Berkeley researchers identify photosynthetic dimmer switch

“If this were the murder mystery board game Clue, you could say that we had found the weapon (zeaxanthin) but didn’t know how and precisely where the crime took place,” Fleming said.

Fleming and his colleagues knew where to look, however. Recently they’d used near-infrared absorption spectroscopy to demonstrate that the generation of the zeaxanthin radical cation occurs exclusively in the three minor light-harvesting proteins, C29, CP26 and CP24. To determine whether one or all of the minor complexes were responsible for production of the energy-quenching cation, they expressed CP29, CP26 and CP24 in bacteria and reconstituted them in vitro with chlorophyll, zeaxanthin and other Photosystem II proteins. They then used ultrafast pump-probe spectroscopy to follow the energy trail on the femtosecond timescale (a femtosecond is one millionth of a billionth of a second) of the energy-quenching process.

“Our findings showed that energy-quenching occurs within all three minor complexes, which is consistent with the results of previous genetic and spectroscopic analyses that indicated no single antenna protein is specifically required for quenching to take place,” said Fleming.

To learn more about how the energy quenching process works, Fleming and his colleagues focused their efforts on C29 because the genetics and molecular architecture of this protein have been well characterized and the supply of known mutations is ample. Their findings suggest that the change in pH as a result of excess solar energy results in CP29 undergoing a conformational change which alters the reduction potential of excitonically-coupled chlorophyll molecules. This then promotes the charge-transfer with zeaxanthin that produces radical cations. Furthermore, it appears that this conformational change is reversible, which opens the possibility of being able to “tune” the electronic coupling between the chlorophylls and thereby modulate the energy of the chlorophylls-zeaxanthin charge-transfer state. In other words, they should be able to switch the energy-quenching process on or off.

“The next step is to examine the energy quenching mechanism in the rest of the Photosystem II complex to see how it is used to regulate the flow of energy throughout the light harvesting system,” said Fleming.

Support for this research came from the U.S. Department of Energy's Office of Basic Energy Sciences through its Chemical Sciences, Geosciences, and Biosciences Division.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov/