The first artificial Mn4Ca-cluster mimicking the oxygen-evolving center in photosystem II

Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosystem II (PSII) of oxygenic photosynthetic organisms is a unique heterometallic-oxide Mn₄CaO₅-cluster that catalyzes water splitting into electrons, protons, and molecular oxygen through a five-state cycle (S_n, n = 0 ~ 4). It serves as the blueprint for the developing of the man-made water-splitting catalysts to generate solar fuel in artificial photosynthesis. Understanding the structure-function relationship of this natural catalyst is a great challenge and a long-standing issue, which is severely restricted by the lack of a precise chemical model for this heterometallic-oxide cluster. However, it is a great challenge for chemists to precisely mimic the OEC in a laboratory. Recently, significant advances have been achieved and a series of artificial Mn₄XO₄-clusters (X = Ca/Y/Gd) have been reported, which closely mimic both the geometric structure and the electronic structure, as well as the redox property of the OEC. These new advances provide a structurally well-defined molecular platform to study the structure-function relationship of the OEC and shed new light on the design of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

Mimicking the Catalytic Center for the Water-Splitting Reaction in Photosystem II

The oxygen-evolving center (OEC) in photosystem II (PSII) of plants, algae and cyanobacteria is a unique natural catalyst that splits water into electrons, protons and dioxygen. The crystallographic studies of PSII have revealed that the OEC is an asymmetric Mn4CaO5-cluster. The understanding of the structure-function relationship of this natural Mn4CaO5-cluster is impeded mainly due to the complexity of the protein environment and lack of a rational chemical model as a reference. Although it has been a great challenge for chemists to synthesize the OEC in the laboratory, significant advances have been achieved recently. Different artificial complexes have been reported, especially a series of artificial Mn4CaO4-clusters that closely mimic both the geometric and electronic structures of the OEC in PSII, which provides a structurally well-defined chemical model to investigate the structure-function relationship of the natural Mn4CaO5-cluster. The deep investigations on this artificial Mn4CaO4-cluster could provide new insights into the mechanism of the water-splitting reaction in natural photosynthesis and may help the development of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

Natural and Artificial Mn4 Ca Cluster for the Water Splitting Reaction

The oxygen-evolving center (OEC) in photosystem II (PSII) is a unique biological catalyst that splits water into electrons, protons, and O₂ by using solar energy. Recent crystallographic studies have revealed that the structure of the OEC is an asymmetric Mn₄ Ca cluster, which provides a blueprint to develop man-made water-splitting catalysts for artificial photosynthesis. Although it is a great challenge to mimic the whole structure and function of the OEC in the laboratory, significant advances have recently been achieved. In this Minireview, recent progress on mimicking the natural OEC is discussed. New strategies are suggested to construct more stable and efficient new generation of catalytic materials for the water splitting reaction based on the artificial Mn₄ Ca cluster in the future.

Artificial Mn4-oxido complexes mimic the oxygen-evolving center in photosynthesis

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 [Mn^III₄] 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 Bu^tOOH 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.

Function of terahertz spectra in monitoring the decomposing process of biological macromolecules and in investigating the causes of photoinhibition

Chlorophyll a and β-carotene play an important role in harvesting light energy, which is used to drive photosynthesis in plants. In this study, terahertz (THz) and visible range spectra of chlorophyll a and β-carotene and their changes under light treatment were investigated. The results show that the all THz transmission and absorption spectra of chlorophyll a and β-carotene changed upon light treatment, with the maximum changes at 15 min of illumination indicating the greatest changes of the collective vibrational mode of chlorophyll a and β-carotene. The absorption spectra of chlorophyll a in the visible light region decreased upon light treatment, signifying the degradation of chlorophyll a molecules. It can be inferred from these results that the THz spectra are very sensitive in monitoring the changes of the collective vibrational mode, despite the absence of changes in molecular configuration. The THz spectra can therefore be used to monitor the decomposing process of biological macromolecules; however, visible absorption spectra can only be used to monitor the breakdown extent of biological macromolecules.