A Structure-Consistent Mechanism for Dioxygen Formation in Photosystem II

In recent DFF studies a new mechanism for O-O bond formation at the oxygen evolving center (OEC) in photosystem II has been suggested. With the structure of the S-4 state required for that mechanism. the structures of the lower S states are investigated herein by adding protons and electrons. A model was used including the full amino acids for the ones ligating the OEC. and in which the backbone positions were held fixed from the X-ray structure. The only charged second-shell ligand Arg357 was also included. An optimized structure for the S, state was reached with a large similarity to one of those suggested by EXAFS. A full catalytic cycle was derived which can rationalize the structural relaxation in the S-2 to S-3 transition, and the fact that only all electron leaves in the transition before. Water is suggested to bind to the OEC in the S-2 to S-3 and S-4, to S-0 transitions. A new possibility for water exchange is suggested from the final energy diagram. The optimal O-O bond formation occurs between an oxygen radical and an oxo ligand. The alternative mechanism, where the oxygen radical reacts with an external water, has a barrier about 20 kcal mol(-1) higher.