May 14 2009
A new discovery concerning the properties of green sulphur bacteria might prove useful in the development of a new generation of solar cells. Chlorosomes, nature's largest and most efficient light-harvesting antennae, accommodate up to a quarter of a million chlorophyll molecules in this type of bacteria. The team of international researchers, reporting in the Proceedings of the National Academy of Sciences (PNAS), believes that the structure of these molecules can be exploited to develop new ways of creating energy.
Chlorosomes are made up of hundreds of thousands of self-assembled bacteriochlorophylls (BChl). Debate about the structure of chlorosomes has lingered for decades, and unlocking its enormous potential for providing insight into energy production has remained just out of arm's reach.
Although the mechanisms of light-harvesting antennae in some organisms that convert sunlight into chemical energy (e.g. plants and algae) are fairly well understood, understanding chlorosomes has proved to be a challenge. They are heterogeneous in their molecular composition, so X-ray crystallography could not be used to solve the structure riddle. Furthermore, information gathered through the use of biochemical and microscopic techniques has so far produced contradictory results.
'Because they [chlorosomes] form very large and compositionally heterogeneous organelles, they had been the only photosynthetic antenna system for which no detailed structural information was available,' the study reads. 'In our approach, the structure of a member of the chlorosome class was determined and compared with the wild type (WT) to resolve how the biological light-harvesting function of the chlorosome is established.'
The researchers used genetic techniques along with two sophisticated bio-imaging techniques: cryo-electron microscopy and solid-state NMR (nuclear magnetic resonance). They discovered that the structure of a chlorosome is made up of a combination of concentric nanotubes, which produce a robust framework for the light-producing antennae.
According to the study, 'The basis for the efficient and ultrafast light harvesting is a helical exciton delocalisation pathway.' In other words, super-fast energy migration to proteins in the cell membrane takes place via helices formed by the chlorophyll molecules, producing the chemical conversion.
The coordinator of the research, Professor Huub de Groot of the Netherlands' Leiden University, suggests that the new findings could be used to develop similar structures for 'artificial leaves', i.e. solar cells which turn energy from the sun into fuels.
When it comes to solar conversion devices, chlorosomes are an attractive model to follow because of their simple composition and their ability to work well even in low-light conditions (green bacteria live under extremely low-light intensity). One of nature's processes, as unlocked by the scientists, may now provide new ways of approaching the challenge of translating nanostructured materials for the conversion of sunlight into fuel.