Seeing how plants split water could provide key to our future energy needs

The possibility of using the Earth’s abundant supply of water as a cheap source of hydrogen is a step closer thanks to researchers from Imperial College London. By mimicking the method plants use to split water, researchers say that a highly energy efficient way to form cheap supplies of hydrogen fuel may be possible in the future.

Reporting online in the journal Science today Imperial researchers reveal the fine detail of the protein complex that drives photosynthesis – the process that converts atmospheric carbon dioxide into organic matter and oxygen (O2) by using sunlight to split water (H2O).

Using X-ray crystallography, the researchers describe for the first time the mechanism that underpins the photosynthetic water-splitting reaction. By analysing these findings the researchers believe it may be possible to learn how to recreate the process on an industrial scale, allowing hydrogen to be manufactured as a fuel.

Professor Jim Barber of Imperial’s Department of Biological Sciences explains:

“Without photosynthesis life on Earth would not exist as we know it. Oxygen derived from this process is part of the air we breathe and maintains the ozone layer needed to protect us from UV radiation. Now hydrogen also contained in water could be one of the most promising energy sources for the future. Unlike fossil fuels it’s highly efficient, low polluting and is mobile so it can be used for power generation in remote regions where it’s difficult to access electricity.

“But the problem is hydrogen doesn’t exist on Earth by itself. Instead it combines with other elements such as oxygen to form water, or with carbon to form methane, coal and petroleum. However, water is very stable and for this reason cannot be used directly as a fuel. Researchers have investigated using electrolysis to split water into oxygen and hydrogen but today it costs ten times as much as natural gas, and is three times as expensive as gasoline.

Yet nature figured out how to split water using sunlight in an energy efficient way 2.5 billion years ago. By revealing the structure of the water splitting centre we can begin to unravel how to perform this task in an energy efficient way too.”

Photosynthesis occurs in plants, some bacteria and algae and involves two protein complexes, photosystem I, and photosystem II – which contains the water-splitting centre.

While previous models of PSII function have sketched out a picture of how the water splitting centre might be organised, the Imperial team were able to reveal the structure of the centre at a resolution of 3.5 angstroms (or one hundred millionth of a centimetre) in the cyanobacterium, Thermosynechococcus elongatus by combining the expertise of Professor So Iwata in solving protein structures and Professor Jim Barber in the photosynthetic process.

“Results by other groups, including those obtained using lower resolution X-ray crystallography at 3.7 angstroms have shown that the splitting of water occurs at a catalytic centre that consists of four manganese atoms (Mn),” explains Professor So Iwata of Imperial’s Department of Biological Sciences.

“We’ve taken this further by showing that three of the manganese atoms, a calcium atom and four oxygen atoms form a cube like structure, which brings stability to the catalytic centre. The forth and most reactive manganese atom is attached to one of the oxygen atoms of the cube. Together this arrangement gives strong hints about the water-splitting chemistry.

“Our structure also reveals the position of key amino acids, the building blocks of proteins, which provide a details of how cofactors are recruited into the reaction centre.”

Professor Barber added: “PSII is truly the ’engine of life’ and it has been a major challenge of modern science to understand how it works. Manufacturing hydrogen from water using the photosynthetic method would be far more efficient than using electrolysis and if we can learn how to use even a fraction of the 326 million cubic miles of water on the planet we can begin to address the world’s pressing need for new and environmentally friendly energy sources.”