Understanding the role chloride ions play in photosynthesis may lead to better solar cells, claim UK scientists.

If we could use sunlight efficiently to split water into hydrogen and oxygen like plants do, mankind would have an unlimited supply of hydrogen to use as fuel, says James Barber of Imperial College London. He has found new insight into the photosynthesis mechanism, bringing this dream a little closer.

Two halide binding sites in the oxygen evolving center of Photosystem II located by X-ray crystallography

Barber's team has located the binding sites of two chloride ions in photosystem II (PSII), the complex that catalyses the water splitting reaction in plants. The location of the ions suggests that they play a role in transport of protons and substrates into and out of the active centre, says the team.

'We have used a trick whereby heavier bromide ions were substituted for chloride within PSII, which maintains full activity,' Barber says. This allowed the team to use an x-ray diffraction technique sensitive to heavy atoms to study crystals of the enzymes and locate the binding sites for bromide, and therefore chloride.

'It has been known for a long time that chloride ions play a role in the chemistry of PSII, but the molecular mechanism has been unclear,' says Gary Brudvig of Yale University, New Haven, US, who himself studies the water splitting reaction in PSII. He says that Barber's results represent 'a significant step forward,' as the exact role of the chloride ions can now be studied based on the new structural information.

'Natural photosynthesis provides a working example of how sunlight can be used for fuel production,' says Brudvig. 'With a better understanding of how nature does the very difficult water-oxidation chemistry, chemists can use this information to design efficient systems for solar energy conversion'.

Danièle Gibney