Lasers reveal the hidden lives of biomolecules

by Angela Herring of news@Northeastern

All around and inside us, an elab­o­rate dance of mol­e­c­ular vibra­tions is con­stantly taking place. From the water in your glass to the fluid sur­rounding your blood cells, mol­e­cules exchange energy with their envi­ron­ment. Large mol­e­cules, such as the heme group of hemo­globin, can respond with waving, sad­dling or ruf­fling motions, said Paul Cham­pion, pro­fessor and chair of the Depart­ment of Physics. These motions, which can also be excited with a fem­tosecond laser pulse into so-​​called “non-​​stationary coherent states,” are the dar­lings of his research.

Cham­pion was recently elected as a fellow to the Amer­ican Asso­ci­a­tion for the Advance­ment of Sci­ence for his “dis­tin­guished con­tri­bu­tions to the struc­ture and dynamics of bio­mol­e­cules using novel laser tech­niques.” These tech­niques have allowed Cham­pion and his team to probe those non-​​stationary coherent states in ways never before possible.

“I am delighted that Paul’s pio­neering work has been rec­og­nized by this honor from AAAS — one of the only sci­en­tific soci­eties that embraces the full spec­trum of sci­ence, with a mis­sion to bring  out­standing sci­ence to the wider public,” said Murray Gibson, dean of the Col­lege of Sci­ence.

In the case of the pancake-​​shaped heme mol­e­cule that Cham­pion has spent nearly two decades studying, these low-​​energy vibra­tions take place out of the plane, as if someone were set­ting up vibra­tions on a drum.

Stan­dard ana­lyt­ical and spec­tro­scopic tech­niques bom­bard mol­e­cules with optical waves of much higher fre­quency (or energy), which probe con­for­ma­tions and allow bonds between indi­vidual atoms to be iden­ti­fied. The lower energy vibra­tions that exchange energy with the sur­round­ings are com­pletely hidden from these stan­dard ana­lyt­ical spec­tro­scopic methods.

But these low energy vibra­tions are nec­es­sary for bio­mol­e­cules to interact with one another and to drive bio­chem­ical reac­tions. For instance, the heme mol­e­cule is the body’s pri­mary oxygen trans­porter. It binds oxygen in hemo­globin, the func­tional unit of the blood, and acts as a sig­naling reg­u­lator when it binds nitric oxide. “There all kinds of things heme groups do, including impor­tant bio­cat­alytic reac­tions,” said Cham­pion. But we cannot under­stand how these bio­mol­e­c­ular activ­i­ties so effi­ciently uti­lize avail­able thermal energy using stan­dard, com­mer­cially avail­able instruments.

So instead, Cham­pion builds his own instru­ments using spe­cial lasers. “You nor­mally think of laser light as having a pure, single fre­quency with one wave­length,” he explained. “But when you make lasers into short, fem­tosecond pulses, as the pulses get shorter, the fre­quency con­tent or band­width becomes larger.”

This allows the team to gen­erate wave­lengths with dif­fer­ences in their fre­quen­cies that are very adept at exciting mol­e­cules into those non-​​stationary, low-​​energy vibra­tional states. Essen­tially, Champion’s lasers allow him to stretch the heme pan­cake in par­tic­ular direc­tions, so to speak. He studies the dif­ferent vibra­tional states in an effort to under­stand how they might change when pro­teins interact with other pro­teins and how they are involved in pushing reac­tions over bar­riers to form products.

“It’s a long-​​studied, com­plex and not fully under­stood sub­ject regarding how ele­men­tary par­ti­cles like elec­trons and pro­tons are trans­ferred in bio­log­ical sys­tems,” he said. “We know that they are trans­ferred, but we don’t know how nature has tuned itself up to do it so effi­ciently and so selectively.”

For example, Cham­pion thinks that there is one type of vibra­tion that helps turn on or off a heme’s ability to give or receive an elec­tron to another pro­tein. This is the type of thing Cham­pion is able to study with his fem­tosecond pulsed laser beams.

The Amer­ican Asso­ci­a­tion for the Advance­ment of Sci­ence (AAAS) is the world’s largest gen­eral sci­en­tific society, serving 10 mil­lion indi­vid­uals. The tra­di­tion of AAAS Fel­lows began in 1874 and honors mem­bers for their sci­en­tif­i­cally or socially dis­tin­guished efforts to advance sci­ence and its applications.

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