Interfacial Properties of Model Membranes and Plasma Lipoproteins Containing Ether Lipids†

The interfacial properties of synthetic ester and ether phosphatidylcholines (PCs were investigated by using the polarity-sensitive fluorescent probes 6-propionyl-2-(dimethylamino)naPhthalene (Prodan) and pyrene. The physical state of the phospholipid matrix was determined by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH). Single-bilayer phospholipid vesicles formed by sonication and model high-density lipoproteins were studied. On the basis of a number of spectroscopic and thermodynamic criteria, the interfacial regions of PCs and their ether analogues are similar. The fluorescence properties of Prodan in model lipoproteins or single-bilayer vesicles were independent of the phospholipid fatty acyl chain length and polar head group, as well as the substitution of ether linkage for ester bonds in the phospholipid. The spectral shifts collated mainly with the physical state of the phospholipid. The emission spectrum of Prodan appeared at shorter wavelengths upon transfer from water to liquid-crystalline phospholipid and blue shifted further when the lipid was cooled to its gel phase. The effect of cholesterol in model high-density lipoproteins on the emission spectrum of Prodan was dose dependent and, at 18 mol % cholesterol the spectrum was similar to that observed in a pure gel-phase lipid and was independent of temperature. The quantum yield of Prodan fluorescence in an ether-PC matrix was similar to that observed in water, whereas in an ester-PC matrix it was enhanced by a factor of about 5. Phospholipid-water partition coefficients of Prodan were independent of the physical state of 1,2-dimyristoyl-sn-glycero-3-phosphocholine 1,2-tetradecyl-sn-glycero-3-phosphocholine. In ether and ester PCs, changes in the enhancement ratio of the vibronic bands of pyrene corresponded with the lipid-phase transition. The quantum yield of pyrene was greatly enhanced when an ester-PC matrix was replaced by an ether-PC matrix. Spectral differences of the probes in gel vs. liquid-crystalline phases are consistent with differences in exclusion or entry of water into the interfacial region due to lateral expansion of the head groups. The observed effects with cholesterol are consistent with dehydration of the probe environment. The decreased quantum yield of Prodan, pyrene, and DPH in ether phospholipids suggests that greater water penetration creates a hydrophobic-hydrophilic balance in the interface for ether phospholipids that is comparable to that of ester phospholipids Therefore, proteins and other amphiphiles that associate with the phospholipid surfaces via the interfacial hydrophobic effect should not distinguish between diacyl- and dialkylglycerophosphocholines.