title: What Can We Learn from a Biomimetic Model of Nature's Oxygen-Evolving Complex?
abstract: A recently reported synthetic complex with a Mn4CaO4core represents a remarkable structural mimic of the Mn4CaO5cluster in the oxygen-evolving complex (OEC) of photosystem II (Zhang et al., Science 2015, 348, 690). Oxidized samples of the complex show electron paramagnetic resonance (EPR) signals at g ≈ 4.9 and 2, similar to those associated with the OEC in its S2state (g ≈ 4.1 from an S =5/2form and g ≈ 2 from an S =1/2form), suggesting similarities in the electronic as well as geometric structure. We use quantum-chemical methods to characterize the synthetic complex in various oxidation states, to compute its magnetic and spectroscopic properties, and to establish connections with reported data. Only one energetically accessible form is found for the oxidized "S2state" of the complex. It has a ground spin state of S =5/2, and EPR simulations confirm it can be assigned to the g ≈ 4.9 signal. However, no valence isomer with an S =1/2ground state is energetically accessible, a conclusion supported by a wide range of methods, including density matrix renormalization group with full valence active space. Alternative candidates for the g ≈ 2 signal were explored, but no low-spin/low-energy structure was identified. Therefore, our results suggest that despite geometric similarities the synthetic model does not mimic the valence isomerism that is the hallmark of the OEC in its S2state, most probably because it lacks a coordinatively flexible oxo bridge. Only one of the observed EPR signals can be explained by a structurally intact high-spin one-electron-oxidized form, while the other originates from an as-yet-unidentified rearrangement product. Nevertheless, this model provides valuable information for understanding the high-spin EPR signals of both the S1and S2states of the OEC in terms of the coordination number and Jahn-Teller axis orientation of the Mn ions, with important consequences for the development of magnetic spectroscopic probes to study S-state intermediates immediately prior to O-O bond formation.
authors.affiliation: Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Satadal
authors.initials: S
authors.lastname: Paul
authors.affiliation: Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.|Research School of Chemistry, Australian National University , Canberra ACT 2601, Australia.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Cox
authors.affiliation: Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Dimitrios A
authors.identifier: http://orcid.org/0000-0002-2146-9065
authors.initials: DA
authors.lastname: Pantazis
mesh_terms: D032701:Biomimetics; D017610:Calcium Compounds / Q000737:chemistry; D056831:Coordination Complexes / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D007536:Isomerism; D017895:Manganese Compounds / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011789:Quantum Theory; D014867:Water / Q000737:chemistry
chemical_list: D017610:Calcium Compounds; D056831:Coordination Complexes; D017895:Manganese Compounds; D010087:Oxides; D045332:Photosystem II Protein Complex; D014867:Water