Paul Champion, professor and chair, Department of Physics, poses for a portrait in the Egan Research building at Northeastern University on August 22, 2016. Photo by Matthew Modoono/Northeastern University

Research sheds new light on proton behavior, draws praise from science community

by Thea Singer

Con­sider: You’ve always thought that the only way to travel from northern New Jersey to New York City was over the Hudson River via the George Wash­ington Bridge. Then one day there’s a news flash: The Lin­coln Tunnel through the Hudson is actu­ally much more efficient.

North­eastern physics pro­fessor Paul Cham­pion and his col­leagues have made a com­pa­rable dis­covery deep in the sub­atomic world of pro­tons, the positively-​​charged par­ti­cles found in the nucleus of every atom.

The paper was pub­lished Wednesday in the journal Nature Chem­istry. Sci­ence mag­a­zine, struck by the results, high­lighted it in its “Editor’s Choice” column upon its pub­li­ca­tion online.

Pro­tons play a major role in many bio­chem­ical sys­tems crit­ical to sus­taining life, including pho­to­syn­thesis and cel­lular respiration—the process by which cells release the energy stored in the chem­ical bonds of food molecules.

Clas­sical physics posits that pro­tons travel over ther­mo­dy­namic barriers—that is, they hop­scotch from one mol­e­c­ular com­pound to another within a system, sparking those bio­chem­ical reac­tions only when the tem­per­a­ture is high enough to kick them over the barrier.

Now Champion’s team—using an ultra­fast pulsed laser system designed at Northeastern—has revealed that pro­tons can actu­ally tunnel through those bar­riers, even at room tem­per­a­ture, sparking the reac­tions at a much faster rate than would be pos­sible if they waited to be ther­mally kicked over the barrier.

Paul Champion, professor and chair, Department of Physics, left, Bridget Salna, physics PhD candidate, and Research Associate Abdelkrim Benabbas, pose for a portrait in the Egan Research building at Northeastern University

Paul Champion, professor and chair, Department of Physics, left, Bridget Salna, physics PhD candidate, and Research Associate Abdelkrim BenabbasPhoto by Matthew Modoono/Northeastern University

The dis­covery upends the under­standing held for cen­turies of pro­tons’ behavior as well as of the com­pounds involved in their trans­port. The next step is for researchers to mimic the behavior in the lab and then use it to develop new tech­nolo­gies. For example, tun­neling could help trans­port pro­tons across a mem­brane and lead to new types of bat­teries. In fact, cer­tain types of bio­log­i­cally inspired bat­teries are already in the pipeline.

These envi­ron­men­tally clean recharge­able bat­teries are mod­eled after mito­chon­dria, the energy fac­to­ries of animal and plant cells. Just as mito­chon­dria con­vert glu­cose, a simple sugar, into adeno­sine triphos­phate at room tem­per­a­ture to power living cells, bio-​​batteries, when per­fected, could con­vert the energy stored in glu­cose to power devices from lap­tops to cars.

“Biology can serve as an inspi­ra­tion for the mate­rials that researchers are trying to create,” says Cham­pion, chair of the Depart­ment of Physics. “Mito­chon­dria are nature’s own highly evolved bat­tery system, and the cur­rency of that bat­tery system is pro­tons. We found that pro­tons tunnel incred­ibly fast at room tem­per­a­ture to move from one point to another. Indeed, that is their dom­i­nant mode of trans­port. It was a very, very sur­prising result.”

Making a quantum leap

The team’s novel under­standing of how a proton oper­ates under­lies the tun­neling capa­bility: Rather than func­tioning as simply a par­ticle, hop­ping over Point A to reach Point B, according to clas­sical physics, the proton also func­tions as a wave, punc­turing Point A to reach Point B, in line with quantum physics.

“At first we weren’t sure what we were seeing,” says Cham­pion. “And then we finally real­ized the pro­tons were tun­neling at room tem­per­a­ture. The process was remark­ably fast—so much faster than over-​​the-​​barrier clas­sical trans­port. We were shocked.”

For their exper­i­ment, the researchers turned to a pro­tein called green flu­o­res­cent pro­tein, or GFP, as a model system. GFP, first seen in the jel­ly­fish Aequorea vic­toria, is com­monly used as a marker in bio­med­ical research because it emits a green glow. By inserting DNA from GFP into other pro­teins, researchers can follow GFP’s col­orful glow to track processes from nerve cell growth to cancer progression.

“The ele­ments com­prising GFP are well known,” says Bridget Salna, the paper’s first author and a doc­toral stu­dent in physics. “The pro­tons move inter­nally a short dis­tance in one direc­tion and then they move back, among just four ele­ments.” They are: three compounds—a glu­tamic acid, a serine, and a water—and the chro­mophore, which deter­mines the green color. The proton’s journey, says Salna, is what sets off the green glow.

The researchers used light from their custom-​​designed lasers to trigger the proton-​​transport process, expec­tantly watching the par­ti­cles’ travels over broad time and tem­per­a­ture scales.

“By nar­rowing down the dura­tion of the pulse we could actu­ally see and track the mol­e­c­ular dynamics over the entire cycle,” says Salna, PhD’17. “It was fascinating.”

Among those rec­og­nizing the break­through was Martin Karplus, winner of the 2013 Nobel Prize in Chem­istry with Michael Levitt and Arieh Warshel. “It cer­tainly makes clear the impor­tance of ‘deep tun­neling’ in proton transfer reac­tion at room tem­per­a­ture,” Karplus wrote in an email to Cham­pion. “Congratulations!”

Originally published in news@Northeastern on August 24, 2016.

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