Manganese clusters with relevance to photosystem II.

ion model is not a true H-abstracting process but rather a special case of proton-coupled electron transfer where reduction of YZ by the Mn4 cluster occurs with the proton emanating from solvent water. These two theories will now be described briefly. A hydrogen atom abstraction process by the YZ residue in conjunction with a dimer-of-dimers structural template has led to an interesting hypothesis wherein an O atom is proposed as a terminal ligand to Mn (see Figure 8 (top)).175,188 The Clion is proposed to migrate in S1 to S2 and S2 to S3 state transitions while YZ serves as the H atom abstractor and Ca2+ ion binds to Clin the lower S states. In another proposal two dinuclear Mn complexes perform diverse functions: one oxidizes H2O to peroxide while the other converts peroxide to water.16,189-191 The final O-O bond forming step in the S4 state is predicted to result from attack of the hydroxo group bound to Ca2+ to a strongly electrophilic MnVdO species. In a similar hypothesis, a Clbridging between a Mn and a Ca2+ is responsible for modulating the nucleophilic attack of the hydroxide ion.181,192 This was first predicted from mass spectrometric studies.176 The product of this step has been argued to be similar to the ferric hydroperoxide in oxyhemerythrin. Oxygen release is envisioned in a manner similar to that of oxyhemerythrin concomitant with reduction of Mn and protonation of μ-oxo bridges.192 On the basis of XAS and EPR data, another mechanism incorporating a “C-shaped” cluster has also been suggested recently involving one redox-active Mn per [Mn2(μ-O)2] dimeric moiety (see Figure 8 (bottom)).17,193,194 Here, the O-O bond is formed between an oxyl radical and a μ-oxo atom. A hybrid density functional theory has been utilized to put forward a triangular Mn moiety closely coupled by μ-oxo groups as a potential model for the WO site. The previously suggested oxyl radical mechanism14 has been reexamined in this work with an oxyl radical placed in a bridging fashion between two Mn atoms. Consistent with earlier studies, only one Mn is redox active in this model and the Ca2+ ion has been shown as playing the role of a bridging metal Manganese Clusters with Relevance to Photosystem II Chemical Reviews, 2004, Vol. 104, No. 9 3991