Scientists Uncover a Photosynthetic Puzzle
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Researchers observed quantum effects in photosynthesis, a process that until recently has been understood largely in classical, rather than quantum, terms.
Quantum physics and plant biology seem like two branches of science that could not be more different, but surprisingly they may in fact be intimately tied.
Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the Notre Dame Radiation Laboratory at the University of Notre Dame examined this connection using ultrafast spectroscopy to investigate what happens at the subatomic level during the very first stage of photosynthesis, when the plant, algae, or bacteria first capture sunlight energy from the sun.
“If you think of photosynthesis as a marathon, we’re getting a snapshot of what a runner looks like just as he leaves the starting line,” said Argonne biochemist David Tiede. “We’re seeing the potential for a much more fundamental interaction than a lot of people previously considered.”
While different species of plants, algae, and bacteria have evolved a variety of different mechanisms to harvest light energy, they all share a feature known as a photosynthetic reaction center. Pigments and proteins found in the reaction center help organisms perform the initial stage of energy conversion.
These pigment molecules, or chromophores, are responsible for absorbing the energy carried by incoming light. After a photon hits the cell, it excites one of the electrons inside the chromophore. As they observed the initial step of the process, Argonne scientists saw something no one had observed before: a single photon appeared to excite different chromophores simultaneously.
“The behavior we were able to see at these very fast time scales implies a much more sophisticated mixing of electronic states,” Tiede said. “It shows us that high-level biological systems could be tapped into very fundamental physics in a way that didn’t seem likely or even possible.”
We typically think of reactions taking place linearly, meaning that one reaction occurs, then the next reaction occurs, and so on in an orderly manner, much like dominoes lined up in a row and falling in series. We normally understand biology in this linear fashion—a gene is turned on, producing a protein, which in turn results in a specific outcome, such as a bacterium producing a protein that causes it to luminesce, or glow in the dark. This is the “classical” understanding of physical processes.
However, the results of this experiment showing one photon can be found in numerous places at the same time—i.e., exciting several chromophores simultaneously—goes against this logical, linear path; this is indicative of a quantum effect.
“It leaves us wondering: how did Mother Nature create this incredibly elegant solution?
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Quantum mechanics looks at atomic particles—e.g., photons, electrons, muons, and quarks—as waves of energy, like the ripple from a pebble dropped into a pond spreading out from the start and affecting every rock, insect, and plant in its path. Instead of having a single event producing one thing or specific effect (the domino hitting another domino and causing a cascade to occur), as in classical physics, there is the potential in quantum mechanics for multiple things to occur based on a single event happening.
The quantum effect events observed by the Argonne team happen very quickly—at the speed of less than a trillionth of a second. To “catch” the effect, the researchers pulsed the chromophore with lasers multiple times at very low temperatures, slowing the reaction down to enable the scientists to follow the photon to see where it goes or—in this case—all the places that it goes at the same time.
Through this wave-like behavior, AKA “quantum effects,” the photon in essence “assesses” the most efficient path to transfer the energy to in the next step in photosynthesis. Once that path is determined, the other options are not needed and the photon goes down that single path, allowing for the next stage to proceed.
The quantum effects observed in the course of the experiment hint that the natural light-harvesting processes involved in photosynthesis may be more efficient than previously indicated by classical biophysics, said chemist Gary Wiederrecht of Argonne’s Center for Nanoscale Materials. “It leaves us wondering: how did Mother Nature create this incredibly elegant solution?” he said.
This work builds a body of evidence, starting from the pioneering work by Graham Fleming and his colleagues at the Lawrence Berkley National Laboratory, demonstrating the role for quantum mechanics in the earliest steps of photosynthesis.
The result of the study could significantly influence efforts by chemists and nanoscientists to create artificial materials and devices that can imitate natural photosynthetic systems. Researchers still have a long way to go before they will be able to create devices that match the light harvesting efficiency of a plant.
One reason for this shortcoming, Tiede explained, is that artificial photosynthesis experiments have not been able to replicate the molecular matrix that contains the chromophores. “The level that we are at with artificial photosynthesis is that we can make the pigments and stick them together, but we cannot duplicate any of the external environment,” he said. “The next step is to build in this framework, and then these kinds of quantum effects may become more apparent.”
Because the moment when the quantum effect occurs is so short-lived, scientists will have a hard time ascertaining biological and physical rationales for their existence in the first place. But the quantum observation of these researchers opens up a new window on a process—photosynthesis—that until now has largely been understood in classical, rather than quantum, terms.
“It makes us wonder if these quantum effects are really just there by accident, or if they are telling us something subtle and unique about these materials,” Tiede said. “Whatever the case, we’re getting at the fundamentals of the first step of energy conversion in photosynthesis.”
The DOE Office of Science supported this research, which made use of instrumentation at the Argonne National Laboratory Center for Nanoscale Materials, one of five DOE Nanoscale Science Research Centers supported by the Office of Science at National Laboratories around the nation. An article based on the study appeared online in the Proceedings of the National Academy of Sciences.
—Jared Sagoff, Argonne National Laboratory, [email protected], and Dawn Adin, DOE Office of Science, [email protected]
Funding
Department of Energy, Office of Basic Energy Sciences
Publication
Libai Huang, Nina Ponomarenko, Gary P. Wiederrecht, and David M. Tiede. “Cofactor-specific photochemical function resolved by ultrafast spectroscopy in photosynthetic reaction center crystals,” Proceedings of the National Academy of Sciences 109, 4851 (2012).
Related Links
Chemical Sciences and Engineering Division, Argonne National Laboratory
Center for Nanoscale Materials, Argonne National Laboratory
