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Trapping Single Molecules Under a Microscope

If realism in art is about representing form, then biology is chock full of real art. For decades, scientists have probed some of the tiniest structures of life's basic building blocks (such as DNA or proteins), rendering full-color ball-and-stick models of them that fill the pages of journals and adorn the trophy cases of biology departments everywhere. While these representations reveal some of the most intricate molecular details of life, they often fall short in depicting how a single molecule moves. Just as the perfect picture of a horse cannot convey the fluidity of it gallop, so does a frozen picture of DNA fail in describing its intricate dance. "These are wet, warm, squishy things," says Adam Cohen of Harvard University. They jiggle, they flap, they twist, they turn, and they randomly "walk" about.

Studying how a single molecule moves is hard, however, because of these very motions. Like a horse, if you set a single molecule free, it will wander away. You can tie it down, ensuring that it no longer wanders, but then you can't necessarily observe how it moves. Now, thanks to a machine built by Adam Cohen and his colleagues at Harvard, it may be possible to confine a single molecule and study its motions at the same time.

The machine basically uses a variable electric field to trap a single molecule under a microscope. It does this by tracking the molecule's motion and then rapidly applying tiny electric pulses to counter this motion and zap the molecule back into place. At the AVS 55th International Symposium & Exhibition, Cohen will describe how he and his colleagues can use this machine to look at things like virus particles or single pieces of DNA. Recently they made a movie by capturing 60,000 high-speed frames of a DNA molecule dancing. The studies show the nature of the molecule's internal forces, says Cohen, and these properties give information about how DNA interacts in a biological setting.

Cohen's talk, "Trapping Single Molecules in Water at Room Temperature" is at 8:20 a.m. on Monday, October 20, 2008, in Room 312 of the Hynes Convention Center.

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