Evidence for the presence of a component of the Mn complex of the photosystem II reaction centre which is exposed to water in the S-2 state of the water oxidation complex

The interaction of water oxidising photosystem II preparations with the aqu eous environment has been investigated using electron spin echo envelope mo dulation spectroscopy in the presence of (H2O)-H-2. The spectra show intera ction of H-2 of (H2O)-H-2 with the preparation in the S-2 state. The compon ent interacting with water decays during 1-4 weeks storage at 77 K, No inte raction of water with the classical multiline S-2 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 access ible to water and may be the water binding site for photosynthetic water ox idation, (C) 2000 Federation of European Biochemical Societies, Published b y Elsevier Science B.V. All rights reserved.