The goals of the research are to characterize the structural, electronic, and vibrational properties of tetrapyrroles in various environments including solutions, proteins, and protein maquettes (small synthetic proteins). The principal investigative tool is resonance Raman spectroscopy, although UV-Vis, infrared, and electron paramagnetic resonance spectroscopies; electrochemical techniques; and vibrational analysis methods also play important roles. The long-term objective is to relate the physical properties of the tetrapyrroles to their functional characteristics (ligand binding and transport, electron and energy transfer, catalysis) in both natural and synthetic proteins. The first objective is (1) to elucidate the vibrational (and other) properties of phlorins (porphyrins with saturated methine positions) and hydrocorphinoids (porphyrins with saturated methine and beta-pyrrole positions) and (2) to map out the potential energy surface of the metal/axial-ligand core motions of tetrapyrroles. The goal of the first study is to raise the level of understanding of the structural/elctronic/vibrational properties of phlorins/hydrocorphinoids to one comparable to that currently available for porphyrins and establish benchmarks for characterizing the former types of cofactors in proteins. The goal of the second is to rectify deficiencies in force fields that are widely used for structure refinement and molecular dynamics simulations. The second objective is to examine the vibrational (and other) spectroscopic properties of the active sites of two metalloproteins, hydroxylamine oxidoreductase (HAO) and methyl-CoM reductase (MCR), that significantly impact the composition of the biosphere and hence, the general health of the population. HAO, a key enzyme in the global nitrogen cycle, contains an iron phlorin (P460) that catalysis the final step in the conversion of ammonia to nitrite. MCR, a key enzyme in methanogenesis, contains a nickel hydrocorphinoid (F430) that catalysis the final step in the conversion of carbon sources to methane. The goal is to elucidate the unique features of the P460 and F430 active sites that mediate their novel chemistry. The third objective is to characterize the vibrational properties of the metatallotetrapyrrole cofactors in a variety of protein maquettes. The maquettes contain tetrapyrroles with different metal ions and peripheral substituents and peptides with a variety of amino acid sequences. Monolayer assemblies of the maquettes will also be investigated. The goal is to elucidate the structure of the cofactors in the synthetic proteins and explore how their redox and electronic properties are influenced by the protein matrix.
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