Research sheds new light on proton behavior

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 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 journalNature 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.

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 jel­ly­fishAequorea 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!”

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