Infrared study of the polymorphic phase behavior of dioleoylphosphatidylethanolamine and dioleoylphosphatidylcholine mixtures

Abstract

Fourier transform infrared spectroscopy has been used to examine the structural dynamics of lipid fatty acyl chains, lipid/water interface and phosphate headgroup of phospholipid mixtures containing dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) in lamellar liquid crystalline (L α) and inverted hexagonal (H II) phases. The CH 2 symmetric stretching, CO stretching and OPO symmetric and antisymmetric stretching bands of the lipids were measured at different lipid compositions (0–100% DOPE) and temperatures (0–75°C). For pure DOPE, a significant increase in the CO and a slight increase in the CH 2 stretching frequencies were found at 5–10°C which corresponds to the known L α-H II transition temperature of DOPE. No significant changes were observed for the OPO symmetric and antisymmetric frequencies. Upon increasing the content of DOPC in the DOPE/DOPC binary mixtures, the L α-H II transition became broadened and shifted to higher temperatures. This behavior was further quantitated by the use of a contour plot of CO frequency as a function of temperature and PE%. Ternary mixtures of DOPE/DOPC/tetradecane were also studied at a fixed DOPE/DOPC ratio (75:25) and at 23°C. The CO stretching frequency increased as the oil concentration varied from 0% to 6% (oil/lipid weight%) and remain constant as the oil content increased from 6–20%. A slight decline in the CH 2 frequency was also noticed at 6–8% tetradecane. These results agreed with the known H II promotion effect of tetradecane in the binary DOPE/DOPC mixtures.