Researchers from the Research School of Chemistry have made an important step towards unlocking the potential of a plant’s photosynthetic powerhouse, moving closer to a renewable source of clean hydrogen fuel.
Professors Rob Stranger and Ron Pace used computer modelling to reveal the molecular structure of the photosynthesis reaction site in plants, where sunlight is used to convert water into its components - hydrogen and oxygen. For the first time, they have identified the specific water molecules in a plant’s photosystem which are converted to oxygen. Their results are detailed in the recent edition of the journal, Angewandte Chemie.
“The part of the plant’s photosystem that is important to this process is called the oxygen-evolving-complex (OEC),” says Professor Stranger. “We know the OEC contains four manganese atoms and a calcium atom, but for decades scientists have been trying to determine the structure of the system and how it works.”
In a process called oxidation, the manganese atoms strip water molecules of electrons (tiny, negatively charged particles) breaking water down into oxygen molecules and positively charged hydrogen particles. There has been debate in the field as to how much oxidising power the manganese in the OEC has, with many research groups thinking the manganese operates at maximum oxidising power.
“What happens in the OEC can be likened to setting a fire in a wicker basket without burning the basket,” says Professor Pace. ”Working at a maximum level is dangerous in the plant photosystem as it could damage the surrounding protein.”
Armed with this knowledge, Stranger and Pace were able to clearly understand a controversial high-resolution X-ray image of the OEC structure published in 2011. “There were some chemically puzzling features in the X-ray image that caused many people to reject the image as flawed,” explains Professor Pace. “When you believe manganese is working at maximum oxidation capacity, this image appeared to conflict with earlier experimental results, in particular lower resolution X-ray studies.”
Using computer modeling, Stranger and Pace recreated the structure of the OEC shown in the image. They then put the computerised OEC through its paces, demonstrating the manganese was not working at maximum oxidation capacity and showing the image did in fact agree with experimental data and previous lower resolution images.
“We were able to show that this structure was completely chemically reasonable,” states Professor Pace.
Further to confirming the OEC structure, their model showed for the first time how the critical water molecules in the OEC are positioned– a big step towards creating a renewable source of hydrogen fuel.
“If you can steal nature’s secrets and understand how the OEC performs this chemistry, then you can learn to make hydrogen much more efficiently. And hydrogen is the fuel for a totally renewable fuel future,” concludes Professor Stranger.