RIKEN addresses structure of photosystem II

From a post at R&D Magazine

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The structure of photosystem II has been examined before at very high resolution by x-ray diffraction crystallography, and detailed models of the system have been obtained. However, the high-energy x-rays used in such analyses tend to damage the structure of the complex, resulting in some discrepancy among findings and a lack of clarity for certain atomic structures.

The research team avoided the problem of radiation damage by using a series of ultrashort femtosecond x-ray pulses generated by RIKEN's 'SACLA' x-ray free-electron laser facility. The short pulses prevented structural damage and the more accurate results allowed the researchers to identify several new features of the catalytic center of photosystem II from the bacterium Thermosynechococcus vulcanus. The results indicate that catalysis occurs in a cluster of manganese and oxygen atoms with one calcium atom. With more accurate measurement of the distances between the atoms, the researchers revealed that one of the oxygen atoms may actually form part of a hydroxide ion and could be derived from a split water molecule.

"We solved the structure in the 'dark-stable' state before the water-splitting reaction," explains Jian-Ren Shen, one of the researchers from Okayama Univ. "We now hope to use the same method with light illumination to follow the process through the reaction cycle." Such a study would provide a complete picture of how the biological photosynthesis reaction proceeds and greatly assist efforts to use the process in a fuel cycle.

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