Over billions of years, plants have perfected the process of using water and sunlight to produce an endless supply of hydrogen. The secrets to this clean, green energy source have remained locked within the flora world – until now.
Using computer modeling at the National Computational Infrastructure, Professors Rob Stranger and Ron Pace 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.
Relying on the NCI's supercomputing facilities, Stranger used a quantum chemical package, the Amsterdam Density Function (ADF) 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, prior to carbon dioxide fixation. For the first time, they have identified the specific water molecules which are converted to oxygen.
"The part of the plant's photosystem that is important to this process is called the oxygen-evolving-complex (OEC)," says Professor Stranger. "We have known for a long time that it 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 (negatively charged particles smaller than an atom) breaking the water down into separate oxygen molecules (which sustains all animal life) and hydrogen (as protons).
There has been debate as to how much oxidising power the manganese in the OEC have, with many research groups thinking the manganese operate at a higher oxidising power. However, both Professors Stranger and Pace say that this is dangerous in the plant photosystem as it could damage the surrounding protein. The problem is like "setting a fire in a wicker basket without burning the basket" but nature finesses this at the limit of chemical possibility.
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 confused many people in field, causing them 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 high-resolution X-ray image. They then put the computerized OEC through its paces, demonstrating the manganese were not working at maximum oxidation capacity and showing the high-resolution 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 to react efficiently – a big step towards creating a renewable source of hydrogen fuel.
"Photosynthesis itself involves hundreds of thousands of atoms", says Pace. "The processes that we're interested in look at only 150 – 200 atoms, and these calculations can take anywhere from a few days to a few weeks on the supercomputer. Our next step is to make them bigger – more realistic, more accurate – which is why we depend on NCI and the valued staff. The new petascale machine will allow us to expand our models".
Professor Stranger stated that "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 than it can be done at the moment".
"We're going to need chemically storable fuels for many years yet and hydrogen is the one for a totally renewable fuel future", says Professor Pace.
