Studies of Lipid-Lipid Interactions in Phospholipid Bilayer Membranes

Citation

Petersen, Nils Overgaard (1978) Studies of Lipid-Lipid Interactions in Phospholipid Bilayer Membranes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/V9WW-2180. https://resolver.caltech.edu/CaltechETD:etd-09222004-135757

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Lipid-lipid interactions in bilayer membranes are studied in relation to their influence on (i) the state of motion of phospholipid molecules in the liquid crystalline phase, (ii) the miscibility of different lipids in the various bilayer phases, and (iii) the aggregation and fusion of sonicated bilayer vesicles. Proton and deuterium order parameters measured for the liquid crystalline phase of unsonicated lipid bilayer membranes are interpreted in terms of two types of acyl chain motions, rapid isomerizations and slower reorientations. The observed order parameters are compatible with 50[degree] angular deflections of the chain with respect to the bilayer normal, coupled with a probability of a trans orientation of a methylene segment in the upper two-thirds of the chain of about 0.8 0.9. The motional model can be shown to account for the dynamic properties of the membrane system as measured by nuclear magnetic relaxation measurements, assuming that the chain isomerization occurs at a rate of [...] while chain reorientation occurs at [...]. Analysis of proton and deuterium linewidth data in terms of this model shows that a bilayer curvature, as induced by sonication, causes an increase in the rate and amplitude of chain reorientation without substantially affecting the isomerization motion along the acyl chain. Phase diagrams for lecithin mixtures are inferred from Differential Thermal Analysis measurements. In the liquid crystalline phase all the lecithins form ideal solutions whereas in the gel phases immiscibility occurs when the lecithin acyl chains differ by four or more methylene groups in length. Cholesterol-lecithin interactions are discussed in terms of the motional model for the lecithin bilayer. A phase diagram for cholesterol-lecithin-water mixtures is proposed and discussed in relation to the currently available data on these mixtures. The condensing effect of cholesterol is interpreted as a decrease in lecithin chain fluctuations. Changes in light scattering properites of DRL vesicles at their phase transition are interpreted using the above model of chain motion. The results are compatible with the conclusions derived from the NMR data. Time dependent absorbance measurements of DPL vesicles are explained by the colloidal properties of the suspension. It is shown that aggregation consists of both coagulation and flocculation, the latter of which is abolished with di- or trivalent cations. The possibility of controlling vesicle-vesicle fusion by coagulation is discussed.

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