The understanding of the structure-function relationship of the oxygen-evolving center (OEC), a Mn4Ca-cluster, in photosystem II is impeded mainly due to the complexity of the protein environment and lack of rational chemical models as a reference. In this study, two novel Mn4-oxido complexes have been synthesized and characterized, in which the peripheral ligands of the [MnIII4] core are provided by eight μ2-carboxylate groups and two neutral terminal ligands (pyridine or isoquinoline). This type of peripheral ligation is very similar to the Mn4Ca-oxide model complexes recently reported to mimic the OEC. The new Mn4-oxide complex can catalyze the oxygen-evolving reaction in the presence of ButOOH as an oxidant. The structure and redox properties comparison of the Mn4-oxido and Mn4Ca-oxido complexes provide important clues to understanding the functional role of Ca in the OEC in natural photosynthesis, and develop more efficient artificial catalysts for the water-splitting reaction in the future.Download high-res image (260KB)Download full-size image

A manganese(II) complex has been prepared with a proline-derived pentadentate ligand (Pro3Py), and it can be converted to a peroxomanganese(III) complex in the presence of H2O2 and triethylamine. The resulting peroxomanganese(III) complex was well characterised by UV-vis, EPR and ESI-MS techniques, and the geometric structure was discussed based on DFT calculations.

A novel family of heteronuclear MnIVCa–oxido complexes containing MnIVCa–oxido cuboidal moieties and reactive water molecules on Ca2+ have been synthesized and characterized to mimic the oxygen-evolving complex (OEC) of photosystem II (PSII) in nature.

A novel heterometallic MnSr complex containing the Mn3SrO4 cuboidal moiety and all types of μ-O2− moieties observed in the oxygen-evolving complex (OEC) in Sr2+-containing photosystem II (PSII) has been synthesized and characterized, which provides a new synthetic model of the OEC.

The detailed structure of catalytic center of water oxidation, Mn4Ca-cluster, in photosystem II (PSII) has been reported recently. However, due to the radiation damage induced by X-ray and the complexity of the Mn4Ca-cluster, the assignment of the μ4-O5 atom coordinated by three Mn and one Ca2+ ions is still lack of essential evidences. In this article, we synthesized one Mn complex containing two μ4-O atoms. It is found that the lengths of all μ4-O-Mn bonds in this Mn complex are in the range of 1.89–2.10 Å, which are significantly shorter than 2.40–2.61 Å distance of μ4-O5-Mn bonds in Mn4Ca-cluster observed in the crystal structure of PSII. In addition, DFT calculations have been carried out on the Mn4Ca-cluster. It is found that the O atom of μ4-O or μ4-OH always trends to deviate from the center position of four metal ions, resulting in unequal bond lengths of four μ4-O-M (M=Mn or Ca), which is obviously different with larger and nearly equal distances between μ4-O and four metal ions observed in the crystal structure. Based on these results, we suggest that the μ4-atom in Mn4Ca-cluster of PSII is unlikely to be a μ4-O, μ4-OH or μ4-OH2, and its assignment is still an open question.

The secondary electron donor, TyrZ, is implicated in tuning the primary charge separation and the water oxidation in active photosystem II (PSII). Two types of mechanisms have been proposed to explain the function of TyrZ. One is that TyrZ tunes the water oxidation through the direct interaction with substrate water molecules; the other is that TyrZ is located in a hydrophobic environment without interacting with H2O, and controls the water oxidation by tuning the strength of the hydrogen bond between TyrZ and His190. Here, methanol was used as a probe to study the possible relationship between TyrZ and H2O by monitoring the TyrZ oxidation and TyrZ• reduction at cryogenic temperatures with electron paramagnetic resonance spectroscopy. The oxidation of TyrZ and reduction of TyrZ• in both S2 and S0 states at 10 K were accelerated by addition of a small amount of methanol (6%). Theoretical studies indicate that Tyr oxidation becomes more difficult if it interacts directly with the methanol molecule; while the decrease of the polarity of its environment accelerates the oxidation of Tyr. Accordingly, CH3OH does not directly interact with TyrZ in active PSII, and the accelerative effect of methanol is caused by the strength increase of the hydrogen bond between TyrZ and His190, resulting from the decrease of polarity of their environment after the displacement of H2O by CH3OH inside PSII. Considering the similarity between methanol and water, the results in this study support the model in which TyrZ does not interact with H2O in active PSII.

Tyrosine Z (TyrZ) oxidation observed at liquid helium temperatures provides new insights into the structure and function of TyrZ in active Photosystem II (PSII). However, it has not been reported in PSII core complex from higher plants. Here, we report TyrZ oxidation in the S1 and S2 states in PSII core complex from spinach for the first time. Moreover, we identified a 500 G-wide symmetric EPR signal (peak position g = 2.18, trough position g = 1.85) together with the g = 2.03 signal induced by visible light at 10 K in the S1 state in the PSII core complex. These two signals decay with a similar rate in the dark and both disappear in the presence of 6% methanol. We tentatively assign this new feature to the hyperfine structure of the S1TyrZ• EPR signal. Furthermore, EPR signals of the S2 state of the Mn-cluster, the oxidation of the non-heme iron, and the S1TyrZ• in PSII core complexes and PSII-enriched membranes from spinach are compared, which clearly indicate that both the donor and acceptor sides of the reaction center are undisturbed after the removal of LHCII. These results suggest that the new spinach PSII core complex is suitable for the electron transfer study of PSII at cryogenic temperatures.

A novel heterometallic MnSr complex containing the Mn3SrO4 cuboidal moiety and all types of μ-O2− moieties observed in the oxygen-evolving complex (OEC) in Sr2+-containing photosystem II (PSII) has been synthesized and characterized, which provides a new synthetic model of the OEC.

A manganese(II) complex has been prepared with a proline-derived pentadentate ligand (Pro3Py), and it can be converted to a peroxomanganese(III) complex in the presence of H2O2 and triethylamine. The resulting peroxomanganese(III) complex was well characterised by UV-vis, EPR and ESI-MS techniques, and the geometric structure was discussed based on DFT calculations.

A novel family of heteronuclear MnIVCa–oxido complexes containing MnIVCa–oxido cuboidal moieties and reactive water molecules on Ca2+ have been synthesized and characterized to mimic the oxygen-evolving complex (OEC) of photosystem II (PSII) in nature.