| Abstract
| - The membrane-forming properties of four cationic lipids (1−4) having varying number of oxyethylene(−CH2−CH2−O−)n units at the linkage region between the pseudo-glyceryl backbone and the hydrocarbonchains have been described. Lipids 1 and 2 have an equal number of oxyethylene units attached to both C-1and C-2 positions of the pseudo-glyceryl backbone, while the other two, 3 and 4, are unsymmetrical in termsof the number of oxyethylene linkages. The membrane characteristics of these lipids were compared with acontrol lipid, 5, bearing just an ether functionality at both the C-1 and C-2 positions. All the lipids (1−5)formed stable suspensions in water which showed the presence of membranous aggregates as revealed byelectron microscopy (TEM). The lipids 1, 2, and 4 showed distinct spherical aggregates in TEM, whereas 3exhibited particles of irregular morphologies. The lipid suspensions were further characterized by dynamiclight scattering and zeta potential measurements. Except for lipid 3, zeta potentials of such lipid aggregateswere found to be substantially lower than their diether analogue, 5. X-ray diffraction studies with the lipidcast films revealed that incorporation of oxyethylene units at the linkages increased the unit bilayer thicknessof the membranous assemblies. Temperature-dependent fluorescence anisotropy experiments using the lipid-soluble probe 1,6-diphenylhexatriene (DPH) suggested a disordered environment in these lipid aggregateseven in their gel states. The aggregates formed by lipid 3 showed the lowest DPH anisotropy values. Clearthermotropic phase transitions typical of membranous assemblies were observed for all the lipid suspensionsby differential scanning calorimetry. The phase transitions were reversible and exhibited large hysteresisindicating that the observed phase transitions were of first order.
|
