Dynamics of the Hydrocarbon Layer in Liposomes of Lecithin and Sphingomyelin Containing Dicetylphosphate

Physical properties of the hydrocarbon region in lipid bilayers were studied in a series of liposomes of lecithin and sphingomyelin containing different concentrations of dicetylphosphate. The technique used was described previously and is based on fluorescence polarization analysis of a specific probe embedded in the analyzed region. The two probes employed in this study were perylene and 1,6-diphenyl-1,3,5-hexatriene, which simulate a rotating disc and a rotating rod, respectively. The determined dynamic properties of the hydrocarbon region in the lecithin liposomes differ markedly from those of the sphingomyelin liposomes. The hydrocarbon region of the lecithin liposomes is of an invariant phase between 0° and 60° characterized by a microviscosity at 25°, η (25°), of 0.8 ± 0.1 poise and a fusion activation energy, ΔE, of 8 ± 2 Cal per mole. In contradistinction to lecithin, the hydrocarbon region of the sphingomyelin liposomes displays a distinct phase transition at 32° ± 2°. The phase at temperatures above the transition point, presumably a liquid crystalline phase, is characterized by ΔE = 16 ± 4 Cal per mole, whereas the phase below it, presumably a gel state, possesses a ΔE value lower than 3 Cal per mole. In addition to that, the·hydrocarbon layer in sphingomyelin liposomes is much more viscous than in lecithin liposomes as shown by η (25°) = 6 ± 1 poise. All of the above characteristics are only slightly and irregularly affected by the presence of dicetylphosphate, despite the strong effects it exerts on the surface charge potential of the liposomes. This indicates that the forces which dictate the dynamic properties of the hydrocarbon region in lipid bilayers predominantly originate from hydrophobic interactions.