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The 'oxygen evolving complex' is a water oxidizing enzyme involved in the photooxidation of water during the light reactions of photosynthesis. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states: S₀ to S₄. Photons trapped by photosystem II move the system from state S₀ to S₄. S₄ is unstable and reacts with water to produce oxygen. Currently the functional mechanism of the complex is not completely understood. Much of the known data have been collected from flash experiments, EPR, and X-ray spectroscopy.
Reference:
Kok, B., B. Forbush, and M. McGloin. 1970. Cooperation of charges in photosynthetic O2 evolution. I. A linear four-step mechanism. Photochem. Photobiol. 11:467–475.