Researchers use LCLS x-ray laser to view simultaneously the structure and chemical behavior of Photosystem II catalyst; major step in studying catalytic processes
14 February 2013
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| Conceptual view shows a Photosystem II crystal hit by a femtosecond X-ray pulse. The resulting diffraction pattern (left) is used to map the overall protein structure. The emission spectrum (right) is simultaneously collected and used to probe the intactness and oxidation state of the crystal’s key Mn complex. (Credit: Greg Stewart, SLAC) Click to enlarge. |
An international team of researchers has used an X-ray laser at the Department of Energy’s (DOE) SLAC National Accelerator Laboratory to look simultaneously at the structure and chemical behavior of the Photosystem II catalyst involved in photosynthesis for the first time. The work, made possible by the ultrafast, ultrabright X-ray pulses at SLAC’s Linac Coherent Light Source (LCLS), is a breakthrough in studying atomic-scale transformations in photosynthesis and other biological and industrial processes that depend on catalysts, which efficiently speed up reactions.
This pioneering experimental technique can be used to further study photosynthesis and other catalytic reactions, the researchers said in a paper published in the journal Science.
We have established that simultaneous XRD and XES studies using ultra-short ultra-bright x-ray pulses at LCLS can probe the intact atomic structure of PS II microcrystals, and the intact electronic structure of its Mn4CaO5 cluster at room temperature. This technique can be used for future time-resolved studies of light-driven structural changes within protein and cofactors, and of chemical dynamics at the catalytic metal center under functional conditions. We expect that this method will be applicable to many metalloenzymes, including those that are known to be very sensitive to x-ray photo-reduction and radiation damage, and over a wide range of time scales, starting with femtoseconds.
—Kern et al.
| LCLS |
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| The Linac Coherent Light Source (LCLS) uses SLAC’s two-mile-long linear accelerator (linac) to produce X-ray pulses of unprecedented brilliance—more than a billion times brighter than the most powerful existing sources (synchrotron sources which are also based on large electron accelerators). |
| Today’s synchrotron facilities produce X-rays that are millions of times brighter than medical X-rays. Scientists use these highly focused, intense beams of X-rays to reveal the identity and arrangement of atoms in a wide range of materials. |
| However, atoms are constantly moving or vibrating, and synchrotron X-ray sources produce long pulses which yield only blurred images of these motions. LCLS is the first source to produce X-rays that are both very intense and clumped into ultrafast pulses. |
| Chemical reactions can take place in quadrillionths of a second (femtoseconds). The ultrafast LCLS X-ray flash captures images of these events with a “shutter speed” of less than 100 femtoseconds. |
Catalysts are vital to many industrial processes, such as the production of fuels, food, pharmaceuticals and fertilizers, and represent a $12 billion-per-year market in the United States alone. Natural catalysts are also key to the chemistry of life; a major goal of X-ray science is to learn how they function in photosynthesis, which produces energy and oxygen from sunlight and water.
The LCLS experiment focused on Photosystem II, a protein complex in plants, algae and some microbes that carries out the oxygen-producing stage of photosynthesis. This four-step process takes place in a simple catalyst—a cluster of calcium and manganese atoms (Mn4CaO5). In each step, Photosystem II absorbs a photon of sunlight and releases a proton and an electron, which provide the energy to link two water molecules, break them apart and release an oxygen molecule.
Past studies have been able to freeze crystals of the catalyst at various stages of the process and see how it looked. But scientists wanted to see the chemistry take place. This was not possible at other X-ray facilities, because the fragile crystals had to be frozen to protect them from radiation damage.
However, the LCLS X-ray laser comes in such brief pulses—measured in quadrillionths of a second—that they could probe the crystals at room temperature in a chemically active state before any damage set in, and generate data on two of the four steps in oxygen generation.
In future LCLS experiments, the researchers hope to study all the steps carried out by Photosystem II in higher resolution, revealing the full transformation of water molecules into oxygen molecules—considered a key to unlocking the system’s potential use in making alternative fuels.
Besides scientists from Berkeley Lab, SLAC and Stanford University, researchers from Technical University Berlin in Germany, Umea and Stockholm universities in Sweden and the European Synchrotron Radiation Facility in France also participated in the research. This work was supported by the DOE’s Office of Science, the National Institutes of Health, the German Research Foundation (DFG), the Alexander von Humbolt Foundation, Umea University, the K&A Wallengberg Foundation, and the Swedish Energy Agency.
LCLS is supported by DOE’s Office of Science. SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research.
Resources
Jan Kern, Roberto Alonso-Mori, Rosalie Tran, Johan Hattne, Richard J. Gildea, Nathaniel Echols, Carina Glöckner, Julia Hellmich, Hartawan Laksmono, Raymond G. Sierra, Benedikt Lassalle-Kaiser, Sergey Koroidov, Alyssa Lampe, Guangye Han, Sheraz Gul, Dörte DiFiore, Despina Milathianaki, Alan R. Fry, Alan Miahnahri, Donald W. Schafer, Marc Messerschmidt, M. Marvin Seibert, Jason E. Koglin, Dimosthenis Sokaras, Tsu-Chien Weng, Jonas Sellberg, Matthew J. Latimer, Ralf W. Grosse-Kunstleve, Petrus H. Zwart, William E. White, Pieter Glatzel, Paul D. Adams, Michael J. Bogan, Garth J. Williams, Sébastien Boutet, Johannes Messinger, Athina Zouni, Nicholas K. Sauter, Vittal K. Yachandra, Uwe Bergmann, and Junko Yano (2013) Simultaneous Femtosecond X-ray Spectroscopy and Diffraction of Photosystem II at Room Temperature. Science doi: 10.1126/science.1234273
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