Most people understand that for plants to survive, water, carbon dioxide and sunlight must go in, and oxygen comes out through photosynthesis. What happens in between, however, was something of a mystery until now: Using femtosecond X-ray crystallography combined with laser pulse excitation on an artificial leaf, researchers at ASU’s Center for Bio-Inspired Solar Fuel Production have finally captured water oxidation in action.
“This study is the first step towards our ultimate goal of unraveling the secrets of water splitting and obtaining molecular movies of biomolecules,” said Petra Fromme, professor of chemistry and biochemistry at ASU.
Every living thing on Earth today depends on this process. Until 2.5 billion years ago, Earth’s atmosphere contained no oxygen, but was slowly converted as plants “invented” the photosynthetic process. Now, with the artificial leaf process, scientists are able to further explore the mechanisms at work in the hopes of creating systems that match (or even exceed) the efficiency of natural photosynthetic organisms.
“The driving force for the oxidation of water is provided by the Photosystem II reaction center which is coupled to the water oxidation complex. By absorbing a quantum of light the molecule of the primary donor, called P680, in the reaction center launches a transfer of an electron along a series of redox active molecules. As a result the photooxidized primary donor P680+ acquires a strong oxidation potential and removes one electron from the water oxidation catalyst. After four turnovers of the reaction center the cycle of oxidation of two molecules of water and replenishing of the Mn cluster is complete by releasing one molecule of oxygen.”
How they were able to capture these processes is a miracle itself: Traditional X-ray crystallography produces only static images, and the subject of those images can become damaged by the X-rays. The laser pulses are what enabled the breakthrough.
“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,” said Fromme. “Extremely fast femtosecond (10 -15 second) laser pulses record snapshots of the PSII crystals before they explode in the X-ray beam, a principle called ‘diffraction before destruction.’”
In the future, the team hopes to make an actual movie of the processes in action in the hopes of further understanding them.