PHASE-BEHAVIOR AND ARRANGEMENT OF MOLECULAR-SPECIES IN MIXTURES OF A MIXED CHAIN AND A SYMMETRICAL 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, ditetradecanoylphosphatidylet hanolamine (C(14:0)C(14:0)PE) packs without significant interdigitatio n of the phospholipid acyl chains across the bilayer center. In this r eport, the gel-fluid transition temperatures of (18:0)C(10:0)PE and C( 14:0)C(14:0)PE in multilamellar dispersions were determined by fluores cence anisotropy of cis-parinaric acid and trans-parinaric acid with a descending temperature scan rate of 0.67-degrees-C/min. The transitio n mid-points detected for C(18:0)C(10:0)PE with cis-parinaric acid wer e 19-degrees-C in water, 18-degrees-C at pH 8.1, and 14-degrees-C at p H 10. The phase diagram for C(14:0)C(14:0)PE and C(18:0)C(10:0)PE at p H 10 suggests complete mixing in the fluid phase and considerable immi scibility in the gel phase. Cross-linking of equimolar mixtures of C(1 4:0)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, c onfirming the notion that C(18:0)C(10:0)PE and C(14:0)C(14:0)PE are mi scible in the fluid phase, as determined from the phase diagram. In co ntrast, 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-rando m arrangement, demonstrating and confirming immiscibility in the gel p hase.