Physicists have found the strongest evidence yet of quantum effects fueling photosynthesis.

Multiple experiments in recent years have suggested as much, but it’s been hard to be sure. Quantum effects were clearly present in the light-harvesting antenna proteins of plant cells, but their precise role in processing incoming photons remained unclear.

In an experiment published Dec. 6 in Proceedings of the National Academy of Sciences, a connection between coherence — far-flung molecules interacting as one, separated by space but not time — and energy flow is established.

“There was a smoking gun before,” said study co-author Greg Engel of the University of Chicago. “Here we can watch the relationship between coherence and energy transfer. This is the first paper showing that coherence affects the probability of transport. It really does change the chemical dynamics.”

The new findings are the latest in a series that have, piece by piece, promised to expand scientific understanding of photosynthesis, one of life’s fundamental processes. Until a few years ago, it seemed a straightforward piece of chemistry.

Then came observations of coherence in antenna-protein chlorophylls from green sulfur bacteria. They lasted far longer than anyone expected, long enough to hint at functional role. Those observations were, however, made at unrealistically ultracold temperatures; then they were made at room temperatures, and in antenna proteins found in plants everywhere.

Confronted with this unexpected coherence, researchers hypothesized a role in enabling ultra-efficient energy transfer. Energy from incoming photons could simultaneously explore every possible chlorophyll route from a protein’s surface to the reaction center at its core, then settle on the shortest path.

An antenna protein complex. The protein scaffolding is grey and chlorophyll molecules are green. Image: Greg Engels

To see if that happened, a team led by Engel and Shaul Mukamel of the University of California, Irvine analyzed the fluctuation of lasers as they passed through antenna proteins. Depending on how they shifted, the researchers could track what happened inside.

They found a clear mathematical link between energy flows and fluctuations in chlorophyll coherence. The link was so clear it could be described in derivative sines and cosines, mathematical concepts taught in college trigonometry.

“The mounting evidence that quantum effects can be seen in natural systems when excited by lasers is compelling,” said Greg Scholes, a University of Toronto biophysicist who first found quantum effects in room temperature photosynthesis.

Further research is needed to understand the full role of quantum physics, said Scholes. “How much do they change our understanding? How much are they needed?” he said.

Engel sees a lesson in the importance of the antenna proteins in which chlorophyll molecules are embedded. “The protein does a lot more for this system than we thought,” he said. “It’s not just a simple structural element.”

Molecular biologists “are trained to look at the molecule,” Engel said. “We don’t usually design systems. We design molecules. The question becomes: Which aspects of this do we strive to recreate? We are very interested in the design principles. How could you design one of these?”

Image: Stephen Heron/Flickr

Citation: “Direct evidence of quantum transport in photosynthetic light-harvesting complexes.” By Gitt Panitchayangkoon, Dmitri V. Voronine, Darius Abramavicius, Justin R. Caram, Nicholas Lewis, Shaul Mukamel, and Gregory S. Engel. Proceedings of the National Academy of Sciences, Vol. 108 No. 49, Dec. 6, 2011.