Through photosynthesis, green plants and certain bacteria are able to transfer solar energy almost instantaneously from light-capturing pigment molecules into molecular reaction centers where solar energy is converted into chemical energy (see Photosynthesis goes quantum, Credit: Illustration by Nicolle Rager Fuller). The energy transfer happens so fast and is so efficient that less than five percent is lost as heat.
How nature manages to pull off this stunt was a long-standing mystery until 2007, when a study led by Graham Fleming, Deputy Director of Berkeley Lab and a UC Berkeley chemistry professor, found the first direct evidence of what he calls a “remarkably long-lived wavelike electronic quantum coherence.” This quantum mechanical wavelike effects can explain the extreme efficiency of the energy transfer because it enables a plant’s photosystem to simultaneously sample all the potential energy pathways from pigment molecules to reaction centers and choose the most efficient one.
Fleming 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.”
However, as is so often the case in science, solving one mystery led to another. What is the source of this remarkably long-lived quantum coherence? A second team, again led by Fleming, believes it has found the answer. From their investigation, they conclude that the protein environment in the reaction center works collectively to keep the fluctuations of excited electronics states of pigment molecules in phase, and therefore protects quantum coherence. This is a brand-new function of the protein in the reaction center.
Fleming and his group have found that in the tightly packed complex formed by pigments and proteins, the light-induced energy excitations in the pigment molecules also set off the vibrational modes of the proteins. This creates a resonance that enhances energy transfer efficiency and is responsible for the long lifetime of the electronic coherence.
“Our results suggest that correlated protein environments preserve electronic coherence in photosynthetic complexes and allow the excitation to move coherently in space, enabling highly efficient energy harvesting and trapping in photosynthesis,” says Fleming. “Rather than simply serving as a static structure that holds the pigments in the proper geometry for efficient energy transfer to the reaction centers, as was anticipated, we find that the protein environment in the reaction centers plays a dynamic role in optimizing the efficiency of the energy transfer.”
The astonishing efficiency by which photosynthesis can transfer solar energy has made the process a prime target for scientists who would like to emulate it to help meet human energy needs. If we could effectively harness even a tiny fraction of the total solar energy available each year, we would have a clean, sustainable, and carbon-neutral source of energy to meet all our needs.