Differential scanning calorimetric studies of the thermotropic phase behaviour of membranes composed of dipalmitoyllecithin and mixed-acid unsaturated lecithins

Abstract

The lecithins 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) were synthesized by reacylation of the appropriate lysolecithins with fatty acid anhydrides. These lecithins were used to make model membranes in mixtures with dipalmitoyllecithin (DPPC), and phase diagrams of the 2 bilayer systems were constructed. These diagrams show that there is essentially no gel-state miscibility in the POPC-DPPC bilayers at any composition, and that SOPC-DPPC bilayers show gel-state immiscibility at DPPC concentrations of less than 50 mol%, and partial miscibility above 50 mol% DPPC. Analysis of the POPC-DPPC phase diagram on the assumption of athermal solution in the liquid-crystalline phase shows that the 2 lipids mix nearly randomly above the phase transition. The liquidus curve of SOPC-DPPC bilayers showed deviations from calculated ideal behavior, which indicated that there is a small excess tendency for the formation of pairs of like molecules in SOPC-DPPC bilayers in the liquid-crystalline phase. In the liquid-crystalline phase, SOPC and DPPC do not pack quite as well as do POPC and DPPC.