Posted: May 14, 2009

Bacteria's light-harvesting antennae explored for developing artificial leaves

(Nanowerk News) An international team of scientists, including researchers from two Dutch universities, Leiden and Groningen, has resolved the structure of chlorophyll in chlorosomes of green bacteria. Chlorosomes are the light-harvesting antennae of these bacteria. They are elongated small pockets which can accommodate up to 250,000 chlorophyll molecules.
‘Artificial leaves’
According to Leiden Huub de Groot, Professor of Biophysical Organic Chemistry in Leiden and co-oprdinator of the group, such structures can be useful in the future for the development of 'artificial leaves': new generations of solar cells for the conversion of energy from sunlight into fuels, because green bacteria can collect sunlight with a high efficiency for conversion to chemical energy ("Alternating syn-anti bacteriochlorophylls form concentric helical nanotubes in chlorosomes")..
The structure proves to be a combination of concentric nanotubes. This produces a robust yet plastic framework for the light-harvesting antennae. Within the nanotubes, the chlorophyl molecules form helices along which superfast energy migratin to proteins in the cell membrane occurs where the chemical conversion takes place.
The flexible structure of the chlorosomes provides the freedom to vary the dimensions dependent on the intensity of the light, to be able to make larger antennae at lower light intensity, and to organise the chlorophyll in a very heterogenous manner inside the antennae. The researchers discovered that this heterogeneity is very effective for the optimum absorption of photons at different wavelengths. This combination of a robust framework and freedom in the accommodatian of chlorophyll molecules has ensured that the bacteria have the possibility to adapt to low light intensity in biological evoltion, for ecample at a depth of 100 metres under the sea.
Last known antenna structure
For plants and algae - the other organisms that convert sunlight into chemical energy - it has been known for some time how their light-harvesting antennae worked. Chlorosomes are, however, very heterogeneous in their molecular composition; no two chlorosomes are the same. As a result, solving the structure using X-ray crystallography is not an option. Biochemical and microscopic techniques have for decades produced contradictory information.
Novel strategy
The research team developed a novel strategy to solve the problem using a combination of genetic techniques and two sophisticated bio-imaging methods: cryo-electron microscopy and solid-state NMR (nuclear-magnetic resonance).
Late genes
The first thing they did was to remove three genes from the bacterium which developed late in the evolutionary process. The biologists in the team, researcher workers at the Pennsylvania State University in the USA, suspected that these ‘late’ genes are responsible for the high level of efficiency with which the bacterium absorbs light. The chlorosomes of these mutants appeared more uniform and simpler in structure than those of the wild type. Moreover, they proved to be much less efficient. The heterogeneity is obviously one of the secrets behind the efficiency.
Precise stacking
The next step was to grow the mutant for enrichment with stable carbon-13 isotopes for solid-state NMR. This work was performed in the Max-Planck Institute for Bioanorganic Chemistry in Germany. According to De Groot, it was clear after the very first NMR measurements that a breakthrough had been made: 'For the first time we had a clear indication that our measuarements would allow us to determine the exact of the chlorophyl. We were able to determine the stacking of the chlorophyl very precisely. We were able to determine the distances between the molecules very precisely and we found that the molecules were stacked alternately with their tails either outwards or inwards.'
Rings and tubes
To get from the microstructure to the nanotubes, yet another technique had to be used: cryo-electronmicroscopy, in Groningen. The Groningen researchers discovered very distinct patterns in the images, that can only be explained with a helical arrangement of the molecules. De Groot: ‘Once we realised that, it was possible to combine the dimensions from the EM with the precise measurements at molecular scale from the solid-state NMR to produce a very detailed structure of the chlorosome.' The result was a structure in which chlorophyll forms stacks and rings that self-assemble into concentric nanotubes. The corresponding structure of the wild type is less uniform, and has the stacks in another direction, approximately perpendicular to the stacking in the mutant. The structural framework thus provides insight into how similar chlorosome systems can be established in different ways. This insight is important for the construction of artificial sysems in a followng step.
New generations of solar energy collectors
De Groot: ‘The reason why chlorosomes are an attractive model for new generations of solar conversion devices is that they have a simple composition and work very well, even at very low light intensity. In natural photosynthesis the amount of sunlight is generally not the limiting factor. Green bacteria live, however, under extremely low light intensity, sometimes with only a few photons per chlorophyll molecule per day. To be able to survive on solar energy is a challenge and thanks to the PNAS research we now know much more about how nature has solved this problem. With the dense packing of chlorophyll molecules, there are strong links between the molecules and it is possible to combine thee nergy that is caught by more than the hundred thousand molecules joined together to generate sufficient flow for the conversion to chemical energy. Moreover, the structure protects itself against a surplus of light. The disorder in the stacking appears to help: a messier structure in this case happens to be better for biology. To apply the knowledge and new insights obtained from the biology in the translation to nanostructured materials fo the conversion of sunlight to fuel is now the next challenge.
Source: University of Leiden
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