An evaluation of structural models for the photosynthetic water-oxidizing complex derived from spectroscopic and X-ray diffraction signatures

Abstract.

Four of the five intermediate oxidation states (S-states) in the catalytic cycle of water oxidation used by O₂-evolving photoautotrophs have been previously characterized by EPR and/or ENDOR spectroscopy, with the first reports for the S₀, S₁, and S₃ states available in just the last three years. The first electron density map of the Mn cluster derived from X-ray diffraction measurements of single crystals of photosystem II at 3.8–4.2 Å resolution has also appeared this year. This wealth of new information has provided significant insight into the structure of the inorganic core (Mn₄O _x Ca₁Cl_1–2), the Mn oxidation states, and the location and function of the essential Ca²⁺ cofactor within the water-oxidizing complex (WOC). We summarize these advances and provide a unified interpretation of debated structural proposals and Mn oxidation states, based on an integrated analysis of the published data, particularly from Mn X-ray absorption spectroscopy (XAS) and EPR/ENDOR data. Only three magnetic spin-exchange models for the inter-manganese interactions are possible from consideration of the EPR data for the S₀, S₁, S₂ and S_{– N} (NO-reduced) states. These models fall into one of three types denoted butterfly, funnel, or tetrahedron. A revised set of eight allowed chemical structures for the Mn₄O _x core can be deduced that are shown to be consistent with both EPR and XAS. The popular "dimer-of-dimers" structural model is not compatible with the possible structural candidates. EPR data have identified two inter-manganese couplings that are sensitive to the S-state, suggesting two possible bridging sites for substrate water molecules. Spin densities derived from ⁵⁵Mn hyperfine data together with Mn K-edge energies from Ca-depleted samples provide an internally consistent assignment for the Mn oxidation states of Mn₄(3III,IV) for the S₂ state. EPR and XAS data also provide a consistent picture, locating Ca²⁺ as an integral part of the inorganic core, probably via shared bridging ligands with Mn (aqua/hydroxo/carboxylato/chloro). XAS data reveal that the Ca²⁺ cofactor increases the Mn(1s→4p) transition energy by 0.6–1 eV with minimal structural perturbation versus the Ca-depleted WOC. Thus, calcium binding appears to increase the Mn-ligand covalency by increasing electron transfer from shared ligands to Mn, suggesting a direct role for Ca²⁺ in substrate water oxidation. Consideration of both the XAS and the EPR data, together with reactivity studies on two model complexes that evolve O₂, suggest two favored structure types as feasible models for the reactive S₄ state that is precursor to the O₂ evolution step. These are a calcium-capped "cuboidal" core and a calcium-capped "funnel" core.