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

The role of the chloride ion in the oxygen evolving complex. Hans van Gorkom^*,1, Charles Yocum², ¹ Department of Biophysics, Huygens Laboratory, Leiden, Netherlands² Departments of Biology and of Chemistry, Ann Arbor, MI, USA

ABSTRACT- A hypothesis is proposed that reconciles apparently conflicting results on the role of the chloride ion in the oxygen evolving complex and accounts for several unexplained properties of chloride-depleted photosystem II preparations. The inactivation of the S2 state by chloride depletion after removal of the extrinsic 17 and 23 kDa polypeptides is attributed to its replacement as a manganese ligand by a hydroxyl ion, resulting in a decreased Mn(IV)/Mn(III) midpoint potential and hence in a different valence distribution in the 4-Mn cluster in the S2 state that blocks its further oxidation to the S3 state. The oxygen evolution activity of extrinsic polypeptide-retaining PSII after chloride-removal by prolonged dialysis (Andreasson team) is proposed to result from the absence of hydroxyl binding at pH 6.3 in the dark-adapted state, and its delay beyond the time scale of oxygen evolution measurement by the presence of the polypeptides during illumination. In this view, chloride serves to maintain a high Mn potential by blocking access of hydroxyl ions to its binding site in high S-states.

KEY WORDS: oxygen evolution, chloride, photosystem II