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A detailed electronic model of the catalytic water splitting Mn4O5Ca cluster in Photosystem II of oxygenic photosynthesis is presented that has been developed based on structural data from X-ray crystallography(1) and magnetic resonance (EPR and ENDOR) techniques.(2-5) High field ELDOR-detected NMR (EDNMR) is introduced as an alternative method for the detection of electron-nuclear hyperfine (hf) interactions of insensitive nuclei like 17O and 14N for which it is superior to the more standard ENDOR and ESEEM (HYSCORE) techniques. We have applied EDNMR at W-band (94 GHz) to detect the interaction of the water oxidizing cluster (S2 state, Seff = ½) with magnetic nuclei of amino acids and attached water molecules. In samples exchanged with H217O, 17O hyperfine couplings of three different types of water molecules were detected.(5) They could be assigned based on the structural model from X-ray crystallography(1) which has been refined using density functional theory(4) and comparison with model complexes. Additional experiments like Ca/Sr exchang2(3) and the use of inhibitors (NH3) helped in this endeavor. This data further refine the reaction pathway for O-O bond formation supporting an oxo/oxyl coupling mechanism in the catalytically active state (S4) of the cycle.(6)