Fluorine NMR Studies on the Dynamics and Structure of Biologically Active Peptides.

Abstract

Fluorine NMR is an excellent technique for studying changes in chemical environments due to its high sensitivity and large chemical shift dispersion. Since fluorine is not found in most biological systems, there is no competition from background signals, a problem that often afflicts measurements using proton, carbon, and nitrogen NMR. Therefore, it has been broadly applied to investigate protein conformational changes and dynamics as well as protein-ligand and protein-membrane interactions. In this thesis, fluorine NMR has been used to examine the structure and dynamics of biologically active peptides, including an antimicrobial peptide and amyloidogenic proteins. Using a series of fluorinated MSI-78 antimicrobial peptides, a detailed examination how the structure and dynamics of the peptide changes on binding lipid bicelles was performed using fluorine chemical shifts, solvent isotope effects on chemical shifts, and changes in fluorine longitudinal and transverse relaxation rates. These results provide a detailed picture of the changes in the local chemical environment and peptide dynamics that occur when MSI-78 binds to the lipid bilayer. Also described are direct, real-time measurements of amyloid formation for fluorinated islet amyloid polypeptide (IAPP) and Amyloid-beta, for which fiber formation is linked to type II diabetes and Alzheimer’s disease respectively. By using fluorine NMR, the consumption of monomeric peptides, rather than the formation of fibrils can be monitored. This study showed by comparing monomer depletion followed by fluorine NMR to fiber formation followed by thioflavin T (ThT) fluorescence assay that IAPP fibrillizes without accumulation of oligomeric intermediates. Inhibitory assays using fluorine NMR have suggested that ThT strongly competes with the amyloid inhibitor, epigallocatechin-3-gallate, for binding sites on IAPP fibers. In contrast to IAPP, time course studies using fluorine NMR have shown that small oligomers of Amyloid-beta accumulated. These types of experimental measurements are difficult to perform by other techniques, and the additional level of detail obtained by measuring the rate of monomer consumption by fluorine NMR might prove fruitful for further characterization of amyloid formation. Studies described in this thesis should contribute to develop the utility of fluorine NMR to investigate biological system as well as to help find potential therapeutic compounds.