Spectroscopic Evidence for a Redox-Controlled Proton Gate at Tyrosine D in Photosystem II

Tyrosine D (TyrD) is one of two well-studied redox active tyrosines in Photosystem II. TyrD shows redox kinetics much slower than that of its homologue, TyrZ, and is normally present as a stable deprotonated radical (TyrD^•). We have used time-resolved continuous wave electron paramagnetic resonance and electron spin echo envelope modulation spectroscopy to show that deuterium exchangeable protons can access TyrD on a time scale that is much faster (50–100 times) than that previously observed. The time of H/D exchange is strongly dependent on the redox state of TyrD. This finding can be related to a change in position of a water molecule close to TyrD

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

Stability of the S₃ and S₂ State Intermediates in Photosystem II Directly Probed by EPR Spectroscopy

The stability of the S₃ and S₂ states of the oxygen evolving complex in photosystem II (PSII) was directly probed by EPR spectroscopy in PSII membrane preparations from spinach in the presence of the exogenous electron acceptor P<i>p</i>BQ at 1, 10, and 20 °C. The decay of the S₃ state was followed in samples exposed to two flashes by measuring the split S₃ EPR signal induced by near-infrared illumination at 5 K. The decay of the S₂ state was followed in samples exposed to one flash by measuring the S₂ state multiline EPR signal. During the decay of the S₃ state, the S₂ state multiline EPR signal first increased and then decreased in amplitude. This shows that the decay of the S₃ state to the S₁ state occurs via the S₂ state. The decay of the S₃ state was biexponential with a fast kinetic phase with a few seconds decay half-time. This occurred in 10–20% of the PSII centers. The slow kinetic phase ranged from a decay half-time of 700 s (at 1 °C) to ∼100 s (at 20 °C) in the remaining 80–90% of the centers. The decay of the S₂ state was also biphasic and showed quite similar kinetics to the decay of the S₃ state. Our experiments show that the auxiliary electron donor Y_D was oxidized during the entire experiment. Thus, the reduced form of Y_D does not participate to the fast decay of the S₂ and S₃ states we describe here. Instead, we suggest that the decay of the S₃ and S₂ states reflects electron transfer from the acceptor side of PSII to the donor side of PSII starting in the corresponding S state. It is proposed that this exists in equilibrium with Y_Z according to S₃Y_Z ⇔ S₂Y_Z^• in the case of the S₃ state decay and S₂Y_Z ⇔ S₁Y_Z^• in the case of the S₂ state decay. Two kinetic models are discussed, both developed with the assumption that the slow decay of the S₃ and S₂ states occurs in PSII centers where Y_Z is also a fast donor to P₆₈₀⁺ working in the nanosecond time regime and that the fast decay of the S₃ and S₂ states occurs in centers where Y_Z reduces P₆₈₀⁺ with slower microsecond kinetics. Our measurements also demonstrate that the split S₃ EPR signal can be used as a direct probe to the S₃ state and that it can provide important information about the redox properties of the S₃ state

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

Electronic Structure of Oxidized Complexes Derived from <i>cis</i>-[Ru^II(bpy)₂(H₂O)₂]²⁺ and Its Photoisomerization Mechanism

The geometry and electronic structure of <i>cis</i>-[Ru^II(bpy)₂(H₂O)₂]²⁺ and its higher oxidation state species up formally to Ru^VI have been studied by means of UV–vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru–O bond distance decreases, indicating increased degrees of Ru–O multiple bonding. In addition, the O–Ru–O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, <i>cis</i>-[Ru^IV(bpy)₂(OH)₂]²⁺ (d⁴) has a singlet ground state and is EPR-silent at low temperatures, while <i>cis</i>-[Ru^V(bpy)₂(O)(OH)]²⁺ (d³) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O–Ru–O fragment. In addition, the photochemical isomerization of <i>cis</i>-[Ru^II(bpy)₂(H₂O)₂]²⁺ to its <i>trans</i>-[Ru^II(bpy)₂(H₂O)₂]²⁺ isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the <i>trans</i> isomer