PS-II

Photosystem-II (PS-II), found in the thylakoid membranes of higher plants, algae and cyanobacteria, is the only biological machinery, which harvests solar energy for water oxidation. Molecular O2 is produced as a by-product during the water oxidation reaction. PS-II consists of about 20 protein subunits, numerous electron transport cofactors, two photo-antenna systems (containing pigments like chlorophyll and carotenes), and a CaMn4O5 catalytic core, also called the Oxygen-Evolving Complex (OEC).

In PS-II, absorption of photon triggers an efficient charge separation across the membrane. In the charge-separated state, the primary oxidant in PS-II, P680+•, is a very strong oxidant with an E0 of about 1.25 V. P680+• oxidizes the OEC via a redox-active tyrosine residue, YZ. Step-wise oxidation of the OEC cycles it through multiple redox states called the S states. The S0 state is the most reduced state of the OEC. Four sequential oxidations (S0-->S1-->S2-->S3-->S4) of the S0 state put the OEC into the S4 state. An O-O bond formation takes place in the S4 state upon which the OEC relaxes back to the S0 state.

The S₀ to S₁ transition of the oxygen-evolving complex (OEC) within Photosystem-II (PS-II) is one of the least understood steps in the S-state cycle of water oxidation. By using a combined Monte-Carlo and Quantum Mechanics/Molecular Mechanics (QM/MM) approach coupled with Extended X-ray Absorption Fine Structure (EXAFS) simulations, we propose the structure of the S₀ state of the OEC. This model is consistent with the experimental EXAFS and X-ray diffraction (XRD) data. Our study suggests that during the S₀ to S₁ proton-coupled electron transfer (PCET) step, one Mn ion gets oxidized and a proton from the O5-µ-hydroxo bridge is removed. The hydrogen bonding network encompassing the Mn-bound water molecules, other surrounding water molecules, and the D1-D61 residue is proposed to facilitate deprotonation of the O5-µ-hydroxo bridge.