Steven Chu:
Uncovering the secret life of molecules
BY DAVID F. SALISBURY
Ten years ago, while he
was working at AT&T Bell Laboratories, Steven Chu and
two of his colleagues invented a marvelous tool called
optical tweezers -- a kind of microscopic version of Star
Trek tractor beams. By specially tuning a laser beam, the
scientists found that they could grip and manipulate a
number of different kinds of microscopic objects immersed
in water, including bacteria.
Since coming to Stanford
in 1987, the physicist has found an interesting way to
use the capabilities of this instrument: studying the
physical properties of individual polymer molecules. In
two recently published papers, Chu and his students have
demonstrated that studying polymers one-by-one can
provide important new insights into the way in which the
properties of polymeric materials arise from the
collective action of large numbers of individual
molecules.
Polymers are large
spaghetti-like molecules that are constructed from large
numbers of identical, smaller molecules strung together
like beads on a string. Polymers figure in everything
from plastics to synthetic fabrics to the DNA in living
cells. Previously, scientists had been limited to
studying polymers in bulk, by the millions and billions.
Chu's laboratory is providing some of the first detailed
studies of the behavior of individual polymers that, not
surprisingly, are revealing that they don't act in
exactly the way that scientists had expected.
Studying the properties of
polymers in bulk is something like trying to determine
the nature of animals in a zoo using only information
about averages, Chu explained. "If someone did a
series of experiments that only measured the average
size, weight and number of legs of the animals, he would
get a distorted picture. For example, he might find that
the average number of legs on the animals is 2.7, and
then look for a theory of animal development that could
explain his finding. Only by looking at individual
animals can you get a true sense of the diversity of
species."
The study that was
published in the July 10 issue of the journal Nature
was conducted with former student Steven Quake, now an
assistant professor of physics at the California
Institute of Technology. He and Chu found that a single
strand of polymer immersed in water obeys the same simple
law of motion as a plucked guitar string. That in itself
is not so surprising, says Chu, but what intrigues him is
the remarkable precision of the molecule's adherence to
this 200-year-old law. Previously, scientists had thought
that such molecules would exhibit much more complex
behavior. "We decided to take the analogy to the
guitar string very seriously and see how well it held up.
It turned out to be much closer than we expected,"
Chu said.
In the second study, Chu
and two doctoral students also discovered that these
molecules appear to express a surprising degree of
individuality. When forced to unravel in a strong
current, apparently identical polymers unwind in highly
individual and unpredictable ways.
Guitar analogy
The idea that the motions
of a polymer can be described by a set of frequencies
corresponding to a fundamental tone and its higher
harmonics, similar to the vibrations of a musical string,
is an old one. But most researchers have considered this
simple "linear theory" to be only a rough
description of the actual motion. In the real world,
polymers are submerged in a solution and capable of
forming knots, so scientists have thought that their
behavior must be more complex and less easy to predict.
To study individual
polymer vibrations, the scientists used strands of DNA 20
microns long. (A strand of human hair is about 25 microns
across.) Previous experiments indicate that DNA strands
act as generic polymers. That is, DNA behaves in the same
way as any other polymer in these types of experiments.
It is also readily available, can be labeled with
fluorescent dye, and the researchers in Chu's lab have
successfully developed a method that allows them to
securely attach the ends of DNA to tiny plastic spheres,
enabling them to manipulate the strands using optical
tweezers.
For the harmonics study,
the researchers attached the tiny spheres, which are
about one-third of a micron in diameter, to both ends of
the DNA molecules. Then, using a pair of optical
tweezers, they gripped the spheres at each end of a DNA
strand and pulled them far enough apart to stretch the
molecules to about three-quarters of their full length.
DNA is too tiny to see
with an optical microscope. But the dyed strands showed
up clearly, so the scientists were able to videotape
their vibrations. The thermal agitation of the water
molecules, called Brownian motion, acted like tiny
fingers plucking at the strand. When the researchers
analyzed these movements, they found that they could be
described by the motion of a set of independent harmonic
tones to an accuracy of better than 1 percent. They
carried their analysis up to the eighth harmonic.
Harmonic motion first was
described by the French mathematician D'Alembert in the
1700s. He discovered that the motion of a string held
taut at both ends could be fully described by
superimposing a series of simple sine waves with
wavelengths that fit evenly into its length. In 1954
American scientist Bruno Zimm suggested that the motion
of a polymer in solution can actually be explained by
D'Alembert's mathematical description.
Normally, a scientist
would not even try to use such a linear theory to
describe the movement of a polymer in solution. If you
move one segment of a single strand that is submerged in
water, that movement generates water waves that then
exert forces on all the other segments. The force exerted
on closer segments is greater than that exerted on
segments farther away. Since the distance of the segments
depends on the instantaneous configuration of the entire
polymer, the mathematics to solve this problem becomes
intractable.
To simplify the math, Zimm
replaced actual distances between segments with average
distances. Strictly speaking this assumption is not
mathematically rigorous. In his treatment he left out a
number of complicated effects: For example, his model
allowed the polymer to pass through itself as a
"phantom-like" strand. Despite the shaky
derivation, the basic conclusion that polymer motions can
be described by a linear set of equations may still be
correct, Chu said.
Twenty years later
Pierre-Gilles de Gennes, professor of the Collège de
France and winner of the Nobel Prize for his
contributions to polymer science, emphasized that
collective polymer motion was far more complicated than
had been assumed by Zimm and others. As an alternative,
he developed a "scaling" theory that describes
the dynamics of a polymer without having to linearize the
equations that describe its motion.
"Because we can
actually see the molecules move, we can directly observe
the higher-order vibrations for the first time. When we
started this work, we sided with de Gennes and felt that
polymer motion cannot be perfectly linear. But we looked
very hard for non-linearity and found no evidence for
it," Chu added. The researchers are continuing their
search for a breakdown of the harmonic model.
How DNA strands unravel
Chu's second study,
performed with graduate students Thomas T. Perkins and
Douglas E. Smith, was published in the June 27 issue of
the journal Science.
It shows that identical polymers in identical
conditions act quite differently, indicating that small
random conditions play an unexpectedly important role in
the way polymers unravel.
In that experiment Chu and
his two doctoral students observed how immersed DNA
strands unravel when exposed to microscopic currents.
Such
currents, or flow fields, occur as a fluid passes through
any constriction or nozzle. Understanding how polymers
deform in these fields is necessary to understand how
polymers can reduce drag in pipelines and how they behave
during processes such as injection molding.
In this case, the
researchers did not use the optical tweezers. Instead,
they manufactured microscopic currents by using
microfabrication techniques to etch perpendicular flow
channels only 650 microns wide and 220 microns deep on a
small plate. Fluorescently labeled DNA molecules flowed
down one channel until they reached the center of the
cell, where they moved into a cross-current. The
researchers videotaped the molecules as they reacted to
the cross-current by unraveling to a greater or lesser
extent.
Despite taking great care
to use identical strands of DNA in identical flow
conditions, the experimenters observed a great diversity
in the way that they unraveled. "We have found that
random thermal fluctuations in the initial starting point
of the elongation get magnified into dramatic differences
in the way each molecule unravels," Chu said.
Until now theorists have
described the elongation of these molecules according to
a "mean-field" theory that assumes the
description of the average behavior is adequate. Since
all the previous polymer experiments probed the behavior
of a large collection of molecules, an average or
so-called "mean-field" theory was developed
that fit the experimental data. The new results indicate
that any such "averaged" theory is incorrect.
Instead, the observation
of thousands of individual molecules showed the
researchers that the elongation process follows a number
of different scenarios and the rate at which they unwind
depends greatly on the initial shape of the partially
coiled polymers.
An even more surprising
outcome of the experiment was that even when identically
coiled polymers are exposed to the same currents, they do
not behave in the same fashion. This unpredictable
behavior is what de Gennes has called "molecular
individualism." As the Nobel laureate states in a
comment on the Stanford paper in the same issue of
Science, "Normally, the average coil shape is enough
to describe many features. But not here."
This individualism
apparently arises from small random conditions. "We
have found that random thermal fluctuations in the
initial starting point of the elongation get magnified
into dramatic differences in the way each molecule
unravels," Chu said. "In the 'nature versus
nurture' debate, no one pays attention to the importance
of tiny random influences. Perhaps such influences play
an important role in how our lives evolve as well."
SR
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