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Optical tweezers provide insights into polymer physics using DNA
STANFORD -- How does Crazy Glue work? Why does Silly Putty bounce when thrown but melt when it sits on a tabletop? What can you learn about both by uncoiling and stretching individual strands of DNA?
All these materials are polymers and researchers in Professor Steven Chu's physics laboratory at Stanford University have begun providing new insights into their physical properties at the molecular level. They are doing so with a recently developed tool called "optical tweezers," which allow scientists to directly manipulate individual molecules - in this case strands of polymer - for the first time.
Polymers are long spaghetti-like molecules that are made of large numbers of smaller chemical units called monomers. Plastics, synthetic fibers and a number of biological materials are made from polymers. Previous studies on the physical characteristics of polymers have been limited to examinations of the bulk properties of these materials.
"Although these traditional methods have provided a great deal of insight, there are a lot of questions that can only be answered by the direct observation of single molecules. Besides, there is something very satisfying in seeing individual atoms and molecules: They are not so small after all," Chu said.
In two papers that appear in the May 6 issue of the journal Science, Chu's group reports on the results of detailed studies of the way in which individual strands of polymer snap back, or relax, after they are fully extended. These results also provide new support for an elegant yet somewhat controversial theory for how individual polymers move within a dense tangle of polymers.
The development of optical tweezers, which made these studies possible, is an offshoot of the use of lasers to cool down groups of atoms to extremely low temperatures (a few tens of millionths of degrees above absolute zero) developed by Chu's group at AT&T Bell Laboratories in 1985. By precisely tuning their lasers, the scientists were able to create a condition that they dubbed "optical molasses" because the motion of atoms within were slowed from supersonic velocities down to a few centimeters per second. Once they had slowed atomic motions down to a crawl, they were able to use another laser beam to trap groups of atoms for relatively long periods of time. Then, in 1986, they successfully demonstrated that a laser beam can be used in similar fashion to trap and manipulate much larger, but still microscopic, particles immersed in water.
"It's like a video game. We move the beam around with a joystick. It's as if you were down there at the molecular scale, gripping and moving things with your hands," said graduate student Thomas T. Perkins.
Although the optical tweezers work very well on tiny plastic spheres a 25,000th of an inch in diameter, which are too small to be seen by the eye but clearly visible in an optical microscope, the effect does not work directly on individual molecules like polymer chains floating in water. So the physicists began searching for a way to attach polymer chains to the spheres. They finally adapted a method used by biotechnologists that allowed them to attach strands of viral DNA to a sphere's surface. In addition to being the master molecule of life, DNA happens to be a polymer. They also found an existing dye that attaches selectively to DNA and makes it fluoresce brightly enough to be seen clearly with a high resolution video camera. In addition, the DNA was readily available in a variety of lengths ranging from four to 40 times the width of the spheres.
"Basically, we are using DNA as a representative of all polymers. According to theory, a number of the properties of polymers depend on their length and flexibility, not on their chemical makeup," said Douglas E. Smith, graduate student and team member.
In the first of two papers, the researchers measured the time it took DNA strands of different lengths to snap back, or relax, after they were extended to their full length. To slow the relaxation times so that they could be measured, they immersed the DNA strands in a sugar-water solution with the consistency of pancake syrup. Next, they stretched a polymer out by holding the plastic sphere in place with the optical tweezers while moving the microscope stage sideways. When the DNA chain was fully extended, they stopped the stage and allowed the polymer to return to its undisturbed length.
Existing theory, called dynamic scaling, maintains that the time it takes for polymers to relax varies as a power of the length. If the power were two, for instance, then a polymer with strands twice as long as a second polymer would take four times longer to relax.
The researchers report that they found that individual strands of DNA behave in a manner consistent with this theory. In this case, the relaxation times varied as a power of 1.6 with the length, so a polymer strand twice as long as another took about 3 times as long to relax while one three times the length took 5.8 times longer.
The researchers made some interesting additional observations. They found, for example, that the free end of a polymer strand recoiled rapidly until it had shrunk to about 70 percent of its full length, after which it continued to coil up at a much slower rate. They also observed that sometimes the DNA coiled into a compact ball after it was released rather than thickening as it shrinks.
The second group of experiments provided descriptive support for a theory for the movement of tangled bunches of polymers. Developed by 1991 Nobel laureate Pierre-Gilles de Gennes of the College de France, it proposes that individual polymers in such materials must move in a fashion similar to that of a snake crawling through a large tangle of other snakes. The only way it can move easily is forward or backward along its length. Sideways motions are restricted by adjacent polymer strands that push in from every direction. He called his theory "reptation," after the Latin word meaning "to crawl."
Reptation theory helps explain the unusual properties of Silly Putty and Crazy Glue. When Silly Putty is thrown against a surface, the impact comes faster than the polymer chains can rearrange themselves, so they act as if they are solid and the ball bounces. When subjected to forces over longer periods of time, as when pressed by hand, the strands have time to respond so the material can be molded into different shapes.
Reptation also underlies the process of plastic welding, the secret of Crazy Glue. The glue causes the chains from different types of plastic to intertwine and interlock, forming a very strong bond.
By adding large amounts of undyed DNA to an aqueous solution along with a dyed DNA strand attached to a plastic sphere, the researchers were able to confirm the basic assumption upon which reptation theory is based. Using the optical tweezers, they moved the sphere rapidly along complicated, looping paths while the DNA strand unraveled behind. After the sphere was stopped, the polymer began relaxing. But, instead of moving straight toward the sphere, it contracted along the path that the sphere had traced out.
In addition to Chu, Perkins and Smith, Stephen R. Quake from Oxford University contributed to the relaxation studies. Quake did preliminary work on the relaxation studies as a Stanford undergraduate and his senior thesis based on this work received the American Physical Society's Apker Award.
The research was supported in part by grants from the U.S. Air Force Office of Scientific Research, the National Science Foundation and an endowment established by Theodore and Frances Geballe.
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