| The aim of this study is to determine the consequences of placing polar residues in a transmembrane helix. For this study, a series of synthetic, polyleucine peptides that contained one Trp residue in the middle were utilized. The Trp residue is a naturally fluorescent molecule and acts as a probe to study the peptides' behavior. In addition, each peptide had two lysines at each end as well as a single polar residue in the core of the peptide. The polar residues that were placed at this position include: Ser, Asp, Lys, Arg, and Asn. In addition, a peptide that had only leucines in the core was created to use as a benchmark for comparison. In this study, these peptides were incorporated into small unilamellar vesicles. The wavelength of maximum fluorescence emission intensity of the Trp residues was a key parameter used in this study. Fluorescence spectroscopy studies demonstrated that a single, uncharged polar residue did not destabilize transmembrane orientation of an α-helical peptide to any significant degree.; It was also found that the ionizable Lys residues at each end of these peptides had an effect on the ability of these peptides to self-associate. Specifically, the tendency for these helical peptides to oligomerize was pH dependent. It is believed that electrostatic repulsions between the Lys residues prevent oligomerization from occurring. But, these repulsions are removed at high pH, and then the helical peptides can self-associate.; The importance of the flanking Lys residues on the behavior of these polyleucine peptides was the impetus for the second part of this study. In this part, various combinations of ionizable residues were placed at the flanking positions of synthetic polyleucine peptides. In particular, one peptide was created that had one Asp and one Lys residue at each termini. A second peptide was created that had three Asp residues and one Lys residue at each termini. As before, both of these peptides had a polyleucine core with one Trp residue in the middle. These peptides were incorporated into bilayer vesicles and studied with fluorescence spectroscopy. The wavelength of maximum emission intensity for both peptides was pH dependent. However, these two peptides had a greater wavelength of maximum emission intensity at neutral pH than the series of Lys-flanked polyleucine peptides mentioned before. It was demonstrated that the wavelength of maximum emission intensity for these two Asp-flanked peptides was dependent on the peptide concentration in the bilayer vesicle. (Abstract shortened by UMI.)... |
