Dancing molecules “trapped�

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Oct. 22, 2008

Courtesy American Institute of Physicsand World Science staffBiology is chock full of art. For decades, scientists have probed some of the tiniest structures of life's bas ic build ing molecular blocks, such as DNA or pro teins, rendering full-color ball-and-stick models of them that fill the pages of jour nals and adorn the trophy cases of biology depart ments eve ry where.

While these representations reveal some of the most intricate molecular details of life, they of ten fall short in depicting how a molecule moves.Just as the perfect picture of a horse can not convey the fluidity of it gallop, so does a frozen pic ture of DNA fail in describing its intricate dance.

"These are wet, warm, squishy things," said 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, how ever, 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 Cohen and colleagues at Harvard, it may be possible to con fine a single molecule and study its motions at the same time. Cohen presented his findings this week in Boston at the annual symposium and exhibition of the American Vacuum Society, a part of the American Institute of Physics.

The machine basically uses a variable electric field to trap a single molecule under a microscope, Cohen said. It does this by tracking the molecule's motion and then rapidly applying tiny electric pulses to counter this motion and zap the molcule back in to place. Cohen described how he and his colleagues can use this machine to look at things like virus particles or single pieces of DNA.

Cohen reported that his group recently 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, said Co hen, and these properties give in formation about how DNA interacts in a biological setting.

* * *rt. For dec ades, sci en tists have probed some of the ti ni est struc tures of life's bas ic build ing

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