A five-coordinate Mn(<scp>iv</scp>) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding

Among the four photo-driven transitions of the water-oxidizing tetramanganese-calcium cofactor of biological photosynthesis, the second-last step of the catalytic cycle, that is the S₂ to S₃ state transition, is the crucial step that poises the catalyst for the final O-O bond formation. This transition, whose intermediates are not yet fully understood, is a multi-step process that involves the redox-active tyrosine residue and includes oxidation and deprotonation of the catalytic cluster, as well as the binding of a water molecule. Spectroscopic data has the potential to shed light on the sequence of events that comprise this catalytic step, which still lacks a structural interpretation. In this work the S₂-S₃ state transition is studied and a key intermediate species is characterized: it contains a Mn₃O₄Ca cubane subunit linked to a five-coordinate Mn(iv) ion that adopts an approximately trigonal bipyramidal ligand field. It is shown using high-level density functional and multireference wave function calculations that this species accounts for the near-infrared absorption and electron paramagnetic resonance observations on metastable S₂-S₃ intermediates. The results confirm that deprotonation and Mn oxidation of the cofactor must precede the coordination of a water molecule, and lead to identification of a novel low-energy water binding mode that has important implications for the identity of the substrates in the mechanism of biological water oxidation.