Investigation of Peptide Folding by Nuclear Magnetic Resonance Spectroscopy
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Understanding structure and folding of a protein is the key to understanding its biological function and potential role in diseases. Despite the importance of protein folding, a molecular level understanding of this process is still lacking. Solution-state nuclear magnetic resonance (NMR) is a powerful technique to investigate protein structure, dynamics, and folding mechanisms, since it provides residue-specific information. One of the major contributions that govern protein structure appears to be the interaction with the solvent. The importance of these interactions is particularly apparent in membrane proteins, which exist in an amphiphilic environment. Here, individual peptide fragments taken from the disulfide bond forming protein B (DsbB) were investigated in various solvents. The alpha-helical structures that were obtained, suggest that DsbB follows the two-stage model for folding. However, side chains of polar residues showed different conformations compared to the X-ray structure of fulllength protein, implying that polar side-chains may re-orient upon helix packing in order to form the necessary tertiary interactions that stabilize the global fold of DsbB. Model peptides in general represent attractive systems for the investigation of non-covalent interactions important for protein folding, including those with the solvent. NMR structures of the water soluble peptide, BBA5, were obtained in the presence an organic co-solvent, methanol. These structures indicate that the addition of methanol stabilizes an alpha-helix segment, but disrupts a hydrophobic cluster forming a beta-hairpin. Since dynamic effects reduce the ability for experimental observation of individual, bound solvent molecules, results were compared with molecular dynamics simulations. This comparison indicates that the observed effects of NMR structures are due to preferred binding of methanol and reduction of peptide-water hydrogen bonding. NMR structures, such as those determined here, represent a distribution of conformations under equilibrium. The dynamic process of protein unfolding can nevertheless be accessed through denaturation. A method was developed to probe thermal denaturation by measuring the temperature dependence of NOE intensity. Applied to a model peptide, trpzip4, it was confirmed that the beta-hairpin structure of this peptide is stabilized by the hydrophobic cluster formed by tryptophan residues. Together, the peptides investigated here illustrate the important roles that solvent-peptide interactions and side chain-side chain hydrophobic interactions play in forming stable secondary and tertiary structures.
Hwang, SoYoun (2012). Investigation of Peptide Folding by Nuclear Magnetic Resonance Spectroscopy. Doctoral dissertation, Texas A&M University. Available electronically from