Evidence for a direct manganese-oxygen ligand in water binding to the S(2) state of the photosynthetic water oxidation complex

The interaction of water with the water oxidizing Mn complex of photosystem II has been investigated using electron spin-echo envelope modulation spectroscopy in the presence of H 2 17O. The spectra show interaction of the 1 7 O with the preparation in the S 2 state induced by 200 K illumination. The modulation is observed only in the center of the multiline spectrum. The inferred hyperfine coupling terms are compatible with water (not hydroxyl) oxygen bound to a particular quasi-axial Mn I I I center in a coupled Mn cluster.