An international team of scientists has determined the structure of the chlorophyll molecules in green bacteria that are responsible for harvesting light energy. The team’s results one day could be used to build artificial photosynthetic systems, such as those that convert solar energy to electrical energy. A research paper about the discovery will be published on 4 May 2009 in the Proceedings of the National Academy of Sciences.
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The scientists found that the chlorophylls are highly efficient at harvesting light energy. “We found that the orientation of the chlorophyll molecules make green bacteria extremely efficient at harvesting light,” said Donald Bryant, Ernest C. Pollard Professor of Biotechnology at Penn State and one of the team’s leaders.
According to Bryant, green bacteria are a group of organisms that generally live in extremely low-light environments, such as in light-deprived regions of hot springs and at depths of 100 meters in the Black Sea. The bacteria contain structures called chlorosomes, which contain up to 250,000 chlorophylls. “The ability to capture light energy and rapidly deliver it to where it needs to go is essential to these bacteria, some of which see only a few photons of light per chlorophyll per day.”…
“The NMR data revealed to us that the individual chlorophyll molecules in green bacteria are arranged in dimers — molecules consisting of two identical simpler molecules — with their long hydrophobic, or water-repellent, tails sticking out of either side,” said Bryant. “We also learned precisely how the chlorophyll molecules attach to one another, and we were able to measure the distance between chlorophyll molecules. The cryo-electron microscopy pictures showed gross structural details and distances, and the NMR results allowed us to quantify these distances as well, and confirmed to us that what were were seeing was, in fact, stacks of the chlorophyll molecules all lined up,” he said. The NMR results also enabled the scientists to determine that the chlorophyll molecules in green bacteria are arranged in helical spirals. In the mutant bacteria, the chlorophyll molecules are positioned at a nearly 90-degree angle in relation to the long axis of the nanotubes, whereas the angle is less steep in the wild-type organism. “It’s the orientation of the chlorophyll molecules that is the most important thing here,” said Bryant. The last steps for the team were to pull together all of their data and to create a detailed computer model of the structure.
“At first it seems counterintuitive that green bacteria have managed to evolve a better light-harvesting system by increasing disorder in the chlorosome structure,” said Bryant. “Most people would think that if you make something that is more highly ordered, you’ll end up with something that works better. But this is clearly a case where that isn’t true. If all of the chlorophylls are identically arranged in a chlorosome, then the energy from the photon, once it is absorbed, is going to wander around over all of those chlorophylls, which could take a long time.