Synthesis and characterization of novel manganese-oxo aggregates of biological relevance using polypyridyl-based nitrogen-donor ligands and m-terphenyl-derived carboxylate ligands

Photosystem II (PSII), the membrane-spanning enzyme present in green plants, algae and cyanobacteria catalyzes the light-driven water oxidation process to generate molecular oxygen. The oxygen evolving complex (OEC) consists of a tetramanganese (Mn₄) cluster, the cofactors Ca2+ and Cl− and redox active tyrosine side chain, Y _z. The oxo-bridged Mn₄ cluster plays the key role in O ₂ evolution. The structural properties of the Mn₄ species and the mechanistic details of water oxidation are not well understood. In absence of well-resolved X-ray diffraction data a multitude of biophysical techniques have been put to use in order to explain the structure and function of the Mn₄ cluster. It has been found that the OEC complex has very distinct EPR signatures for different intermediate states.; Our research focuses on the synthesis of Mn₄ model clusters in order to mimic the structural and functional properties of the PSII active site. We have been using a heptadentate ligand, N,N,N ′,N′-tetra(2-methylpyridyl)-2-hydroxypropanediamine (abbreviated as Htphpn) to synthesize tetramanganese complexes that showed great potentials in modeling the structural and spectroscopic properties of the enzyme active site. Our recent results in this project will be discussed in Chapter 2, which include generation of a hyperfine EPR signal relevant to the PSII S₀ state. The bidentate N-donor ligand, bipyridyl (bpy) has been used extensively in enzyme modeling chemistry, but the same with methyl substituted ligands remains unexplored in this field. We have synthesized several high valent Mn clusters with 4,4′-dimethyl-2,2 ′-bipyridyl (dmb) ligand including a novel tetranuclear manganese complex containing an Mn₄(μ-O)₅ unit. The results will be illustrated in Chapter 3 and 4. As the chemical modification studies including site-directed mutagenesis show that the PSII Mn₄ cluster is ligated to a greater number of oxygen donor residues, e.g., aspartate and glutamate, compared to N-donor histidine, it is important to employ carboxylate ligands in synthesizing Mn model complexes. Past studies have shown that the use of small carboxylate donors often results in aggregation of higher number of Mn atoms, which are non-relevant in biology. Currently, we are using ortho-substituted bulky benzoate ligands to eliminate the possibility of higher aggregation. A few groups have pioneered in using this type of ligands in modeling of di-iron enzyme active sites. The recent developments using these ligands will be discussed in Chapter 5 and 6.