Evidence that the Mn complex of the photosystem II reaction centre is exposed to water in the S1 and S2 states .

Abstract

The interaction of water oxidising photosystem II preparations with the aqueous environment has been investigated using ESEEM spectroscopy in the presence of 2H2O and deuterated CH3OH. The spectra show modulation indicating interaction of 2H of 2H2O and methanol with the preparation in the S2 state. The modulation frequency observed is essentially at the Larmor Frequency of2 H indicating that the H is not directly coupled to the Mn and is at a distance of 3-5A from the Mn. The component interacting with water, which has a multiline EPR spectrum somewhat narrower than the stable multiline Mn complex signal, decays during 1-4 weeks storage at 77K. No interaction of water with the rest of the multiline S2 Mn signal, which is more stable on storage at 77K, 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. The results of similar experiments using 17O labelled water will be presented.