Solar power: Crystallographic clues

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  • Published: Aug 27, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Solar power: Crystallographic clues

Systemic understanding

Purdue physics professor Yulia Pushkar (left) and postdoctoral researcher Lifen Yan work in Pushkar's laser lab. Pushkar and Yan are part of an international team using spinach to study the proteins involved in photosynthesis. (Purdue University photo/Tim Brouk)

An international research team is testing its intellectual strength using serial femtosecond crystallography with a view to teasing apart the proteins involved in photosynthesis, the process by which plants convert solar energy into carbohydrates.

In research funded by the US National Science Foundation and the US Department of Energy, the team of Yulia Pushkar and postdoctoral researcher Lifen Yanin at Purdue University in West Lafayette, Indiana, working with international colleagues have focused their attention on the unicellular marine cyanobacterium Thermosynechococcus elongatus with a view to understanding photosynthesis.

Photosynthesis is the well known process by which plants, algae and cyanobacteria exploit the energy from sunlight to catalyse the conversion of carbon dioxide and water into carbohydrates and thus allow them to grow and ultimately feed other life forms. There are two large membrane protein complexes involved in photosynthesis: photosystem I and II (PSI and PSII). These systems act in series to catalyse the light-driven reactions in photosynthesis. PSII uses light to split water molecules releasing oxygen as a side product that helps maintain the composition of the Earth's atmosphere. The water-splitting process involves the cycling of the so-called oxygen-evolving complex (OEC) of PSII through five different states, S0 to S4, in which, the researchers explain, four electrons are sequentially extracted from the OEC in four light-driven charge separation events.

Free laser faciliitation

The team has now used the recently developed technique of serial femtosecond crystallography in which a series of single-shot diffraction patterns can be collected from a stream of nanocrystals, using femtosecond pulses from an X-ray Free Electron Laser (XFEL) rather than relying on the ability to produce the mesoscopic crystals required of conventional diffraction techniques. This circumvents the problem facing many protein scientists hoping to discover the inner workings of membrane proteins. Pushkar and colleagues have used this relatively new approach to carry out time-resolved experiments on PSII nano- and micro-crystals from T. elongatus.

The team has obtained structures from PSII in the dark S1 state and following excitation with a double laser pulse that gives rise to a putative S3 state, with 5 and 5.5. angstrom resolution. "The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC," the team explains. This results in a concomitant elongation of the metal cluster as well as changes in the protein environment. On the basis of electron paramagnetic resonance and X-ray spectroscopic studies and computational work, the team suggests that these changes should facilitate binding of the second substrate water molecule between the now dangling manganese ion and the Mn3CaOx cubane structure as the system makes the transition from S2 to S3. In Pushkar's laboratory, researchers have also extracted the equivalent PSII protein complex from shop-bought spinach (Spinacia oleracea) to corroborate the universality of the system in nature.

Alternative energy

"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 unrivalled 60 percent efficiency," explains Pushkar. "Understanding this system is indispensible for alternative energy research aiming to create artificial photosynthesis." Emulating nature's prowess with an artificial photosynthetic system might usurp all the current technology based on semiconductor materials and allow us to develop machines for converting solar energy into renewable, environmentally friendly hydrogen-based fuels.

While the X-ray crystallography work reveals the structural changes taking place in a "diffraction before destruction" approach, the electron paramagnetic resonance spectroscopy is needed to reveal the electronic configurations of the molecules as they evolve. "The electronic configurations are used to confirm what stage of the process Photosystem II is in at a given time," Pushkar explains. "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."

Related Links

Nature 2014, online: "Serial time-resolved crystallography of photosystem II using a femtosecond X-ray laser"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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