FTIR Study of Manganese Dimers with Carboxylate Donors As Model Complexes for the Water Oxidation Complex in Photosystem II

The carboxylate stretching frequencies of two high-valent, di-μ-oxido bridged, manganese dimers has been studied with IR spectroscopy in three different oxidation states. Both complexes contain one monodentate carboxylate donor to each Mn ion, in one complex, the carboxylate is coordinated perpendicular to the Mn-(μ-O)₂-Mn plane, and in the other complex, the carboxylate is coordinated in the Mn-(μ-O)₂-Mn plane. For both complexes, the difference between the asymmetric and the symmetric carboxylate stretching frequencies decrease for both the Mn₂^IV,IV to Mn₂^III,IV transition and the Mn₂^III,IV to Mn₂^III,III transition, with only minor differences observed between the two arrangements of the carboxylate ligand versus the Mn-(μ-O)₂-Mn plane. The IR spectra also show that both carboxylate ligands are affected for each one electron reduction, i.e., the stretching frequency of the carboxylate coordinated to the Mn ion that is not reduced also shifts. These results are discussed in relation to FTIR studies of changes in carboxylate stretching frequencies in a one electron oxidation step of the water oxidation complex in Photosystem II

Room-Temperature Energy-Sampling Kβ X‑ray Emission Spectroscopy of the Mn₄Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O₂ Formation

In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn₄Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn Kβ_1,3 line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The Kβ_1,3 line energies suggest a high-valence configuration of the Mn₄Ca complex with Mn­(III)₃Mn­(IV) in S₀, Mn­(III)₂Mn­(IV)₂ in S₁, Mn­(III)­Mn­(IV)₃ in S₂, and Mn­(IV)₄ in S₃ and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex. There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S₄ state unlikely. The difference spectra of both manganese Kβ_1,3 emission and K-edge X-ray absorption display different shapes for Mn­(III) oxidation in the S₂ → S₃ transition when compared to Mn­(III) oxidation in the S₁ → S₂ transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S₂ → S₃ transition, which could be oxidation of five-coordinated Mn­(III) to six-coordinated Mn­(IV). Conceivable options for the rearrangement of (substrate) water species and metal–ligand bonding patterns at the Mn₄Ca complex in the S₂ → S₃ transition are discussed

Kα X‑ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S₃ State

The unique manganese–calcium catalyst in photosystem II (PSII) is the natural paragon for efficient light-driven water oxidation to yield O₂. The oxygen-evolving complex (OEC) in the dark-stable state (S₁) comprises a Mn₄CaO₄ core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S₃) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn Kα X-ray emission spectra of the S₀, S₁, S₂, and S₃ intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S₃ unlikely

Cobaloxime-Based Artificial Hydrogenases

Cobaloximes are popular H₂ evolution molecular catalysts but have so far mainly been studied in nonaqueous conditions. We show here that they are also valuable for the design of artificial hydrogenases for application in neutral aqueous solutions and report on the preparation of two well-defined biohybrid species via the binding of two cobaloxime moieties, {Co­(dmgH)₂} and {Co­(dmgBF₂)₂} (dmgH₂ = dimethylglyoxime), to apo <i>Sperm-whale</i> myoglobin (<i>Sw</i>Mb). All spectroscopic data confirm that the cobaloxime moieties are inserted within the binding pocket of the <i>Sw</i>Mb protein and are coordinated to a histidine residue in the axial position of the cobalt complex, resulting in thermodynamically stable complexes. Quantum chemical/molecular mechanical docking calculations indicated a coordination preference for His93 over the other histidine residue (His64) present in the vicinity. Interestingly, the redox activity of the cobalt centers is retained in both biohybrids, which provides them with the catalytic activity for H₂ evolution in near-neutral aqueous conditions