Roles of the Flexible Primary Coordination Sphere of the Mn₄CaO_xCluster: What Are the Immediate Decay Products of the S₃State?

The primary coordination sphere of the multinuclear cofactor (Mn₄CaO_x) in the oxygen-evolving complex (OEC) of photosystem II is absolutely conserved to maintain its structure and function. Recent time-resolved serial femtosecond crystallography identified large reorganization of the primary coordination sphere in the S₂to S₃transition, which elicits a cascade of events involving Mn oxidation and water molecule binding to a putative catalytic Mn site. We examined how the crystallographic fields, created by transient conformational states of the OEC at various time points, affect the thermodynamics of various isomers of the Mn cluster using DFT calculations, with an aim of comprehending the functional roles of the flexible primary coordination sphere in the S₂to S₃transition and in the recovery of the S₂state. The results show that the relative movements of surrounding residues change the size and shape of the cavity of the cluster and thereby affect the thermodynamics of various catalytic intermediates as well as the ability to capture a new water molecule at a coordinatively unsaturated site. The implication of these findings is that the protein dynamics may serve to gate the catalytic reaction efficiently by controlling the sequence of Mn oxidation/reduction and water binding/release. This interpretation is consistent with EPR experiments; g ∼5 and g ∼3 signals obtained after near-infrared (NIR) excitation of the S₃state at 4 K and a g ∼5 only signal produced after prolonged incubation of the S₃state at 77 K can be best explained as originating from water-bound S₂clusters (S_total= 7/2) under a S₃ligand field, i.e., the immediate one-electron reduction products of the oxyl-oxo (S_total= 6) and hydroxo-oxo (S_total= 3) species in the S₃state.