Bioenergy, Solar

From Photosynthesis Basics to Renewable Energy Breakthrough

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.

Recently, an international team led by scientists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have taken a major step toward understanding this process. Their work, detailed in the November 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 renewable energy technologies. The team, which includes scientists from Germany’s Technical and Free Universities in Berlin, the Max Planck Institute in Muelheim 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. 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. 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. “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,” said Junko Yano of Berkeley Lab’s Physical Biosciences Division, who is one of the lead authors of the study. 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. “We have a real structure now,” added 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. Ultimately, this research will further 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,” said 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.” 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.