When it comes to studying energy transfer in photosynthesis, it's good to think "outside the bun."
That's what Robert Blankenship, Ph.D., professor of biology and chemistry in Arts & Sciences at Washington University in St. Louis, did when he contributed a protein to a study performed by his collaborators at Lawrence Berkeley National Laboratory and the University of California at Berkeley. The protein's moniker? The taco shell protein.
Taco shell protein
It's called bacteriochlorophyl (BChl) a protein, but Blankenship fondly calls it the taco shell protein because of its structure : its ribbon-like backbone wraps around three clusters of seven chlorophylls, just like a taco shell around ground beef. The structure also is referred to as trimeric because of the three clusters.
The protein, which comes from a photosynthetic bacterium that lives in extremely high temperatures, enabled the researches to discover that quantum mechanical effects appear to play a role in photosynthesis.
The taco shell protein is arguably the most studied and understood protein in a complex photosynthesis researchers refer to as the antenna system, molecules that efficiently transfer energy from light in a cascade.
Photosynthesis transforms light, carbon dioxide and water into chemical energy in plants and some bacteria. The wavelike characteristic of this energy transfer process can explain its extreme efficiency, in that vast areas of phase space can be sampled effectively to find the most efficient path for energy transfer.
"We have a very detailed molecular structure of this protein and we understand the electronic properties of it very well, too," said Blankenship. "It's taught us a lot about how chlorophylls interact with proteins. It was ideal for this study."
Blankenship's colleague, Graham R. Fleming, Ph.D., Deputy Director of the U.S. Department of Energy's Lawrence Berkeley National Laboratory and professor of chemistry at the University of California, and colleagues used 2-D spectroscopy to study what happens inside a bacteriochlorophyll complex, and detected a 'quantum beating."
The effect, described in the April 12, 2007 issue of Nature, occurs when light-induced excitations in the complex meet and interfere constructively much like the interactions that occur between the ripples formed by throwing stones into a pond.
The collaboration is a good illustration of interdisciplinary science. The Washington University group's expertise is in photosynthesis, especially antenna systems, and the West Coast group's specialty is advanced laser techniques. The quantum finding would have been impossible without collaboration.
"We have obtained the first direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis," said Fleming, the principal investigator for the study. "This wavelike characteristic can explain the extreme efficiency of the energy transfer because it enables the system to simultaneously sample all the potential energy pathways and choose the most efficient one."
Fleming is also a professor of chemistry at UC Berkeley, and an internationally acclaimed leader in spectroscopic studies of the photosynthetic process. In a paper entitled, "Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems," he and his collaborators report the detection of "quantum beating" signals, coherent electronic oscillations in both donor and acceptor molecules, generated by light-induced energy excitations, like the ripples formed when stones are tossed into a pond.
Electronic spectroscopy measurements made on a femtosecond (millionths of a billionth of a second) time-scale showed these oscillations meeting and interfering constructively, forming wavelike motions of energy (superposition states) that can explore all potential energy pathways simultaneously and reversibly, meaning they can retreat from wrong pathways with no penalty. This finding contradicts the classical description of the photosynthetic energy transfer process as one in which excitation energy hops from light-capturing pigment molecules to reaction center molecules step-by-step down the molecular energy ladder.
"The classical hopping description of the energy transfer process is both inadequate and inaccurate," said Fleming. "It gives the wrong picture of how the process actually works, and misses a crucial aspect of the reason for the wonderful efficiency."
The taco shell is a sort of "middleman" in the antenna system, sandwiched in between a larger antenna and a molecule called the Reaction Center, where all the chemistry in energy transfer takes place, said Blankenship.
"Most of the absorption of light is carried out by a complex called the chlorosome that then transfers the energy to the trimeric protein that in turn transfers to the Reaction Center," Blankenship said.