. Today, researchers are making
rudimentary nanomachines by harnes-sing biological motor proteins
--such as those involved in muscle contraction--and plunking them down
on surfaces in hopes of getting them to do some new types of
work. That work takes energy. "If you're going to do this, you will
need fuel," says Gust, a chemist at Arizona State University in
Tempe. And like Rockefeller, whose Standard Oil provided the juice
that launched Ford's automotive revolution, Gust is ready to prime the
pumps.
Gust and a handful of colleagues have built tiny refineries that
convert the energy in sunlight to chemical fuel. The fuel in this case
is adenosine triphosphate (ATP), the same energy-rich molecule that
powers chemical reactions inside cells. At last August's meeting of
the American Chemical Society (ACS) in Washington, D.C., Gust reported
that he and his colleagues had collaborated with other groups to run
their protein-based molecular machines on little more than
sunlight. "They're like GM [General Motors] and Ford, and we're like
Exxon," Gust says.
Gust didn't start out to make the world's smallest gas stations. For
much of the past 15 years, he has worked with Arizona State colleagues
Thomas and Ana Moore and numerous students to mimic nature's ability
to harvest light energy--an ability on which nearly all life depends
either directly or indirectly. In 1997, Gust and the Moores reported
in Nature that they had developed a unique photosynthesis mimic inside
the protected confines of liposomes, spherical membranes made from two
layers of fatty lipid molecules. Spanning those lipid membranes are
three-part molecules called artificial reaction centers, after the
apparatus in chlorophyll that allows plants to absorb sunlight and put
that energy to work. In this case, a square-shaped porphyrin group
absorbs light, which kicks an electron out of its normal ground state
and into a higher energy level, leaving behind a positively charged
electron vacancy called a hole. The electron and hole are then snapped
up by a pair of chemical groups in the reaction centers, separating
the charges and creating a chemical potential. In a multistep process,
the electron-grabbing molecule then uses this charge separation to
shuttle protons from the outside to the inside of the liposome.
Light-harvesting bacteria and plants use a similar buildup of protons
and a protein called ATP synthase to generate ATP. And following
nature's lead, in another Nature paper in 1998, the Arizona State
researchers showed that they could incorporate ATP synthase proteins
inside their liposomes and generate ATP. As the protons pass through
the ATP synthase molecule to the outside of the liposome, they cause
the protein to spin, a mechanical motion that helps create ATP, which
is dumped outside the liposome.
That set the stage for powering nanotech devices. One such set of
devices--nanopropellers--is being developed by Carlo Montemagno of
Cornell University in Ithaca, New York, and is reported on page 1555
of this issue. Montemagno and his colleagues also use ATP synthase,
which harbors a tiny shaft that spins inside a cylinder. But in this
case they anchor copies of the protein rotor on surfaces. They then
fuse tiny metal bars to the top of the shaft, creating what looks like
a nanoscale version of a helicopter blade that rotates when it's fed
ATP.
To set these minichoppers spinning, Montemagno and his colleagues
normally just spike a solution by covering them with ATP. But if
nanomachines are ever to have a more independent future, researchers
will need simpler ways of providing them with energy. So Gust recently
teamed up with Montemagno to supply the ATP-generating liposomes. By
merely adding them to the nanocopter solution and then shining light
on them, the researchers showed that they could set the blades
spinning. Michael Therien, a chemist at the University of Pennsylvania
in Philadelphia, says he's impressed with the work. "It's a first step
to a purely engineered system" of nanomachines that can work without
human intervention, he says.
Gust has also begun teaming up with Viola Vogel and colleagues at the
University of Washington, Seattle, to help power a series of
nanoshuttles that also make use of ATP-driven biological motors to do
the work. Finally, Gust's team has adapted its artificial
photosynthesis scheme to do what plants do best: convert CO2 into more
complex molecules. At the ACS meeting, Gust reported that his team can
start with a compound called pyruvate and add the carbon from a CO2
molecule to make oxaloacetate. Gust believes he and his team may
eventually be able to coax their tiny refineries to regenerate complex
hydrocarbons such as those found in gasoline from simple sunlight and
CO2. If so, nanorefineries may one day be the key to powering both the
nanoworld and the world at large.
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