By studying spinach using serial femtosecond crystallography and electron paramagnetic resonance, an international team of physicists have come up with a potential method of converting sunlight into an environmentally-friendly alternative to fossil fuel.
In the research, published in Nature this month, the team used the most powerful X-ray laser in the world, the Linac Coherent Light Source (LCLS) located at the National Accelerator Laboratory in the USA, to record snapshots of the proteins involved in photosynthesis.
The study was carried out by Arizona State University (ASU) and Purdue University in the USA. The international team discovered that the proteins in spinach are capable of converting solar energy into chemical energy with an unrivalled efficiency of up to 60 per cent, providing a platform for the study of alternative energy sources and to create artificial photosynthesis. Artificial photosynthesis could allow for the conversion of the sun’s energy into renewable, hydrogen-based fuels.
The physicists first extracted photosystem II (PSII) - a protein complex involved in photosynthesis - from spinach, in a special laboratory that kept the spinach cool and sheltered from light. The team then mimicked the action of the sun by exciting the protein molecules with a laser, and then measured the changes in their electron configuration.
‘These proteins require light to work, so the laser acts as the sun in this experiment,’ said Yulia Pushkar, a Purdue associate professor of physics. ‘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.’
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 explained (see image 1).
The Purdue team recently revealed the structure of the first and third states of this photosynthetic mechanism, using a new technique called serial femtosecond crystallography. Extremely fast femtosecond laser pulses record snapshots of the PSII crystals before they explode in the X-ray beam, a principle called 'diffraction before destruction'.
The researchers performed the time-resolved femtosecond crystallography experiments on Photosystem II nanocrystals. The crystals are hit with two green laser flashes before the structural changes are revealed by the femtosecond X-ray pulses.
In addition to X-ray crystallography, which does not provide details of how the electronic configurations evolve over time, the team used electron paramagnetic resonance to reveal the electronic configurations of the molecules, Pushkar noted: ‘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.’
'This is a major step toward the goal of making a movie of the molecular machine responsible for photosynthesis, the process by which plants make the oxygen we breathe, from sunlight and water,' explained John Spence, ASU Regents’ Professor of physics.
The National Science Foundation and Department of Energy funded the Purdue team’s work, which is now available online in the journal Nature.
Image 1: The oxygen-evolving complex (OEC) of PS II cycles through five states, S0 to S4, where four electrons are sequentially extracted from the OEC in four light–driven, charge-separation events. Credit: Mary Zhu, ASU