Oxygen-evolving complex

[4] Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S0 to S4, with S0 the most reduced and S4 the most oxidized.

S4 reacts with water producing free oxygen: This conversion resets the catalyst to the S0 state.

The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn4Ca1OxCl1–2(HCO3)y.

Many characteristics of it have been examined by flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy.

[5] The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.

The Kok cycle. The oxidation state of the manganese centres is subject to debate. [ 1 ]
X-ray crystal structure of the Mn 4 O 5 Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 Å. [ 2 ]