A calorimetric study of binary mixtures of saturated and monounsaturated mixed-chain phosphatidylethanolamines

Abstract

In this study, we have semisynthesized the following three molecular species of mixed-chain phosphatidylethanolamine: C(22):C(12)PE, C(16):C(18:1Δ 9)PE, and C(10):C(24:1Δ 15)PE. These lipids share a common structural characteristic, that is,t they all have the same total number of carbon atoms in their acyl chains. Aqueous dispersions prepared from three sets of binary lipid mixtures, C(16):C(18:1Δ 9)PE/C(22):C(12)PE,C(10):C(24:1Δ 15)PE/C(22):C(12)PE, and C(16):C(18:1Δ 9)PE/C(10):C(24:1Δ 15)PE, were studied by high-resolution differential scanning calorimetry, leading to the construction of three temperature-composition phase diagrams. A computer program developed on the basis of the thermodynamic equations for non-ideality of mixing (or Brigg-Williams approximation) was applied to fit the calorimetric data, yielding the non-ideality parameters of mixing in the gel and the liquid-crystalline bilayers (ϱ G and g9 L). Based on the shapes of these phase diagrams and the values of ϱ G and ϱ L, it is concluded that any two of the three molecular species of phosphatidylethanolamines under study can mix nearly ideally in the bilayer plane of the liquid-crystalline bilayer. However, these binary lipid mixtures do exhibit the gel-gel phase immiscibility over an extensive compositional region in the gel-state bilayer. By comparison with experimental data obtained with binary mixtures of saturated identical-chain phospholipids, we can conclude that mixed-chain cis-monounsaturated lipid molecules and saturated lipid molecules are highly demixed in the same two-dimensional plane of the gel-state bilayer, although the bilayer thickness difference between the lipid bilayer composed of cis-monounsaturated lipids and that of saturated lipids may be only one or two C C bond lenghts at T < T m.