Veronika A. Szalai

A MECHANISTIC AND STRUCTURAL MODEL FOR THE FORMATION AND REACTIVITY OF A MNV=O SPECIES IN PHOTOSYNTHETIC WATER OXIDATION

Photosynthetic water oxidation is carried out by a tetranuclear Mn ncluster contained in the membrane-bound protein complex photosystem II n(PSII). The mechanism of PSII catalysed water oxidation is unknown; nhowever, several current models invoke a high-valent MnO nspecies as a key intermediate in O–O bond formation. In part, nthese proposals are based on biophysical studies of the protein which nsuggest that the redox-active tyrosine residue, YZ, abstracts nhydrogen atoms directly from substrate water molecules bound to the nMn4 cluster. In this paper, we consider organic oxidation and nO–O bond-forming reactions catalysed by biomimetic Mn and Ru model ncomplexes that are believed to proceed via MO nintermediates. We also interpret biophysical data concerning the roles nof Ca2+ and Cl– in photosynthetic water noxidation, proposing that they are involved in a hydrogen-bonded network nbetween the Mn4 cluster and YZ. Connecting the nobserved reactivities of model complexes containing MO groups to nspectroscopic information on the environment of the Mn4 ncluster in the protein leads us to favour an O–O bond-forming step nin photosynthetic water oxidation that occurs through nucleophilic nattack of a calcium-bound hydroxide ligand on the electrophilic oxygen natom of a MnO intermediate. In addition, a new role for nCl– is proposed in which Cl– tunes the nnucleophilicity of the calcium-bound hydroxide.

A Structural and Mechanistic Model of the O 2 -Evolving Complex of Photosystem II

Acetate-inhibited photosystem II (PSII) membranes illuminated at temperatures above 250 K and rapidly frozen to 77 K display a broadened radical electron paramagnetic resonance (EPR) signal arising from the S2 state of the Mn4 cluster interacting with YZ• (1, 2). Spectral simulation of the S2YZ• EPR signal from acetate-inhibited PSII indicates that the distance between the two paramagnets is 7.7 ± 0.5 A based on a point-dipole approximation (3). To gain insight into the structure of the water-oxidation site, we have constructed a series of structural models which incorporate a Mn4 complex, YZ, Ca2+ and Cl−. We find that a structural model in which YZ is H-bonded to water/hydroxide bound to one Mn atom in the cluster is consistent with the YZ-Mn4 distance, the proposed YZ and Mn4 cluster orientations in the membrane, and proposals that YZ participates in proton-coupled electron transfer with the Mn4 cluster. By considering the chemistry and reactivity of Mn=O species, we propose a mechanism of the O-O bond-forming step which includes new functional roles for Ca2+ and Cl− (4).