The interaction of water oxidising photosystem II preparations with the aqueous environment has been investigated using electron spin echo envelope modulation spectroscopy in the presence of 2H2O. The spectra show interaction of 2H of 2H2O with the preparation in the S2 state. The component interacting with water decays during 1–4 weeks storage at 77 K. No interaction of water with the classical multiline S2 Mn signal, which is more stable on storage at 77 K, was detected. The results show that a component of the water oxidation complex, possibly involving the Mn centre, is accessible to water and may be the water binding site for photosynthetic water oxidation.
