Magazine article Science News

Watching Polymers Wend Their Way Along

Magazine article Science News

Watching Polymers Wend Their Way Along

Article excerpt

How do polymers really move?

Do the long, chainlike molecules float through solutions in a haphazard way? Or do these tangled tubes tend to wriggle through viscous fluids like small snakes slithering through mud?

These questions have puzzled chemists for decades, especially since the "reptation" model appeared in the early 1970s. This theory holds that polymers undulate as they meander through matter.

What has hamstrung chemists trying to test this model thoroughly has been a lack of direct visual evidence. Until several months ago, no one had actually seen a single polymer slither. Then, in the March 17 NATURE, a group of scientists reported seeing the squiggling motion of actin filaments.

Now, Thomas T. Perkins, Douglas E. Smith, and Steven Chu, all physicists at Stanford University, describe observing the wriggling of a single strand of DNA. Their report appears in the May 6 SCIENCE.

Using fluorescence microscopy and stained DNA, the scientists first watched individual DNA strands contort in ways predicted by the reptation theory, then tracked the molecule's twists and turns for up to 2 minutes at a time.

With optical tweezers, a laser method that enables them to manipulate individual DNA strands, the researchers tugged and twisted one strand at a time, deforming it in various ways. "We'd grab it, pull it, and watch it move," says Perkins. "We made a loop and pulled on one end. Then we squeezed it and watched it expand back." One by one, bends, kinks, and loops took hold. …

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