THE PHASE-BEHAVIOR OF AQUEOUS DISPERSIONS OF UNSATURATED MIXTURES OF DIACYLGLYCEROLS AND PHOSPHOLIPIDS

The phase behavior of mixtures of 1-palmitoyl-2-oleoyl-sn-glycerol (1, 2-POG) with 1-palmitoyl-2-oleoyl-sn-glycero3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was studied by using DSC, small-angle X-ray diffraction and P-31-NMR. The results ha ve been used to construct phase diagrams for both type of mixtures, in the 0-45 degrees C range. It is concluded that 1,2-POG form complexes in the gel phases with both POPC and POPS. In the case of POPC, two c omplexes are postulated, the first one at a 1,2-POG/POPC molar ratio o f 40:60, and the second one at 70:30, defining three different regions in the phase diagram. Two eutectic points are proposed to occur: one at a very low 1,2-POG concentration and the other at a 1,2-POG concent ration slightly lower than 70%. In the case of the 1,2-POG/POPS mixtur es, the pattern was similar, but the first complex was seen to happen at a higher concentration, about 50 mol% of 1,2-POG, whereas the secon d was found at 80 mol% of 1,2-POG. This indicated a bigger presence of 1,2-POG in the complexes with POPS than with POPC. In the first regio n of the phase diagram, i.e. at concentrations of 1,2-POG lower than t hat required for the formation of the first complex, and at temperatur es above the phase transition, lamellar phases were seen in all the ca ses. In region 2 of the phase diagram, i.e. at concentrations where th e first and the second complexes coexist, a mixture of lamellar and no n-lamellar phases was observed. Finally, at high concentrations of 1,2 -POG, non-lamellar phases were detected as predominant, these phases b eing of an isotropic nature, according to P-31-NMR. An important concl usion of this study is that, using unsaturated lipids, similar to thos e found in biological membranes, it has been shown that diacylglycerol s are found separated in domains, and that this process starts at very low concentrations of diacylglycerols. The formation of separated dom ains enriched in diacylglycerol is biologically relevant as it will al low them to have important effects on the membrane structure besides t he fact that their concentration in the biomembrane is relatively low. (C) 1998 Elsevier Science B.V. All rights reserved.