First step in converting solar energy using 'artificial leaf'

Two things are needed to produce fuel from sunlight: an antenna that harvests light, and a light-driven catalyst. The most efficient antennae contain bacteria. An international team headed by Huub de Groot imitated them and discovered how they function.

Structure of artificial light harvesters determined

An international team of researchers has modified chlorophyll from an alga so that it resembles the extremely efficient light antennae of bacteria. The team was then able to determine the structure of these light antennae. This is the first step to converting sunlight into energy using an artificial leaf. The researchers will be publishing an article on their research findings in the online Early Edition of the PNAS (Proceedings of the National Academy of Sciences) journal in the week starting 29 June.
Leiden researcher Swapna Ganapathy has obtained her PhD based on this subject, under the supervision of Professor Huub de Groot, one of the initiators of the research.
Forests at nano scale
They are the stuff of dreams: artificial forests at nano scale. Or pavements and motorways where gaps in the surface are filled with pigment molecules that collect sunlight and convert it into fuel and other forms of – clean – energy. But before this can happen, artificial photosynthesis systems first have to be developed that work both quickly and efficiently.  
Fuel from sunlight
Two things are needed to generate fuel from sunlight: an antenna that harvests light, and a light-driven catalyst. The article in PNAS is about the first of these: the antenna.
Imitating light antennae of bacteria
The fastest light harvesters can be found in nature: in green leaves, algae and bacteria. The light antennae of bacteria – chlorosomes – are the fastest of all. They have to be capable of harvesting minimal quantities of light particles in highly unfavourable light conditions, such as deep in the sea. These chlorosomes are made up of chlorophyll molecules. The art is to imitate these systems very precisely. German colleagues in Huub de Groot’s team modified chlorophylls from the alga Spirulina, such that they resembled the pigments of bacteria. De Groot’s Leiden group then studied the structure of these semi-synthetic light antennae.
De Groot: ‘ Nanotechnology and supramolecular systems are becoming increasingly important, but it is very difficult to determine their structure. So-called cartoons are frequently made that give a schematic indication of what their structure could be.’ De Groot and his colleagues successfully determined the detailed molecular and supramolecular structure of their artificial self-assembled light antennae. They did this using a combination of solid state NMR and X-ray diffraction (see attachment). X-ray diffraction enabled them to determine the overall structure and NMR allowed them to penetrate deeply into the molecules. (See explanation of the method).
Stacking of molecules
De Groot: ‘We already knew that the light antennae in bacteria form a structure rather like the annual rings of a tree trunk. The molecules in these semi-synthetic antennae seem to stack in a different way; they are flat. But this, too, is one of four ways we had thought in advance were possible.
New approach
The researchers still have to determine how the light antennae of modified Spirulina chlorophylls work in practice. De Groot: ‘This is a completely new approach in this field.’ The new insights are coming in quick succession. Last month, De Groot, with an international team made up partly of different members, also reported a breakthrough in PNAS. In that article he showed how – also with a combination of NMR and another technique, namely electron microscopy – he had resolved the structure of the light antennae of the bacteria themselves. This allowed the researchers to explain how the antennae were able to function so quickly and so efficiently.

Zinc chlorins for artificial light-harvesting self-assemble into antiparallel stacks forming a microcrystalline solid-state material Swapna Ganapathy, Sanchita Sengupta, Piotr K. Wawrzyniak, Valerie Huber, Francesco Buda, Ute Baumeister, Frank Wurthner, and Huub J. M. de Groot
PNAS online Early Edition <


Also see other articles in the Leiden University newsletters:


The method: a combination of solid state NMR and X-ray diffraction

Solid state NMR and X-ray diffraction are both methods for the structural research of materials, generally with different areas of application in research.  

NMR for micro-level
NMR (nuclear magnetic resonance) is a microscopic technique for researching ordered, but not necessarily  crystalline materials. The nuclei of the atoms in the molecule are made to resonate using radio waves. These resonances are sensitive to the characteristics of the environment around the molecule and can jump between atoms and molecules. They can then be harvested using sensitive antennae and compared with the signal originally transmitted. Using advanced computer calculations, the stacking and ordering of molecules can be determined, but only at micro-level.

Limited number of reflections
X-ray diffraction is based on the principle of scattering of X-rays. Reflected waves produce a regular on-off pattern, and this regularity allows the structure to be determined using careful computer analysis. But for the structure to be adequate, the molecules have to be arranged over a longer distance, otherwise the reflections extinguish one another. Crystalline materials are therefore needed for the application of the diffraction techniques. 
What about if a material is semi-crystalline? The molecules are then spaced regularly, but each molecule is not spaced in precisely the same way. Then diffraction produces a limited number of reflections, not enough to completely clarify the structure at molecular level. 

Filling in the details
Solid state NMR can then be used to fill in the details. Together they generate a model of the packing and the symmetry of the self-assembled semi-crystalline structure, and provide a basis for further research of the functional characteristics. This new concept to clarify the structure of a material in solid state was applied to determine the stacking of zink chlorins that form conductive filaments with possibilities for application in supramolecular electronics and artificial light harvesters.
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Fundamentals of Science is one of the 11 research profile areas of Leiden University
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(30 June 2009/HP)

Last Modified: 17-11-2010