Graduate Thesis Or Dissertation
 

Structural effects of intrinsic and extrinsic perturbations to model membranes

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  • The observed and calculated structural parameters of three systems of phosphatidylcholine bilayers in fully-hydrated multilamellar vesicles (MLVs) were determined using x-ray diffraction techniques. The analysis used the x-ray reflections from the low-, intermediateand wide-angle regions of the diffraction patterns in the subgel, gel, rippled and fluid phases to determine the lamellar repeat distance, projected electron density profile, headgroup and sidechain area, sidechain tilt angle, width of the lipid bilayer, and hydration level based on the known molecular volume of the lipids in each phase. The thermotropic phases of DPPC were reinvestigated to establish a model system and to determine the changes in packing modes through the transitions. Based on changes in the headgroup area and hydration level as the temperature is raised through the subgel phase and on the large discrepancy between the calculated lipid thickness and the peakto- peak distance in the electron density profile, the reflections in the intermediate region of the diffraction pattern were reassigned to obtain a larger headgroup area. The headgroup area and hydration level were found to be larger in the subgel than in the gel phase, contrary to previous analyses. The subgel- and gel-phase sidechain packing modes coexist through the subgel to gel phase transition. No coexistence was observed in the transformation from the gel phase to the subgel phase on annealing at low temperatures. Reflections in the intermediate region of the diffraction patterns from the gel and fluid phases were identified and interpreted as reflections from headgroup packing. To investigate the effects of intrinsic perturbations to the model system, the structural parameters of bilayers of two isobranched lipids (17iPC and 20iPC) were determined. No rippled phase is observed in either lipid. Although the subgel, gel and fluid phases are analogous to those observed in DPPC, the presence of the single methyl group at the penultimate carbon of each side chain affects both the headgroup and sidechain packing and reduces the hydration levels in all phases. To investigate the effects of extrinsic perturbations to the model system, the structural parameters of bilayers of DPPC in the presence of 10, 20 and 30mol% concentrations of bromoform were determined. The main effect of the presence of the solute is to stabilize the rippled phase, extending its range to lower temperatures. The changes induced in the structural parameters are non-monotonic as a function of solute concentration, but hydration levels are generally reduced. Up to 30mol%, the partial molar volume of bromoform in the bilayers is negligible.
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