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Dancing molecules “trapped”

Oct. 22, 2008
Courtesy American Institute of Physics
and World Science staff

Bi­ol­o­gy is chock full of art. For dec­ades, sci­en­tists have probed some of the ti­ni­est struc­tures of life’s bas­ic build­ing mo­lec­u­lar 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. 

A particle two ten-thou­s­andths of a mil­li­meter wide trapped in Co­hen's de­vice, called the Anti-Brow­ni­an El­ec­tro­ki­ne­tic trap. (Image courtesy A. Co­hen)


While these rep­re­senta­t­ions re­veal some of the most in­tri­cate mo­lec­u­lar de­tails of life, they of­ten fall short in de­pict­ing how a 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,” 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, 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 Co­hen and col­leagues at Har­vard, it may be pos­si­ble to con­fine 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 mol­e­cule back in­to 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 Co­hen, and these prop­er­ties give in­forma­t­ion about how DNA in­ter­acts in a bi­o­log­i­cal set­ting.


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Biology is chock full of art. For decades, scientists have probed some of the tiniest structures of life’s basic building molecular 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,” 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, 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 Cohen and colleagues at Harvard, it may be possible to confine 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 molecule back into 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 Cohen, and these properties give information about how DNA interacts in a biological setting.