Issue 9, 1999

A mechanistic and structural model for the formation and reactivity of a MnV[double bond, length half m-dash] O species in photosynthetic water oxidation

Abstract

Photosynthetic water oxidation is carried out by a tetranuclear Mn cluster contained in the membrane-bound protein complex photosystem II (PSII). The mechanism of PSII catalysed water oxidation is unknown; however, several current models invoke a high-valent Mn[double bond, length half m-dash] O species as a key intermediate in O–O bond formation. In part, these proposals are based on biophysical studies of the protein which suggest that the redox-active tyrosine residue, YZ, abstracts hydrogen atoms directly from substrate water molecules bound to the Mn4 cluster. In this paper, we consider organic oxidation and O–O bond-forming reactions catalysed by biomimetic Mn and Ru model complexes that are believed to proceed via M[double bond, length half m-dash] O intermediates. We also interpret biophysical data concerning the roles of Ca2+ and Cl in photosynthetic water oxidation, proposing that they are involved in a hydrogen-bonded network between the Mn4 cluster and YZ. Connecting the observed reactivities of model complexes containing M[double bond, length half m-dash]O groups to spectroscopic information on the environment of the Mn4 cluster in the protein leads us to favour an O–O bond-forming step in photosynthetic water oxidation that occurs through nucleophilic attack of a calcium-bound hydroxide ligand on the electrophilic oxygen atom of a Mn[double bond, length half m-dash]O intermediate. In addition, a new role for Cl is proposed in which Cl tunes the nucleophilicity of the calcium-bound hydroxide.

Article information

Article type
Paper

J. Chem. Soc., Dalton Trans., 1999, 1353-1362

A mechanistic and structural model for the formation and reactivity of a MnV[double bond, length half m-dash] O species in photosynthetic water oxidation

J. Limburg, V. A. Szalai and G. W. Brudvig, J. Chem. Soc., Dalton Trans., 1999, 1353 DOI: 10.1039/A807583B

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