Oxomanganese complexes for natural and artificial photosynthesis

The oxygen-evolving complex (OEC) of Photosystem II (PSII) is an oxomanganese complex that catalyzes water-splitting into O2, protons and electrons. Recent breakthroughs in X-ray crystallography have resolved the cuboidal OEC structure at 1.9 Å resolution, stimulating significant interest in studies of structure/function relations. This article summarizes recent advances on studies of the OEC along with studies of synthetic oxomanganese complexes for artificial photosynthesis. Quantum mechanics/molecular mechanics hybrid methods have enabled modeling the S1 state of the OEC, including the ligation proposed by the most recent X-ray data where D170 is bridging Ca and the Mn center outside the CaMn3 core. Molecular dynamics and Monte Carlo simulations have explored the structural/functional roles of chloride, suggesting that it regulates the electrostatic interactions between D61 and K317 that might be critical for proton abstraction. Furthermore, structural studies of synthetic oxomanganese complexes, including the [H2O(terpy)MnIII(μ-O)2MnIV(terpy)OH2]3+ (1, terpy = 2,2′:6′,2″-terpyridine) complex, provided valuable insights on the mechanistic influence of carboxylate moieties in close contact with the Mn catalyst during oxygen evolution. Covalent attachment of 1 to TiO2 has been achieved via direct deposition and by using organic chromophoric linkers. The (III,IV) oxidation state of 1 attached to TiO2 can be advanced to (IV,IV) by visible-light photoexcitation, leading to photoinduced interfacial electron transfer. These studies are particularly relevant to the development of artificial photosynthetic devices based on inexpensive materials.

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► The dark-stable S1 state of the oxygen evolving complex (OEC) of Photosystem II has been recently modeled using quantum mechanics/molecular mechanics (QM/MM) hybrid methods that explicitly describe the surrounding biomolecular environment consistently with the X-ray structure resolved at 1.9 Å resolution.
► Molecular dynamics and Monte Carlo studies based on the DFT-QM/MM model of the OEC have shown that chloride regulates the formation of a salt-bridge of polar amino acids next to the OEC that are thought to be involved in proton abstraction.
► DFT studies have shown that carboxylate groups can function as redox and acid/base cofactors during oxidation state transitions of oxomanganese complexes.
► DFT QM/MM modeling and experimental studies of Mn catalysts covalently bound to TiO2 semiconductors have characterized the surface attachment mode.