A structure-consistent mechanism for dioxygen formation in photosystem II.

In recent DFT 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(1) 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 an 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.