K. Kanda

Electronic Structure of the CaMn4O5 Cluster in the PSII System Refined to the 1.9 Å X-ray Resolution. Possible Mechanisms of Photosynthetic Water Splitting

Broken-symmetry (BS) UB3LYP calculations have been performed for the CaMn4O5 cluster (1) in the oxygen-evolving complex (OEC) of the PSII system refined to 1.9 A resolution by Umena, Kawakami, Kamiya, Shen to elucidate its electronic structure that is crucial for consideration of possible mechanisms of photosynthetic water splitting. Our UB3LYP computations have elucidated the position of protonated oxygen of the CaMn(III)2Mn(IV)2O4(OH) cluster (1a) at the S1 stage of Kok cycle. Starting from the newly elucidated S1 structure of 1a, we have calculated the electronic structure of proton and electron released CaMn(IV)4O5 cluster (1b) that mimics the S4 stage of the cycle. The LUMOs of 1b are depicted for pictorial understanding of electrophilic oxygen sites that are responsible for nucleophilic attack of hydroxide anion (or water) for the O-O bond formation. Implications of present computational results are discussed in relation to possible mechanisms of photosynthetic water splitting.

Density Functional Study of Manganese Complexes: Protonation Effects on Geometry and Magnetism

Protonation processes are ubiquitous in various biochemical reactions such as the water-oxidizing reaction in photosystem II and detoxications of active oxygen species in Mn catalase and Mn superoxide dismutase. In order to investigate them, experiments to probe protons often need supplementary computational results to support the experimental spectra, for which reliable DFT methods are required for description of protonation processes. In this study, we investigated manganese complexes, [Mn(IV)2O2Hn(salpn)2] n+ (n = 0,1,2), of which geometries and magnetism show systematic changes due to protonations to bridged oxygen anions. We examined the performance of B3LYP, B3LYP-D, BP86, BP86-D, and LC-ωPBE on these changes. With all methods, the observed changes during protonation processes can be reproduced, and the quantitatively best procedure is found to be LC-ωPBE/LACVP* for geometry optimization calculations and LC-ωPBE/chem for calculations of magnetic interactions. This conclusion is expected to be a numerical foundation for theoretical investigation of reaction centers in manganese-containing proteins.

Calculation of Magnetic Properties and Spectroscopic Parameters of Manganese Clusters with Density Functional Theory

Recently, the fundamental structures of the oxygen-evolving complex (OEC) in photosystem II were revealed with the X-ray diffraction experiment. Next problems are elucidation of the protonation mode and oxidation states of OEC that are key points for the oxidation reaction in the OEC. Comparison between electron paramagnetic resonance experimental results and ab initio computational results for the hyperfine coupling constants (HFCs) is helpful to determine them. Although the calculated HFC values strongly depend on the approximated exchange–correlation (XC) term of the ab initio density functional theory (DFT) method, there is little investigation on XC dependence of calculated HFC values. Thus, in this study, we have examined the accuracy of contemporary functionals, which are known to be efficient to describe magnetic interactions and molecular interactions, with implementing a benchmark test of HFCs. For this purpose, we constructed a test set consisting of nine dinuclear Mn complexes and Mn(II) ion ligated with six H2O molecules. The computational results are discussed in relation to nature of XC functionals.