Active sites of bio-inorganic systems usually contain one or more transition metal centers, which act as catalysts for the intended chemical transformation. Nature remarkably often utilizes the element manganese in the metabolism of dioxygen and oxygen-based reactants, such as hydrogen peroxide or water. An illustrative example is the oxidative decomposition of water into molecular dioxygen, which is catalyzed by a tetranuclear manganese cluster at the active site of photosystem II in green plants, algae, and several cyanobacteria. This process is driven by solar light and, hence, received a lot of attention in the context of solar fuels. Due to the size and complexity of the involved protein systems, the molecular mechanisms of these processes are often unknown and the structure elucidation of the active sites is difficult to achieve. EPR (electron paramagnetic resonance) spectroscopy is an important tool for the characterization of such open-shell transition metal systems. Quantum-chemical methods provide substantial assistance for the analysis and interpretation of the complicated spectra of large biological systems. Especially density functional theory (DFT) is used extensively to provide insight into the molecular and electronic structure of the active sites and has become an established tool for the considerably large biological systems. The complexity of the systems and their electronic structure require frequently the use of broken-symmetry DFT techniques. Hence, spin-projection techniques, which may strongly affect the quality of the calculated results, have to be applied to the quantum-chemical data before comparison with experimental EPR data. In this work, DFT is used to study the molecular and electronic structure of several bio-inorganic manganese systems. In particular, it aims at providing high-quality quantum-chemically calculated EPR parameters. In the first results part (Chapter 5), the focus lies on the description of quantum-chemically calculated hyperfine (HFC) interactions in dinuclear MnIIIMnIV model complexes. The quality and limits of the applied spin-projection technique are studied in detail. In the second part (Chapter 6), a study on the active site of manganese catalase in its MnIIIMnIV oxidation state is presented. Several structures are proposed and analyzed on the basis of their EPR properties, and a new semi-empirical scaling of manganese HFCs is proposed. The last part (Chapter 7) presents investigations on the oxygen-evolving complex of photosystem II in its S2 state. The first section describes the evaluation of EPR parameters for different proposed structural models. The quality of spin-projection techniques for such multinuclear systems is discussed in detail. The second section presents a study on the binding of ammonia to the manganese cluster. Calculated 14N EPR parameters are used to determine the binding site and the chemical nature of the ammonia-derived ligand.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Molecular chemistry, Physical chemistry|
|Keywords:||Hyperfine interactions, Bio-inorganic systems|
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