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 S-2 state to the S-3 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 mu(2)-oxo bridge (O5) that is able to link the closed cubane S-2(B) intermediate conformer to the S-3 open cubane structure. This mechanism can reconcile the apparent conflict between recently reported water exchange and electron paramagnetic resonance experiments, and theoretical studies.