Interactions of highly charged cationic peptides and large anions with lipid bilayers
Huang, Huey W.
Master of Science
The first chapter is devoted to the highly charged cationic peptides penetratin (pAntp), a 16 residue peptide belonging to the class of CPP (cell penetrating peptides). The translocation of pAntp across cell membranes is believed to occur through a mechanism that is independent of receptors, transporters, and endocytosis. We studied the state of pAntp bound to lipid bilayers by the method of oriented circular dichroism (OCD). In bilayers composed of mixed lipids (DOPC/DOPG) pAntp shows both conformational and orientational changes. At low peptide concentrations (Peptide/Lipid ratio) and high charge densities, the pAntp tends to adopt alpha-helical conformation. At high peptide concentrations and low charge densities, the pAntp tends to adopt beta-sheet and random coil conformations. The alpha-helical pAntp was observed to change its orientation in membrane as the hydration of the bilayers changes. The effect of the peptide termini on its conformation was also examined. The peptides with three different ending forms were compared. The result seems to suggest that the conformation of the peptide is subject to the variation of the peptide termini. The second chapter investigates on the effect of large chaotropic anions on lipid bilayer structure (Hofmeister effect). X-ray diffraction experiments were done on POPC lipid and its mixture with sodium salts (NaI and NaSCN). The result shows that in the present of the salts, the change in the bilayer structure is primarily the thermal motional range of the phosphate headgroup of lipid. The lipid headgroup undergoes a broader motion range in the presence of I- and a narrower range in the presence of SCN-.
Cell biology; Biophysics; Biology