Jul
24
Yulia Pushkar, a Purdue assistant professor of physics involved in the research said, “The proteins we study are part of the most efficient system ever built, capable of converting the energy from the sun into chemical energy with an unrivaled 60 percent efficiency. Understanding this system is indispensable for alternative energy research aiming to create artificial photosynthesis.”
The process by which plants convert the sun’s energy into carbohydrates is used to power the cellular processes. During photosynthesis plants use solar energy to convert carbon dioxide and water into hydrogen-storing carbohydrates and oxygen. Artificial photosynthesis could allow for the conversion of solar energy into renewable, environmentally friendly hydrogen-based fuels.
In Pushkar’s laboratory, students extract a protein complex called Photosystem II from spinach they buy at the supermarket. It is a complicated process performed over two days in a specially built room that keeps the spinach samples cold and shielded from light, she said.
Physics professor Yulia Pushkar (left) and postdoctoral researcher Lifen Yan work in Pushkar’s laser lab. Click image for the largest view.
Once the proteins have been carefully extracted, the team excites them with a laser and records changes in the electron configuration of their molecules.
“These proteins require light to work, so the laser acts as the sun in this experiment,” Pushkar said. “Once the proteins start working, we use advanced techniques like electron paramagnetic resonance and X-ray spectroscopy to observe how the electronic structure of the molecules change over time as they perform their functions.”
The idea is to figure out how Photosystem II is involved in the photosynthetic mechanism that splits water molecules into oxygen, protons and electrons. During this process a portion of the protein complex, called the oxygen-evolving complex, cycles through five states in which four electrons are extracted from it, Pushkar said.
Oxygen Evolving Complex of PS II Cycle. Click image for more info.
Petra Fromme, professor of chemistry and biochemistry at Arizona State University, leads the international team that recently revealed the structure of the first and third states at a resolution of 5 and 5.5 Angstroms, respectively, using a new technique called serial femtosecond crystallography. A paper detailing the results was published in Nature and is available online.
In addition to Pushkar, Purdue postdoctoral researcher Lifen Yan and former Purdue graduate student Katherine Davis participated in the study and are paper co-authors.
Fromme explained in a statement, “The trick is to use the world’s most powerful X-ray laser, named LCLS, located at the Department of Energy’s SLAC National Accelerator Laboratory. Extremely fast femtosecond (one-quadrillionth of a second) laser pulses record snapshots of the PSII crystals before they explode in the X-ray beam, a principle called ‘diffraction before destruction.’”
While X-ray crystallography reveals structural changes, it does not provide details of how the electronic configurations evolve over time, which is where the Purdue team’s work came in. The Purdue team mimicked the conditions of the serial femtosecond crystallography experiment, but used electron paramagnetic resonance to reveal the electronic configurations of the molecules, Pushkar said.
“The electronic configurations are used to confirm what stage of the process Photosystem II is in at a given time,” she said. “This information is kind of like a time stamp and without it the team wouldn’t have been able to put the structural changes in context.”
The Arizona State University (ASU) press release hints that the team will get to a moving picture over time. The study that shows the first snapshots of photosynthesis in action as it splits water into protons, electrons and oxygen – the process that maintains Earth’s oxygen atmosphere.
The revealing of the mechanism of this water splitting process is essential for the development of artificial systems that mimic and surpass the efficiency of natural systems. The development of an “artificial leaf” is one of the major goals of the ASU Center for Bio-Inspired Solar Fuel Production, which was the main supporter of this study.
ASU Regents’ Professor Devens Gust in a connected paraphrase sums up, “A crucial problem is discovering an efficient, inexpensive catalyst for oxidizing water to oxygen gas, hydrogen ions and electrons. Photosynthetic organisms already know how to do this, and we need to know the details of how photosynthesis carries out the process using abundant manganese and calcium. The research gives us a look at how the catalyst changes its structure while it is working. Once the mechanism of photosynthetic water oxidation is understood, chemists can begin to design artificial photosynthetic catalysts that will allow them to produce useful fuels using sunlight.”
Its known that in photosynthesis, oxygen is produced at a special metal site containing four manganese atoms and one calcium atom, connected together as a metal cluster. This oxygen-evolving cluster is bound to the protein Photosystem II that catalyzes the light-driven process of water splitting. It requires four light flashes to extract one molecule of oxygen from two water molecules bound to the metal cluster.
The ultimate goal of the work is to record molecular movies of water splitting.
For now the researchers discovered large structural changes of the protein and the metal cluster that catalyzes the reaction. The cluster significantly elongates, thereby making room for a water molecule to move in.
Its a large team that is getting to the point they understand what is happening at the level of atoms inside the molecule. Its only a matter of time until a protein can be made to accomplish the job in an artificial way.
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