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ScienceWise - Summer 2013

Stealing Nature’s secrets

Article Illustration
Over billions of years nature has created the most efficient method for oxidising water possible.

How billions of years of practice created the world’s most efficient hydrogen source

Hydrogen represents the ideal green fuel for transport because it burns to create only water vapour in a vehicle exhaust. But one of the key steps in achieving clean, green renewable transport is of course generating that hydrogen in the first place. It can be electrolysed from water but this is a process that requires huge amounts of energy which has to be generated by other means. Alternatively, hydrogen can be obtained by processing hydrocarbons but that’s so inefficient you’d be better off just burning the hydrocarbons in the first place. So what’s to be done?

Professors Rob Stranger and Ron Pace are two chemists from the Australian National University who believe they may have at least part of the answer. “Plants generate their energy by photosynthesis and one key step in this process is the separation of water into hydrogen and molecular oxygen,” Professor Stranger explains.

“Nature’s been practicing this process for two and a half billion years so plants are really, really good at it. So good in fact that the mechanism they employ is almost 100% energy efficient” says Professor Pace.

If humans could replicate this kind of chemistry we’d have the potential for an energy revolution. Large facilities using safe and readily available raw materials could directly convert sunlight and water into clean liquid fuel. The problem is that we don’t yet fully understand all the details of Nature’s amazingly efficient photosynthesis chemistry.

One of the key processes revolves around a manganese and calcium catalyst compound known as the water oxidizing complex or WOC. Until recently the exact structure of WOC was a mystery. However, in 2011 a group of scientists succeeded in crystallising it. Being able to crystallise compounds is vitally important for chemists because it allows them use a technique known as X-ray diffraction to analyse molecular structure.

An individual molecule is too small to reveal its physical structure even in the most powerful electron microscopes. However, that very smallness becomes an advantage when using X-rays. Because the distance between the atoms of the molecule and the wavelength of the X-rays are similar the two strongly interact in the form of diffraction.

The signal from one molecule would be far too small to measure. However, when assembled into a crystal, all the identical molecules line up in orderly rows, so the signals from each one combine to create a characteristic diffraction pattern. By examining this pattern, chemists can determine the structure of that particular molecular species.

The trouble was that the WOC structure revealed by X-ray diffraction didn’t really seem to fit well into what was understood about the plant’s water oxidation mechanism. As a result, many scientists thought that the process of X-raying the crystal had damaged its structure and that the results were therefore misleading. However, the ANU team felt that there might be a way the bizarre structure could be correct if some of the underlying assumptions about the WOC mechanism were re-examined.

“It was generally thought that the manganese atoms in the complex needed to have a very high oxidizing power in order to function,” says Professor Pace, “But we were a little skeptical about that because it could be harmful to surrounding proteins. Having the manganese operating at maximum oxidizing capacity in this context would be a bit like using a wicker basket to contain a campfire, so we began to explore the chemistry of lower power oxidation.”
As with most modern chemists, the ANU team used supercomputer models of the molecules and reactions to test various possibilities, backing up the theoretical calculations with experiments in the lab. Eventually their persistence paid off.

They were able to derive a structure for the complex that not only fitted the X-ray data perfectly but also revealed how the positioning of the water molecules was critical to the efficient operation of the system.

“If you can steal nature’s secrets and understand how plants perform this chemistry, then you can learn to make hydrogen much more efficiently than we can at present” Professor Stranger says. “And the great thing about this system is that it’s already 100% efficient so we don’t need to improve it, we just need to understand it and make it work for us.”

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