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INORGANIC CHEMISTRY
Calcium is an essential component of the catalyst that forms oxygen from water during photosynthesis. It seems that part of calcium's job is to enable the release of oxygen from this catalyst.
The mechanism of oxygen production in photosynthesis remains unresolved, despite many studies. The presence of a calcium ion in the inorganic active site of the complex that produces oxygen is particularly puzzling, because the reaction is a redox pro- cess involving the transfer of multiple electrons, whereas calcium is redox-inactive (it cannot transfer electrons under biological conditions). Writing in Nature Chemistry, Bang et al.1 pro- vide insight into the ion's possible role. They report that, under oxidative conditions, oxygen molecules can be released from iron peroxide complexes - which can be thought of as sim- plified models of the active site - in the pres- ence of calcium ions, but not in the presence of other metal ions that are better Lewis acids (electron-pair acceptors) than calcium.
Oxygenic photosynthesis, the process by which plants convert carbon dioxide to organic matter using solar energy, has generated most of the oxygen (O2 molecules) in the atmosphere, enabling life on Earth as we know it. Oxygen forms when water is oxi- dized, a complex chemical reaction involving the transfer of four electrons and four protons (H+ ions; Fig. 1a). The reaction is catalysed by the oxygen-evolving complex (OEC) - an inorganic cluster of four manganese ions and one calcium ion bridged by oxide ligands2, buried in a photosynthetic protein complex known as photosystem II (PSII). The mecha- nism of water oxidation is debated, particularly the role of calcium and the site at which the oxygen-oxygen (O-O) bond of the dioxygen molecule forms. Further insight might allow...