Chlorophyll in plants absorbs light from the sun, which then becomes energy used by the so-called "oxygen-evolving complex" to catalyse the splitting of water into molecular oxygen. This complex contains four manganese and one calcium atoms that are known to be at the centre of the catalytic reaction. Five intermediate states have been proposed in the process of photosynthesis - a cycle known as "Kok cycle"- but only four had been proved until recently. With the help of the ESRF, scientists have been able to identify the missing state, which is particularly important because it is directly involved in the molecular oxygen formation. They suggest, furthermore, an extension of the "Kok cycle" with an additional intermediate and propose a new reaction mechanism on a molecular basis for the release of dioxygen. This gives new insight into the mechanism of photosynthesis.
In order to study this process, the use of synchrotron light was crucial: "A very intense and stable X-ray beam is necessary to perform this study on such a complex, highly diluted protein present in the investigated spinach sample", explains Pieter Glatzel, head of beamline ID26, where the experiments were carried out. The researchers measured the fluorescence from the sample that is emitted after excitation with X-rays.
They flashed the sample with a laser and registered the change using X-ray fluorescence every 10 microseconds to find out how different oxidation states developed. When carefully analysing the reaction kinetics, they observed a time delay before the O2-evolving step. This delay unambiguously proved the existence of the long-searched for intermediate state.
How far away are we then from using the sun to power our daily lives? Michael Haumann, the main author of the publication, asserts that "these are important results that will make an impact in the photosynthesis community. They help our understanding of how solar energy is used in plants and contribute to the efforts to produce more efficient solar cells for our needs".