Infrared and time-resolved fluorescence spectroscopic studies of the polymorphic phase behavior of phosphatidylethanolamine/diacylglycerol lipid mixtures

Abstract

Fourier transform infrared (FTIR) and time-resolved fluorescence spectroscopy have been employed to examine the structural dynamics of lipid fatty acyl chains and lipid/water interfacial region of a binary lipid mixture containing unsaturated phosphatidylethanolamine (PE) and diacylglycerol (DG). Infrared vibrational frequencies of the CH 2 symmetric stretching and the C  O stretching bands of the lipids were neasured at different lipid compositions and temperatures. For 0% DG, the lamellar gel to lamellar liquid crystalline (L β–L α) and the L α to inverted hexagonal (L α–H Il) phase transitions were observed at ≈ 15° and 55°C, respectively. As the DG content increased gradually from 0% to 15%, the L α–H II phase transition temperature decreased drastically while the L β-L α phase transition temperature decreased only slightly. At 10% DG, a merge of these two phase transitions was noticed at ≈ 10°C. For the composition study at 23°C, the L α–H Il transition occurred at ≈6–10% DG as indicated by abrupt increase in both the CH 2 and C  O streching frequencies at those DG contents. Using time-resolved fluorescence spectroscopy, abrupt decreases in both the normalized long time residual and the initial slope of the anisotropy decay function of lipid probes, 1-palmitoyl-2-[[2-[4-(6-phenyl- trans-1,3,5-hexatrienyl)phenyl]ethyl]carbonyl]-3- sn-phosphatidylcholine, in these PE/DG mixtures were observed at the L α–H II phase transition. These changes in the anisotropy decay parameters suggested that the rotational dynamics and orientational packing of the lipids were altered at the composition-induced L α–H II transition, and agreed with a previous temprature-induced L α–H II transition study on pure unsaturated PE (Cheng (1989) Biophys. J. 55, 1025–1031). The fluorescence lifetime of water soluble probes, 8,1-anilinonapthalenes sulfonate acid, in PE/DG mixtures increased abruptly at the L α–H II phase transition, suggesting that the conformation and hydration of the lipid/water interfacial region also undergo significant changes at the L α–H Il transition.