PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Studies of the S₀ state multiline signal of the oxygen evolving centre in Photosystem II. Karin Ahrling^*,1, M.C.W. Evans², ¹ Photobioenergetics, Research School of Biological Sciences, Canberra, ACT, Australia² Biology Department, University College London, London, United Kingdom

ABSTRACT- The first studies of the S₀ state multiline EPR signal (hrling et al (1997) Biochemistry 36: 13148-13152; Messinger et al (1997) Biochemistry 36: 11055-11060) established that 1-3 % methanol is essential for its observation. The EPR signal is a Mn hyperfine multiline signal and arises from a net S=1/2 ground state. The question remains what the total spin state might be without methanol in the observation buffer. There have been some indication that the centre is also an S=1/2 ground state without methanol (Messinger et al (1997) J Am Chem Soc 119: 11349-11350; Boussac et al (1999) Biochemistry 38:11942-11948). In this CW EPR and ESEEM study we have explored samples given 3 flashes of light (predominantly S₀ state)in the presence of D₂O and deuterated methanol. We find that the line narrowing brought about by the deuterium is sufficient to resolve the Mn hyperfine in samples without methanol. Preliminary ESEEM results indicate that methanol displaces the bound water in the S₀ state.

KEY WORDS: Manganese, EPR, ESEEM, Oxygen Evolving Centre, S0 state