Binary mixtures of saturated and unsaturated mixed-chain phosphatidylcholines. A differential scanning calorimetry study

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Abstract
High-resolution differential scanning calorimetry (DSC) has been used to study the aqueous dispersions of mixed-chain phosphatidylcholines prepared from colyophilized mixtures of C(18):C-(11:1.DELTA.10)PC/C(18):C(10)PC and C(18):C(11:1.DELTA.10)PC/C(18):C(11)PC of various molar ratios. These mixed-chain phospholipids are characterized by a marked disparity in their acyl-chain lengths; however, the sn-1 acyl chain in the fully extended conformation is about twice as long as the sn-2 acyl chain. Their thermotropic behavior was determined, and the phase diagrams of these two mixtures were constructed from the calorimetric data. Results indicate that C(18):C(11:1.DELTA.10)PC/C(18):C(10)PC and C(18):C-(11:1.DELTA.10)PC/C(18):C(11)PC are miscible in all proportions with a near-ideal behavior of mixing in the gel and liquid-crystalline phases. Equimolar mixtures of diC(14)PC/C(18):C(11:1.DELTA.10)PC, diC(14)PC/C(18):C(10)PC, and diC(14)PC/C(18):C(11)PC have also been studied by DSC. These phosphatidylcholines in the 1:1 mixture differ in Tm by less than 11.degree. C; however, they exhibit gel-phase immiscibility in the plane of the bilayer. Taken together, these studies suggest that C(18):C(11)PC and C(18):C(11:1.DELTA.10)PC are packed similarly to C(18):C(10)PC in excess water as mixed interdigitated bilayers, at T < Tm, which transform into partially interdigitated bilayers when heated above Tm.