18O-Water exchange in photosystem II: Substrate binding and intermediates of the water splitting cycle

Photosynthetic water oxidation proceeds via four light-activated intermediate states. The nature of the substrate water in each of these intermediate states is important for understanding the mechanism of the photosynthetic O–O bond formation. Oxygen-18 water exchange measurements have been used to probe the binding of the substrate water at the water oxidation site in photosystem II. In this review we summarize the water exchange behaviour for a range of aqua-metal complexes for comparison with the Photosystem II exchange measurements. The analysis provides a basis for determining the nature of the substrate water in each of the intermediate states in the photosynthetic reaction cycle.