Phase behavior of hydrated bilayer of binary phospholipid mixtures composed of 1,2-distearoylphosphatidylcholine and 1-stearoyl-2-oleoylphosphatidylcholine or 1-oleoyl-2-stearoylphosphatidylcholine

Phase behavior of hydrated bilayer of binary phospholipid mixtures composed of 1,2-distearoylphosphatidylcholine (DSPC) and 1-stearoyl-2-oleoylphospharidylcholine (SOPC), and DSPC and 1-oleoyl-2-stearoylphosphatidylcholine (OSPC) was examined in terms of a pseudo-binary phase diagram. The phase boundaries determined by differential scanning calorimetry (DSC) were analyzed based on a thermodynamic model applying the Bragg-Williams approximation for non-ideality of mixing. The interchange energy parameters derived from this approach for liquid-crystalline and solid-like gel phases were rho(0)((L)) = 2.0 kJ mol(-1) and rho(0)((S)) = 5.4 kJ mol(-1) for SOPC/DSPC mixture and rho(0)((L)) = 1.9 kJ mol(-1) and rho(0)((S)) = 5.5 kJ mol(-1) for OSPC/DSPC mixture. These values for both mixture systems are close to each other, and also close to those obtained for dimyristoylphosphalidylcholine (DMPC)/DSPC mixture. This suggests that the effect of introducing a cis-unsaturated acyl chain into a diacylphosphatidylcholine on the mixing behavior with homologous phospholipid with two identical saturated acyl chains is comparable to that of shortening the length of saturated acyl chain by four methylene units. (C) 1999 Elsevier Science B.V. All rights reserved.