This paper investigates the mechanism of water splitting in photosystem II (PSII) as described by chemically sensible models of the oxygen-evolving complex (OEC) in the S0-S4 states. The reaction is the paradigm for engineering direct solar fuel production systems since it is driven by solar light and the catalyst involves inexpensive and abundant metals (calcium and manganese). Molecular models of the OEC Mn3CaO4Mn catalytic cluster are constructed by explicitly considering the perturbational influence of the surrounding protein environment according to state-of-the-art quantum mechanics/molecular mechanics (QM/MM) hybrid methods, in conjunction with the X-ray diffraction (XRD) structure of PSII from the cyanobacterium Thermosynechococcus elongatus. The resulting models are validated through direct comparisons with high-resolution extended X-ray absorption fine structure spectroscopic data. Structures of the S3, S4, and S0 states include an additional mu-oxo bridge between Mn(3) and Mn(4), not present in XRD structures, found to be essential for the deprotonation of substrate water molecules. The structures of reaction intermediates suggest a detailed mechanism of dioxygen evolution based on changes in oxidization and protonation states and structural rearrangements of the oxomanganese cluster and surrounding water molecules. The catalytic reaction is consistent with substrate water molecules coordinated as terminal ligands to Mn(4) and calcium and requires the formation of an oxyl radical by deprotonation of the substrate water molecule ligated to Mn(4) and the accumulation of four oxidizing equivalents. The oxyl radical is susceptible to nucleophilic attack by a substrate water molecule initially coordinated to calcium and activated by two basic species, including CP43-R357 and the mu-oxo bridge between Mn(3) and Mn(4). The reaction is concerted with water ligand exchange, swapping the activated water by a water molecule in the second coordination shell of calcium.

This paper introduces structural models of the oxygen-evolving complex of photosystem II (PSII) in the dark-stable S1 state, as well as in the reduced S0 and oxidized S2 states, with complete ligation of the metal-oxo cluster by amino acid residues, water, hydroxide, and chloride. The models are developed according to state-of-the-art quantum mechanics/molecular mechanics (QM/MM) hybrid methods, applied in conjunction with the X-ray crystal structure of PSII from the cyanobacterium Thermosynechococcus elongatus, recently reported at 3.5 Å resolution. Manganese and calcium ions are ligated consistently with standard coordination chemistry assumptions, supported by biochemical and spectroscopic data. Furthermore, the calcium-bound chloride ligand is found to be bound in a position consistent with pulsed electron paramagnetic resonance data obtained from acetate-substituted PSII. The ligation of protein ligands includes monodentate coordination of D1-D342, CP43-E354, and D1-D170 to Mn(1), Mn(3), and Mn(4), respectively; η(2) coordination of D1-E333 to both Mn(3) and Mn(2); and ligation of D1-E189 and D1-H332 to Mn(2). The resulting QM/MM structural models are consistent with available mechanistic data and also are compatible with X-ray diffraction models and extended X-ray absorption fine structure measurements of PSII. It is, therefore, conjectured that the proposed QM/MM models are particularly relevant to the development and validation of catalytic water-oxidation intermediates.

A refined computational structural model of the oxygen-evolving complex (OEC) of photosystem II (PSII) is introduced. The model shows that the cuboidal core Mn3CaO4 with a "dangler" Mn ligated to a corner mu4-oxide ion is maximally consistent with the positioning of the amino acids around the metal cluster as characterized by XRD models and high-resolution spectroscopic data, including polarized EXAFS of oriented single crystals and isotropic EXAFS. It is, therefore, natural to expect that the proposed structural model should be particularly useful to establish the structure of the OEC, consistently with high-resolution spectroscopic data, and for elucidating the mechanism of water-splitting in PSII as described by the intermediate oxidation states of the EC along the catalytic cycle.

In recent years, there has been considerable interest in studies of catalytic metal clusters in metalloproteins based on Density Functional Theory (DFT) quantum mechanics/molecular mechanics (QM/MM) hybrid methods. These methods explicitly include the perturbational influence of the surrounding protein environment on the structural/functional properties of the catalytic centers. In conjunction with recent breakthroughs in X-ray crystallography and advances in spectroscopic and biophysical studies, computational chemists are trying to understand the structural and mechanistic properties of the oxygen-evolving complex (OEC) embedded in photosystem II (PSII). Recent studies include the development of DFT-QM/MM computational models of the Mn(4)Ca cluster, responsible for photosynthetic water oxidation, and comparative quantum mechanical studies of biomimetic oxomanganese complexes. A number of computational models, varying in oxidation and protonation states and ligation of the catalytic center by amino acid residues, water, hydroxide and chloride have been characterized along the PSII catalytic cycle of water splitting. The resulting QM/MM models are consistent with available mechanistic data, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction data and extended X-ray absorption fine structure (EXAFS) measurements. Here, we review these computational efforts focused towards understanding the catalytic mechanism of water oxidation at the detailed molecular level.

We apply density functional theory (DFT) and the DFT+U technique to study the adsorption of transition metal porphine molecules on atomistically flat Au(111) surfaces. DFT calculations using the Perdew-Burke-Ernzerhof exchange correlation functional correctly predict the palladium porphine (PdP) low-spin ground state. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with experiments on substituted porphyrins. It exhibits no covalent bonding to Au(111), and the binding energy is a small fraction of an electronvolt. The DFT+U technique, parametrized to B3LYP-predicted spin state ordering of the Mn d-electrons, is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state. Its interaction with the gold substrate is stronger and more site-specific than that of PdP. The binding can be partially reversed by applying an electric potential, which leads to significant changes in the electronic and magnetic properties of adsorbed MnP and approximately 0.1 A changes in the Mn-nitrogen distances within the porphine macrocycle. We conjecture that this DFT+U approach may be a useful general method for modeling first-row transition metal ion complexes in a condensed-matter setting.

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Inexpensive water oxidation catalysts are needed to develop photocatalytic solar cells that mimic photosynthesis and produce fuel from sunlight and water. This paper reports the successful attachment of a dinuclear di-m-oxo manganese water oxidation catalyst [H 2 O(terpy)Mn III (m-O) 2 Mn IV (terpy)H 2 O](NO 3 ) 3 (1, terpy ¼ 2,2 0 :6 0 2 00 -terpyridine) onto TiO 2 nanoparticles (NPs) via direct adsorption, or in situ synthesis. The resulting surface complexes are characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. We conclude that the mixed-valence (III,IV) state of 1 attaches to near-amorphous TiO 2 NPs by substituting one of its water ligands by the TiO 2 NP, as suggested by low-temperature (7 K) EPR data. In contrast, the analogous attachment onto well-crystallized TiO 2 NPs leads to dimerization of 1 forming Mn(IV) tetramers on the TiO 2 surface as suggested by EPR spectroscopy and electrochemical studies.

Abstract:The 1 H and 13 C nuclear magnetic resonance (NMR) spectra of the retinyl chromophore in rhodopsin are investigated by using quantum mechanics/molecular mechanics (QM/MM) hybrid methods at the density functional theory (DFT) B3LYP/6-31G*:Amber level of theory, in conjunction with the gauge independent atomic orbital (GIAO) method for the ab initio selfconsistent-field (