Scientists at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and SLAC National Accelerator Laboratory are working toward a better understanding of how a large complex of proteins, called Photosystem II, is able to split water molecules into oxygen, electrons and hydrogen ions (protons).

Electron Density Map of the Manganese Calcium Cluster. Click image for more info.

Splitting the water for the hydrogen and the oxygen is the solar energy system that makes air breathing animal life on earth possible and provides the source of all the hydrogen enriched carbon form foods like sugar and starches to oils and fats that can be food and fuel as well.

As a matter of process structure that could drive down operating costs, this research is critical.  It might enable science to compress the equivalent of millions of years of oil, gas and coal formation into a growing season and a few hours or days.  Getting carbon or nitrogen is fairly easy; coming up with free hydrogen is an energy intense and large investment demanding matter.

The potential can be an artificial version of photosynthesis that could use sunlight to produce liquid fuels from nothing more than carbon dioxide and water.

Photosystem II could be expected to be the key to unlimited transports liquid fuels.

The DOE and Berkeley Lab lead an international team using ultrafast, intensely bright pulses of X-rays from SLAC’s Linac Coherent Light Source (LCLS)/  The research team produced the first ever images at room temperature of microcrystals of the Photosystem II complex. Previous imaging studies, using X-rays generated via synchrotron radiation sources, required cryogenic freezing, which alters the samples.  The work is described in the Proceedings of the National Academy of Sciences (PNAS) titled “Room Temperature Femtosecond X-ray Diffraction of Photosystem II Microcrystals.

To catalyze its reactions, Photosystem II relies upon an enzyme that contains a manganese-calcium cluster that is highly sensitive to radiation. With the high-intensity femtosecond X-ray pulses of the LCLS, the research team was able to record intact images of these clusters before the radiation destroyed them.

X-ray Diffraction Patterns of Photosystem II Microcrystals. Click image for more info.

Vittal Yachandra, a chemist with Berkeley Lab’s Physical Biosciences Division comments, “We have demonstrated that the ‘probe before destroy’ strategy of the LCLS is successful even for the highly-sensitive oxygen bridged manganese-calcium cluster in Photosystem II at room temperature. This is an important step toward future studies for resolving the composition and atomic structure of the manganese-calcium cluster in the Photosystem II complex during the critical formation of oxygen molecules.”

It astonishes that for more than two billion years, nature has employed photosynthesis to oxidize water into molecular oxygen.  Photosystem II, the only known biological system that can harness visible light for the photooxidation of water, produces most of the oxygen in Earth’s atmosphere through a five-step catalytic cycle (S0-to-S4 oxidation states). Light-harvesting proteins in the complex capture solar photons that energize the manganese-calcium cluster and drive a series of oxidations and proton transfers that in the final S4 state forms the bond between oxygen atoms that yields molecular oxygen.

Yachandra and Junko Yano, a chemist with Berkeley Lab’s Physical Biosciences Division, have led scientific teams in the past that have shed much light on the S0 through S3 oxidation states of the manganese-calcium cluster, which remain stable for several seconds. However, the S4 state is highly reactive and has not yet been fully characterized in experiments.

Yano takes up the explanation with, “Capturing the S4 state in a time-resolved manner will be essential for understanding the water-oxidation mechanism. While X-ray diffraction is clearly the technique of choice for such detailed structural studies, the inherent radiation sensitivity of the manganese-calcium cluster poses a major challenge for protein crystallography on synchrotron radiation sources.”

SLAC’s LCLS is an X-ray laser 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. Yachandra, Yano and their colleagues suspended Photosystem II microcrystals in a liquid that was jet-streamed into the path of the pulsed light. The diffraction of LCLS X-rays passing through the Photosystem II microcrystals created patterns that computers reconstructed into images of the complex’s composition and atomic structure at a resolution of 6.5 angstroms – one ten-billionth of a meter or about the diameter of a hydrogen atom.

Jan Kern, a research scientist at Berkeley Lab and SLAC who is the lead author on the PNAS paper anticipates, “We hope that with improved samples, in the future we will be able to get to a higher resolution – perhaps 3 angstroms or better.”

The experiment to produce the paper used Photosystem II microcrystals (approximately 10 micrometers in diameter) as a matter of efficiency. Molecular reconstruction through X-ray diffraction requires the examination of literally millions of crystals, since each shot from the LCLS destroys the specimen.

Kern explains, “Because it takes months to grow sufficient quantities of the Photosystem II complex in bacterial culture, the use of microcrystals made the most efficient use of time and materials. Also, microcrystals were much easier to direct toward the LCLS X-ray beam using the liquid-stream sample delivery system developed by our collaborators at SLAC.”

Paper co-authors Paul Adams and Nicholas Sauter, also with Berkeley Lab’s Physical Biosciences Division led the data analysis in this study, writing new software to manage the computations.
“Doing this study was a monumental achievement that required a large team to make it happen,” Sauter says. “We injected crystal samples into the beam at a rate of 120 per second, and after a week we had 63 Terabytes of data from which we selected the best 7,000 diffraction images to reconstruct photosystem II’s molecular structure.”
More work is underway at the LCLS using both X-ray diffraction and spectroscopy techniques to investigate the intermediate reaction states formed in the Photosystem II complex as it undergoes photooxidation.

Yano winds up the press release update with, “We hope to learn from nature’s design principles and apply that knowledge to the design and development of artificial photosynthetic systems.”

The puzzle of photosynthesis is getting taken apart such that engineering a mechanical process looks like part of the future.  Just how efficient that might be, how expensive to build and operate are questions still well off in the future.  Many innovators believe they have a synthetic photosynthesis device, but so far the productive values are wanting.

Science needs the photosynthesis of nature understood well enough to work out the fundamentals for creativity to apply.  An endless supply of hydrogen to enrich carbon and nitrogen would solve the paradox of supplying high density liquid fuels.

Life is an amazing thing, and the more we understand it the more complex and beautiful is becomes.


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