Effect of Chloride Depletion on the Magnetic Properties and the Redox Leveling of the Oxygen-Evolving Complex in Photosystem II

Chloride is an essential cofactor in the oxygen-evolution reaction that takes place in photosystem II (PSII). The oxygen-evolving complex (OEC) is oxidized in a linear four-step photocatalytic cycle in which chloride is required for the OEC to advance beyond the S₂ state. Here, using density functional theory, we compare the energetics and spin configuration of two different states of the Mn₄CaO₅ cluster in the S₂ state: state A with Mn1³⁺ and B with Mn4³⁺ with and without chloride. The calculations suggest that model B with an S = 5/2 ground state occurs in the chloride-depleted PSII, which may explain the presence of the EPR signal at g = 4.1. Moreover, we use multiconformer continuum electrostatics to study the effect of chloride depletion on the redox potential associated with the S₁/S₂ and S₂/S₃ transitions.