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