Evidence for the presence of a component of the Mn complex of the photosystem II reaction centre which is exposed to water in the S2 state of the water oxidation complex

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 (H2O)-H-2. The spectra show interaction of H-2 of (H2O)-H-2 with the preparation in the S-2 state. The component interacting with water decays during 1-4 weeks storage at 77 K, No interaction 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 accessible to water and may be the water binding site for photosynthetic water oxidation, (C) 2000 Federation of European Biochemical Societies, Published by Elsevier Science B.V. All rights reserved.