Learning how nature splits water

By Aussiegirl

What a marvelous process! Sunlight comes in, and after a unbelievably complex series of transformations, oxygen is produced -- and after a long while we humans appear. But why did the bacteria "think it necessary" to develop this new way of being? Why didn't they just stay as they were, since they were already alive and functioning? Why become more complicated? It's hard to imagine that any length of time could produce such change.
(The beautiful and complicated illustration "portrays the water-splitting catalytic cycle with the Mn4Ca structure in the middle".)

Learning how nature splits water

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. [....]

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. [....]

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." [....]