Infrared study of the bilayer stability behavior of binary and ternary phospholipid mixtures containing unsaturated...

The bilayer stability of liposomes containing unsaturated phosphatidylethanolamine (PE) has been investigated by measuring the C = O and CH2 stretching frequencies of the lipids at different lipid compositions and temperatures. Binary mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and dilinoleoyl-PE (DLPE) are known to exhibit lamellar liquid crystalline (L alpha), inverted hexagonal (HII) and metastable intermediate (I) phases. Abrupt increases in the C = O and CH2 stretching frequencies at 65-75 and 90-95% PE, respectively, were found as the PE content of the DLPE/POPC mixtures was increased from 0 to 100%. These transitions were associated with the L alpha -I and I-HII phase transitions of the DLPE/POPC mixtures, accordingly. The effects of three lipid perturbants, butylated hydroxytoluene (BHT), diacylglycerol (DG) and cholesterol (CL), on the above L alpha -I and I-HII transitions were also examined. All perturbants were found to be effective in shifting the L alpha -I transition of the DLPC/POPC mixtures to a lower PE% as detected by the C = O stretching frequency measurements. On the other hand, the perturbants appeared to eliminate the I-HII transition of the DLPE/POPC mixtures as detected by the CH2 stretching frequency measurements. The effectiveness of the perturbants in promoting the L alpha -I phase transition of the DLPE/POPC mixtures followed the order of DG > BHT > CL.(ABSTRACT TRUNCATED AT 250 WORDS)