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Green bacteria promise a new path to solar power

EuroPhotonics
Jul 2009
Charles T. Troy, charlie.troy@laurin.com

LEIDEN, Netherlands – An international team of scientists has resolved the structure of chlorophyll in the chlorosome of green bacteria. Chlorosomes are the light-harvesting antennae of these bacteria. They are elongated small pockets that can accommodate up to 250,000 chlorophyll molecules.

According to Huub de Groot, professor of chemistry at Universiteit Leiden and coordinator of the research, similar configurations can be useful in developing “artificial leaves” – new generations of solar cells for the conversion of sunlight to fuel – because green bacteria can collect sunlight with a high efficiency for conversion to chemical energy.

The structure proves to be a combination of concentric nanotubes that produce a robust but nevertheless plastic framework for the light-harvesting antennae. The chlorophyll molecules form helices along which occurs superfast energy migration to proteins in the cell membrane, where the chemical conversion takes place.

Varied dimensions

The flexible structure of the chlorosomes permits them to vary their dimensions according to the intensity of light. The antennae are larger at low-light intensity; inside the antennae, organization of chlorophyll is heterogeneous, which is effective for the optimum absorption of photons at various wavelengths. This combination of a sturdy framework and freedom in accommodating chlorophyll molecules ensures that the bacteria can adapt to low-light intensity during the process of biological evolution – for example, at 100 metres deep in the sea.

Chlorosomes are heterogeneous in molecular composition, so determining their structures with x-ray crystallography is not an option. Biochemical and microscopic techniques used over the years have produced contradictory information. The research team developed a strategy to combine genetic techniques and two sophisticated bioimaging methods: cryoelectron microscopy and solid-state nuclear magnetic resonance (NMR).

First, three genes that developed late in the evolutionary process were removed from the bacterium. The team’s biologists, from Pennsylvania State University in the US, suspected that those “late” genes are responsible for the bacterium’s highly efficient absorption of light. Chlorosomes from these mutants appear much more uniform and have a simpler structure compared with those from the wild type. Moreover, they proved to be less efficient. The heterogeneity is obviously one of the secrets behind the efficiency of the evolved green bacterium.

The next step was growing the mutant so it could be enriched with stable carbon-13 isotopes for solid-state NMR. This work was performed in Germany at the Max Planck Institut für Bioanorganische Chemie. The first solid-state NMR experiments made clear that a new view of the structure could be obtained. Researchers could now determine distances between the molecules very closely and found that the molecules piled up with their tails alternating.

To switch focus from the microstructure to nanotubes, still another technique was used: cryo-electron microscopy. Carried out in Groningen, the method produced images with distinct patterns that can be explained only by a helical arrangement of molecules. Once this structure was established, it was possible to combine the dimensions from the electron microscope with the precise measurements at the molecular scale from the solid-state NMR to produce a detailed structure of the chlorosome. The result was a configuration in which chlorophyll forms stacks and rings that self-assemble into concentric nanotubes. The corresponding structure of the wild type is less uniform, and its stacks go in another direction, approximately perpendicular to those in the mutant. The structural framework thus provides insight into how similar chlorosome systems can be established in different ways. This information is important in the next step for the construction of artificial systems.

Chlorosomes are an attractive model for new generations of solar conversion devices because they have a simple composition and work well at low light intensity. In natural photosynthesis, the quantity of sunlight is generally not the limiting factor. Green bacteria live in extremely low light conditions, with sometimes only a few photons per chlorophyll molecule per day. The next challenge will be translating the knowledge and insights obtained from studying the biology of green bacteria to the creation of nanostructured materials for the conversion of sunlight to fuel.


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