The phase behavior of mixed aqueous dispersions of dipalmitoyl derivatives of phosphatidylcholine and diacylglycerol.
The phases and transition sequences for aqueous dispersions of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycerol (1,2-DPG) have been studied by differential scanning calorimetry, dynamic x-ray diffraction, freeze-fracture electron microscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The results have been used to construct a dynamic phase diagram of the binary mixture as a function of temperature 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 diagram. 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 slightly higher than 66 mol%. At temperatures higher than the transition temperature, lamellar phases were predominant at low 1,2-DPG concentrations, but nonlamellar phases were found to be predominant at high proportions of 1,2-DPG. A very important aspect of these DPPC/1,2-DPG mixtures was that, in the gel phase, they showed a ripple structure, as seen by freeze-fracture electron microscopy and consistent with the high lamellar repeat spacings seen by x-ray diffraction. Ripple phase characteristics were also found in the fluid lamellar phases occurring at concentrations up to 35.6 mol% of 1,2-DPG. Evidence was obtained by Fourier transform infrared spectroscopy of the dehydration of the lipid-water interface induced by the presence of 1,2-DPG. The biological significance of the presence of diacylglycerol in membrane lipid domains is discussed.
López-García
F
F
Departmento de Bioquímica y Biología Molecular (A), Universidad de Murcia, Spain.
Villalaín
J
J
Gómez-Fernández
J C
JC
Quinn
P J
PJ
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning / Q000379:methods; D004075:Diglycerides / Q000737:chemistry*; D005614:Freeze Fracturing; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008563:Membrane Lipids / Q000737:chemistry / Q000502:physiology; D008854:Microscopy, Electron / Q000379:methods; D008954:Models, Biological; D008968:Molecular Conformation; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D014867:Water; D014961:X-Ray Diffraction / Q000379:methods
D004075:Diglycerides; D008563:Membrane Lipids; D014867:Water; D015060:1,2-Dipalmitoylphosphatidylcholine; C005071:dipalmitin