A CALORIMETRIC STUDY OF BINARY-MIXTURES OF SATURATED AND MONOUNSATURATED MIXED-CHAIN PHOSPHATIDYLETHANOLAMINES

In this study, we have semisynthesized the following three molecular s pecies of mixed-chain phosphatidylethanolamine: C(22):C(12)PE, C(16):C (18:1 Delta(9))PE, and C(10):C(24:1 Delta(15))PE. These lipids share a common structural characteristic, that is, they all have the same tot al number of carbon atoms in their acyl chains. Aqueous dispersions pr epared from three sets of binary lipid mixtures, C(16):C(18:1 Delta(9) )PE/C(22):C(12)PE, C(10):C(24:1 Delta(15))PE/C(22):C(12)PE, and C(16): C(18:1 Delta(9))PE/C(10):C(24:1 Delta(15))PE, were studied by high-res olution differential scanning calorimetry, leading to the construction of three temperature-composition phase diagrams. A computer program d eveloped on the basis of the thermodynamic equations for non-ideality of mixing (or Brig g-Williams approximation) was applied to fit the ca lorimetric data, yielding the non-ideality parameters of mixing in the gel and the liquid-crystalline bilayers (rho(G) and rho(L)). Based on the shapes of these phase diagrams and the values of rho(G) and rho(L ), it is concluded that any two of the three molecular species of phos phatidylethanolamines under study can mix nearly ideally in the bilaye r 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 exp erimental data obtained with binary mixtures of saturated identical-ch ain phospholipids, we can conclude that mixed-chain cis-monounsaturate d 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 o r two C-C bond lengths at T < T-m.