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Learning How Nature Splits Water: HighResolution Structure of Photosynthetic Catalyst Holds Promise for Clean Energy

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Posted by lkgeo1 on November 4, 2006, 11:16 pm

Learning How Nature Splits Water: HighResolution Structure of
Photosynthetic Catalyst Holds Promise for Clean Energy

Publication Date:04-November-2006
03:30 PM US Eastern Timezone
BERKELEY, CA - About 3.2 billion years ago, primitive bacteria
developed a way to harness sunlight to split water molecules into
protons, electrons and oxygen, the cornerstone of photosynthesis that
led to atmospheric oxygen and more complex forms of life - in other
words, the world and life as we know it. Today, scientists have taken a
major step toward understanding this process by deriving the precise
structure of a catalyst composed of four manganese atoms and one
calcium atom that drives this water-splitting reaction. Their work,
detailed in the Nov. 3, 2006 issue of the journal Science, could help
researchers synthesize molecules that mimic this catalyst, which is a
central focus in the push to develop clean energy technologies that
rely on sunlight to split water and form hydrogen to feed fuel cells or
other non-polluting power sources.

Specifically, an international team led by scientists from the U.S.
Department of Energy's Lawrence Berkeley National Laboratory
(Berkeley Lab) pieced together high-resolution (approximately 0.15
ngstrom) structures of a Mn4Ca cluster found in a photosynthetic
protein complex (one ngstrom equals one ten-billionth of a meter).
The team, which includes scientists from Germany's Technical and Free
Universities in Berlin, the Max Planck Institute in Mlheim, and the
Stanford Synchrotron Radiation Laboratory, used an innovative
combination of x-ray spectroscopy and protein crystallography to yield
the highest-resolution structures yet of the metal catalyst.

"This is the first study to combine x-ray absorption spectroscopy and
crystallography  in such a detailed manner to determine the structure
of an active metal site in a protein, especially something as
complicated as the photosynthetic Mn4Ca cluster," says Junko Yano of
Berkeley Lab's Physical Biosciences Division, who is one of the lead
authors of the study.

The metal catalyst resides in a large protein complex, called
photosystem II, found in plants, green algae, and cyanobacteria. The
system drives one of nature's most efficient oxidizing reactions by
using light energy to split water into oxygen, protons, and electrons.
Because of its efficiency and reliance on nothing more than the sun,
the catalyst has become a target of scientists working to develop
carbon-neutral sources of energy. Learn the catalyst's structure,
then how it works, and perhaps scientists can develop similarly robust

But until now, the precise structure of the catalyst has eluded all
attempts of determination by x-ray diffraction and various
spectroscopic techniques. Even a 3.0-ngstrom-resolution structure
obtained by the Berkeley Lab group's collaborators at the Technical
and Free Universities in Berlin, using x-ray diffraction, didn't
allow the researchers to pinpoint the exact positions of the
cluster's manganese and calcium atoms and its surrounding ligands.
Part of the problem is the fact that the metal catalyst is highly
susceptible to radiation damage, which rules out extremely
high-resolution x-ray diffraction studies.

To minimize radiation damage, Yano and colleagues combined x-ray
absorption fine structure spectroscopy measurements with x-ray
diffraction data from crystallographic studies, which were obtained at
the Stanford Synchrotron Radiation Laboratory, where the techniques
used in this study were developed in collaboration with the Berkeley
Lab scientists. This technique exposes the Mn4Ca cluster to much lower
doses of radiation, and enabled the team to obtain three similar
structures at a resolution much higher than previously possible.

These three structures shed new light on how the catalyst fits within
the much larger photosystem II protein complex. The x-ray diffraction
structures at a medium resolution are sufficient to determine the
overall shape and placement of the catalyst within the protein complex,
and the spectroscopy measurements provide high-resolution information
about the distances and orientation of the catalyst.

 "We have a real structure now," adds Vittal Yachandra, also with
Berkeley Lab's Physical Biosciences Division and a co-author of the
paper. "It's not just guesswork anymore. Before, there were a lot
of disparate pieces and scientists were forced to speculate on the
catalyst's structure. Now, we can begin to infer how the energy of
sunlight is used to oxidize water to molecular oxygen."

Scientists already know that the catalyst goes through four steps as it
oxidizes water to oxygen, with each step triggered by the absorption of
a photon. Now, they can learn how individual bonds are broken and
formed, and how the water molecule splits apart, step by step. The
group's high-resolution structure is already yielding clues.

"We found that our structure is unlike the 3.0 ngstrom-resolution
x-ray structure and other previously proposed models," says Yano.
"The higher-resolution structures are likely to be important in
gaining a mechanistic understanding of water oxidation."

Ultimately, this research will inform the search for renewable energy
sources. Many of the strategies scientists propose depend on a way to
wrest hydrogen, which is an energy carrier, from water. Unfortunately,
the current methods used to extract hydrogen from water require either
electricity or methane, both of which come at a price.

"That's why the water-splitting complex in photosynthesis is the
basis for a lot of work being done in energy research today," says
Yachandra. "This is the main underpinning for our work. We are trying
to understand how nature works so we can apply the same principles to
clean energy research."
The Science paper is entitled Where Water is Oxidized to Dioxygen:
Structure of the Photosynthetic Mn4Ca Cluster. Co-authoring the paper
with Yano and Yachandra are Ken Sauer and Yulia Pushkar from Berkeley
Lab and UC Berkeley; Jan Kern and Athina Zouni from the Technical
University in Berlin; Johannes Messinger from the Max Planck Institute
in Mlheim; Jacek Biesiadka, Bernhard Loll and Wolfram Saenger from
the Free University in Berlin; and Matthew Latimer from SSRL.

This work is part of Berkeley Lab's Helios program, which seeks to
develop abundant and inexpensive solar-based energy technologies. The
research was supported by the U.S. Dept of Energy, Office of Basic
Energy Sciences, the National Institutes of Health, the Deutsche
Forschungsgemeinschaft, and the Max-Planck-Gesellschaft. Synchrotron
facilities were provided by the Stanford Synchrotron Radiation
Laboratory operated by DOE.

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