The thermotropic behavior of aqueous dispersions of mixed-acid lecithins

Davis, Philip James (1982) The thermotropic behavior of aqueous dispersions of mixed-acid lecithins. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Most phospholipids found in biological membranes are mixed-acid phospholipids. Yet there is little known about the physical properties of mixed-acid phospholipids in bilayers. In this work positional isomers were synthesized using modifications of published procedures. Differential scanning calorimetry was used to study the thermotropic properties of aqueous dispersions of l-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC), l-palmitoyl-2oleoyl-sn-glycero-3-phosphocholine (POPC), l-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (OSPC), l-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), l-oleoyl-2-arachidoyl-sn-glycero-3-phosphocholine (OAPC), and l-arachidoyl-2-oleoyl-sn-glycero-3-phosphocholine (AOPC). The gel to liquid-crystalline phase transition temperatures of these lecithins were different from each other, and from those of the parent single-acid lecithins. For each pair of positional isomers the isomer with the longer chain at the sn-l position had the lower transition temperature. – The thermotropic behavior of mixtures of cholesterol with four mixed-acid lecithins and three single-acid lecithins was studied using differential scanning calorimetry. Asymmetric endotherms were observed, and these were resolved into a narrow and a broad component. The narrow components disappeared at different levels of cholesterol for the different lecithins. The results suggested that for the OSPC-SOPC pair, cholesterol interacted more with OSPC than with SOPC. For the OAPC-AOPC pair, the interaction was greater between AOPC and cholesterol than between OAPC and cholesterol. Structural differences in the hydrocarbon region of the lecithin bilayers may account for these observations. – Phase diagrams were constructed for mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) with some mixed-acid lecithins. The phase diagrams were analyzed using Regular Solution Theory, and excess interaction energies were obtained. – Gel phase immiscibility was observed in the mixtures of DPPC with OPOC and with SOPC, but not in mixtures of DMPC with POPC or with l-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine. The miscibility in the gel phase appeared to be related to the transition temperatures of the lipids. In the liquid-crystal there was complete miscibility in all four mixtures. The estimates of excess interaction energies suggested that transient clusters of like molecules in the liquid-crystal may occur. Computer simulations based on excess interaction energies were also presented.

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