Learning how nature splits water
Posted on Thursday, November 09, 2006 @ 22:26:18 UTC by vlad
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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.
...
"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."
....
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 pieced
together high-resolution (approximately 0.15 Ångstrom) structures of a
Mn4Ca cluster found in a photosynthetic protein complex (one
Еngstrom 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 Mülheim, 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
molecules.
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-Ångstrom-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-Ångstrom-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 Mülheim; Jacek Biesiadka, Bernhard Loll and
Wolfram Saenger from the Free University in Berlin; and Matthew Latimer
from SSRL.
Source: Lawrence Berkeley National Laboratory
Article from: http://www.physorg.com/news82034817.html
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