Reorganization of Substrate Waters between the Closed and Open Cubane Conformers during the S2 to S3 Transition in the Oxygen Evolving Complex

A crucial step in the mechanism for oxygen evolution in the Photosystem II complex resides in the transition from the S2 state to the S3 state of Kok-Joliot's cycle, in which an additional water molecule binds to the cluster. On the basis of computational chemistry calculations on Photosystem II models, we propose a reorganization mechanism involving a hydroxyl (W2) and a μ2-oxo bridge (O5) that is able to link the closed cubane S2B intermediate conformer to the S3 open cubane structure. This mechanism can reconcile the apparent conflict between recently reported water exchange and electron paramagnetic resonance experiments, and theoretical studies. © 2015 American Chemical Society.