www.sciencedaily.com/release...0756.htm

ScienceDaily (July 13, 2009) — 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 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 subject 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.

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 are to 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 from the University of Würzburg 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.

Nanotechnology

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.

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.

Journal reference:

1. Swapna Ganapathy, Sanchita Sengupta, Piotr K. Wawrzyniak,
Valerie Huber, Francesco Buda, Ute Baumeister, Frank Wurthner, and
Huub J. M. de Groot. Zinc chlorins for artificial light-harvesting
self-assemble into antiparallel stacks forming a microcrystalline
solid-state material. PNAS, June 29, 2009

Adapted from materials provided by Leiden University, via EurekAlert!,
a service of AAAS.