THE PHASE-BEHAVIOR OF MIXED AQUEOUS DISPERSIONS OF DIPALMITOYL DERIVATIVES OF PHOSPHATIDYLCHOLINE AND DIACYLGLYCEROL

The phases and transition sequences for aqueous dispersions of mixture s of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalm itoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron micr oscopy, P-31-nuclear magnetic resonance spectroscopy, and Fourier-tran sform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperat ure over the range 20 degrees-90 degrees C. It is concluded that DPPC and 1,2-DPG form two complexes in the gel phase, the first one with a DPPC/1,2-DPG molar ratio of 55:45 and the second one at a molar ratio of approximately 1:2, defining three different regions in the phase di agram. Two eutectic points are postulated to occur: one at a very low 1,2-DPG concentration and the other at a 1,2-DPG concentration slightl y higher than 66 mol%. At temperatures higher than the transition temp erature, lamellar phases were predominant at low 1,2-DPG concentration s, but nonlamellar phases were found to be predominant at high proport ions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixture s was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high la mellar repeat spacings seen by x-ray diffraction. Ripple phase charact eristics were also found in the fluid lamellar phases occurring at con centrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fouri er transform infrared spectroscopy of the dehydration of the lipid-wat er interface induced by the presence of 1,2-DPG. The biological signif icance of the presence of diacylglycerol in membrane lipid domains is discussed.