Browsing by Author "Wang, Wangchen"
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Item Interactions of highly charged cationic peptides and large anions with lipid bilayers(2006) Wang, Wangchen; Huang, Huey W.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-.Item Structural study on lipid membrane by X-ray anomalous diffraction(2008) Wang, Wangchen; Huang, Huey W.The shapes and functions of biological membranes are closely correlated. Under certain conditions, lipid membranes can be induced into long-range ordered lattice structures. X-ray diffraction is a powerful technique in resolving these crystallized membrane structures. To solve the X-ray phase problem for diffraction amplitudes, we developed a novel approach to use the MAD (multi-wavelength anomalous diffraction) method that was routinely used in protein crystallography and applied to the lipid membrane structures. Several lipidic structures were studied including lamellar phase (L), distorted hexagonal phase (H IIδ ), and rhombohedral phase (R). The procedure was first established and demonstrated on the lamellar phase of the brominated lipid di18:0(9,10dibromo)PC. The non-lamellar phases were further investigated. The binary lipid mixture di10:0(9,10dibromo)PC and cholesterol exhibiting a distorted hexagonal phase (H IIδ ) was found to demix locally from their composition ratio. The cholesterol with negative spontaneous curvature preferentially resides at the high-curvature region in the unit cell. The membrane fusion intermediate state (stalk) was studied with two brominated lipids at rhombohedral phase. The density distribution of the bromine label atoms clearly reveals the lipid chains configurations in the unit cell confirming to the hypothesized stalk structure. The reverse-monte carlo (RMC) simulation was used to explain the influence of disorder on the structure resolution of the lipid packing in the unit cell.