Theoretical investigations of structure and mechanism of the oxygen-evolving complex in PSII

Hybrid density functional theory has been used to study a proposed Mn3Ca-Mn cubane-like structure for the oxygen-evolving complex in photosystem II. A refined analysis of the structure is made regarding the position of the heavy atoms, the oxidation states of the manganese centers and the protonation states of the ligands. After finding the most stable S-2 structure, S-0 --> S-1 and S-1 --> S-2 transitions are proposed. Further, proposals for the higher S state transitions, S-2 --> S-3 and S-3 --> S-4 have been investigated, using the O-H bond dissociation energy as a probe. With an intact cubane-like structure, oxidation to a high valent Mn-oxo species has so far not been found to be energetically feasible. Instead, tentative proposals of the S-2 --> S-3 transition involving structural rearrangements are made.