Phase behavior and arrangement of molecular species in mixtures of a mixed chain and a symmetric phosphatidylethanolamine in the gel and fluid phases.

1-Octadecanoyl, 2-decanoylphosphatidylethanolamine (C(18:0)C(10:0)PE) has been reported to exhibit mixed interdigitated gel-phase packing of the phospholipid acyl chains (Mason, J.T. and Stephenson, F.A. (1990) Biochemistry 29, 590-598). In contrast, ditetradecanoylphosphatidylethanolamine (C(14:0)C(14:0)PE) packs without significant interdigitation of the phospholipid acyl chains across the bilayer center. In this report, the gel-fluid transition temperatures of C(18:0)C(10:0)PE and C(14:0)C(14:0)PE in multilamellar dispersions were determined by fluorescence anisotropy of cis-parinaric acid and trans-parinaric acid with a descending temperature scan rate of 0.67 degrees C/min. The transition mid-points detected for C(18:0)C(10:0)PE with cis-parinaric acid were 19 degrees C in water, 18 degrees C at pH 8.1, and 14 degrees C at pH 10. The phase diagram for C(14:0)C(14:0)PE and C(18:0)C(10:0)PE at pH 10 suggests complete mixing in the fluid phase and considerable immiscibility in the gel phase. Cross-linking of equimolar mixtures of C(14:0)C(14:0)PE and C(18:0)C(10:0)PE with dimethylsuberimidate at pH 10 revealed a random arrangement of the two species in the fluid phase, confirming the notion that C(18:0)C(10:0)PE and C(14:0)C(14:0)PE are miscible in the fluid phase, as determined from the phase diagram. In contrast, cross-linking of the equimolar mixture of C(18:0)C(10:0)PE and C(14:0)C(14:0)PE in the gel phase at 0 degrees C revealed a non-random arrangement, demonstrating and confirming immiscibility in the gel phase.