http://www.sciencemag.org/cgi/content/full/290/5496/1524b
Coaxing Molecular Devices to Build Themselves
Dennis Normile
NAGOYA--When Nagoya University chemist Makoto Fujita thinks about how
to store data in tiny spaces, he thinks big--about how nature handles
the problem. Then he tries to emulate the answer. "A living cell is
incomparably smaller than a compact disc, but its DNA carries far more
information," he says. "Handling data at the DNA's molecular scale
could lead to handheld supercomputers. But to do that we need more
efficient ways to manipulate molecules."
Fujita and others are doing exactly that through a technique called
directed self-assembly. By adroitly exploiting the chemical and
electrical bonds that hold natural molecules together, they can get
molecules to form desired nanometer-scale structures. This could lead
to computer logic and memory devices up to 100 times smaller than
their current counterparts. Self-assembled molecules might also serve
as cages to hold and deliver unstable medical compounds, and as
crucibles for chemical reactions.
The technique builds upon two properties of matter. One is the bonding
between hydrogen atoms, which holds the two strands of DNA in its
double helix. The other is the electrical attraction between
positively charged organic ions and negatively charged metal ions. The
organic ions are strategically placed on organic molecules, or
ligands, which are like the rods of Tinkertoy sets. The metal ions are
like the socketed disks that hold the rods together. Unlike
Tinkertoys, however, these metal ions and ligands assemble themselves
if mixed together in solution in the right proportions and under the
right thermodynamic conditions.
Fujita was among those who pioneered the metal-ion technique in the
early 1990s. His first construction used four simple linear ligands
and four metal ions to produce square macromolecules. More recently,
his group has built an eight-sided, three-dimensional structure made
up of two pyramids joined at their bases. The molecule is 3
nanometers across, and the cavity is big enough to hold a C60
molecule, the so-called buckyball. Fujita and other researchers have
also created a variety of grids, tubes, cages, and catenanes, which
are rings interlocked like links of a chain. "Such structures cannot
be synthesized by conventional chemical reactions, but we can very
easily construct them using directed self-assembly," Fujita says.
J. P. Sauvage, a chemist at Louis Pasteur University in Strasbourg,
France, has proposed using catenanes as computing devices. One ring
would be mechanically fixed, allowing the second ring to rotate. If
the second ring has an additional ion, it could be rotated 180 degrees
back and forth in response to an adjacent electrical charge. The
position of the ring would indicate the 1 and 0 of digital data.
The molecular cages can be constructed to have small openings through
which atoms can enter and react with other atoms, forming molecules
that are too big to escape. This arrangement, which can capture and
stabilize molecules that otherwise rapidly react and disappear if free
in solution, could be the basis for a drug-delivery system.
Donald Cram, a Nobel laureate in chemistry who is now retired from the
University of California, Los Angeles, used such a cage to capture
cyclobutadiene, a compound that briefly appears at intermediate stages
of some chemical reactions but which until then had been too unstable
for chemists to isolate. Julius Rebek, director of the Skaggs
Institute for Chemical Biology of the Scripps Research Institute in La
Jolla, California, who works on self-assembly using hydrogen atoms,
explains that the characteristics of cyclobutadiene had been
theoretically predicted but never confirmed. "By encapsulating the
molecule, [Cram kept it] stable long enough to be characterized by
nuclear magnetic resonance," he says. Aside from its value to basic
research, this trick could be used to deliver drugs that are hard to
stabilize.
Fujita predicts that the number of self-assembled molecules and their
uses will rise rapidly in the years to come. Whereas a decade ago
there were only a handful of groups working on self-assembly using
metal ions, he says, "now there are dozens. The field is advancing
very rapidly."
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