Structural study on lipid membrane by X-ray anomalous diffraction
Huang, Huey W.
Doctor of Philosophy
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.
Biophysics; Biological sciences; Anomalous diffraction; Lipid membranes Structure