Oxygen ligands from water are bound to the manganese cluster giving rise to the ‘multiline’ EPR signal from the S2 state of photosystem II, as shown by the observation of hyperfine broadening of the EPR signal in the presence of 17O-enriched water. The binding must occur in the S0, S1 or S2 state. The estimated upper limit for the 17O hyperfine coupling constant, 0.5 mT, excludes Superoxide and hydroxyl radicals in the S2 state, but not peroxide.
