Evolution from S 3 to S 4 States of the Oxygen-Evolving Complex in Photosystem II Monitored by Quantum Mechanics/Molecular Mechanics (QM/MM) Dynamics.

Water oxidation in the early steps of natural photosynthesis is fulfilled by photosystem II, which is a protein complex embedded in the thylakoid membrane inside chloroplasts. The water oxidation reaction occurs in the catalytic core of photosystem II, which consists of a Mn4Ca metal cluster, at which, after the accumulation of four oxidising equivalents through five steps (S0-S4) of the Kok-Joliot cycle, two water molecules are split into electrons, protons, and molecular oxygen. In recent years, by combining experimental and theoretical approaches, new insights have been achieved into the structural and electronic properties of different steps of the catalytic cycle. Nevertheless, the exact catalytic mechanism, especially concerning the final stages of the cycle, remains elusive and greatly debated. Herein, by means of quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations, from the structural, electronic, and magnetic points of view, the S3 state before and upon oxidation has been characterised. In contrast with the S2 state, the oxidation of the S3 state is not followed by a spontaneous proton-coupled electron-transfer event. Nevertheless, upon modelling the reduction of the tyrosine residue in photosystem II (TyrZ ) and the protonation of Asp61, spontaneous proton transfer occurs, leading to the deprotonation of an oxygen atom bound to Mn1; thus making it available for O-O bond formation.