Julian Limburg, Veronika A. Szalai and Gary W. Brudvig
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 MnO
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
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
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
O intermediate. In addition, a new role for
Cl– is proposed in which Cl– tunes the
nucleophilicity of the calcium-bound hydroxide.