Billions of years ago, primitive bacteria developed a way to harness sunlight to split water molecules into protons, electrons and oxygen-the cornerstone of photosynthesis. Now, a team of scientists has taken a major step toward understanding this process by deriving the precise structure of the catalytic metal-cluster center containing four manganese atoms and one calcium atom (Mn₄Ca) that drives this water-splitting reaction. This catalytic center resides in a large protein complex, called photosystem II, found in plants, green algae, and cyanobacteria. The international team was led by scientists from LBNL, and includes scientists from Germany's Technical and Free Universities in Berlin, the Max Planck Institute in Mülheim, and from SSRL.

Until now, the precise structure of the Mn₄Ca cluster has eluded all attempts of determination by x-ray crystallography and spectroscopic techniques, in part because the metal catalyst center is highly susceptible to radiation damage. The team used a novel combination of polarized single crystal x-ray absorption spectroscopy (XAS) and x-ray diffraction measurements at SSRL's BL9-3 to control the radiation dose and thereby obtain XAS data to high resolution. This enabled the team to constrain the possible metal cluster site structure to three similar ones at a resolution much higher (~0.15 Å) than previously possible.

The 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.

To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/research/highlights_archive/psII-06.html

Yano, J.; Kern, J.; Sauer, K.; Latimer, M. J.; Pushkar, Y.; Biesiadka, J.; Loll, B.; Saenger, W.; Messinger, J.; Zouni, A.; Yachandra, V. K. Where Water is Oxidized to Dioxygen: Structure of the Photosynthetic Mn₄Ca Cluster, (2006) Science 314, 821-825.