The goals of the research are to characterize the structural, electronic, and vibrational, properties of tetrapyrrolic macrocycles in various environments including solutions, heme proteins, heme protein maquettes (small synthetic proteins), and supramolecuIar assemblies (synthetic nanostructures). The principal investigative tool is resonance Raman (RR) spectroscopy, although Uv-Vis, infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies; electrochemical techniques; and vibrational analysis methods also play important roles. The long-term objective is to relate the physical properties of the rings to the their functional characteristics (ligand binding and transport, electron transfer, energy transfer, catalysis) in both synthetic and natural proteins.
The specific aims are as follows: (l) Low-frequency vibrational spectra will be obtained and assigned for various five- and six- coordinate porphyrins and hydroporphyrins. Optimum force fields will be developed for the out-of-plane deformations of the rings. These force fields will explicitly include histidine as an axial ligand. (2) RR spectra will be obtained for a series of distal-site myoglobin mutants whose X-ray crystal structures and ligand-binding kinetics have been well characterized. The RR data will be assessed in the context of a complete vibrational model for the low-frequency modes of the heme(axial-ligand unit in order to develop reliable frequency/structure/ligand-binding affinity correlations. The RR data for the mutants will also be used for further refinement of the force field for the out-of-plane modes of the heme/axial-ligand unit. This force field should be more reliable than one developed from model complexes alone and should facilitate more accurate modeling of the heme motions in molecular dynamics simulations. (3) RR spectra will be obtained for a series of small (62 amino acids) synthetic mono- and diheme binding peptides. These data will be used to assess how the structure of the hemes affects their redox chemistry. This information will serve as a benchmark for future synthetic protein design aimed at precise control of the redox properties of the hemes. (4) Optical, RR, and EPR spectra will be obtained for a variety of novel, covalently linked porphyrinic assemblies. These data will be used to determine how the geometrical arrangement of the rings in the assembly affects the electronic communication between the macrocycles.
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