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A dual look at photosystem II using the world's most powerful X-ray laser

teria, is the only known biological system able to harness sunlight for the oxidation of water into molecular oxygen. Photooxidation of water by photosystem II is responsible for most of the oxygen in Earth's atmosphere. At the core of photosystem II is a manganese-calcium (Mn4Ca) complex that when energized by solar photons catalyzes a four photon-step cycle of oxidation states (S1-to-S4) that ultimately yield molecular oxygen. Scientists need to observe intact X-ray crystallography of the Mn4Ca ion in action but the molecule is highly sensitive to radiation.

"X-ray damage to metalloenzyme crystals has been a big issue for scientists even when the crystals were imaged at cryogenic temperatures," Yachandra says. "The LCLS is the world's only source of X-rays at this time capable of providing femtosecond pulses at the high intensities that enabled us to image intact photosystem II crystals before they were destroyed by exposure to the X-ray beams."

SLAC's LCLS is an X-ray free electron laser (XFEL) powered by a two-mile-long linear accelerator (or linac) that generates pulses of X-ray light on a femtosecond timescale. These pulses are more than a billion times brighter than those from the most powerful synchrotrons. SLAC is operated by Stanford University on behalf of DOE.

With their simultaneous XRD/XES experiments, Yano, Yachandra and their colleagues were able to observe the geometric structure and follow changes in the electronic structure of the Mn4Ca catalyst as they pumped the photosystem II crystal with visible-light laser pulses to simulate solar photons.

"We were able to unequivocally show that both the photosystem II complex and the Mn4Ca complex remain intact through the first two steps (S1 and S2) of the photooxidation process," says Yachandra.

Says Nicholas Sauter, Berkeley Lab computer scientist and a co-author of the study, "To be able to draw these conclusions, we've developed new software tools and are l

Contact: Lynn Yarris
DOE/Lawrence Berkeley National Laboratory

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A dual look at photosystem II using the world's most powerful X-ray laser
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