Synthesis and vesicle formation from dimeric pseudoglyceryl lipids with (CH2)(m) spacers: pronounced m-value dependence of thermal properties, vesicle fusion, and cholesterol complexation

Eight new dimeric lipids, in which the two Me2N+ ion headgroups are separat ed by a variable number of polymethylene units [-(CH2)(m)-], have been synt hesized. The electron micrograph (TEM) and dynamic light scattering (DLS) o f their aqueous dispersions confirmed the formation of vesicular-type aggre gates. The vesicle sizes and morphologies were found to depend strongly on the m value, the method, and thermal history of the vesicle preparation. In formation on the thermotropic properties of the resulting vesicles was obta ined from microcalorimetry and temperature-dependent fluorescence anisotrop y measurements. Interestingly, the T-m values for these vesicles revealed a nonlinear dependence on spacer chain length (m value). These vesicles were able to entrap riboflavin. The rates of permeation of the OH- ion under an imposed transmembrane pH gradient were also found to depend significantly on the m value. X-Ray diffraction of the cast films of the lipid dispersion s elucidated the nature and the thickness of these membrane organizations, and it was revealed that these lipids organize in three different ways depe nding on the m value. The EPR spin-probe method with the doxylstearic acids 5NS, 12NS, and 16NS, spin-labeled at various positions of stearic acid, wa s used to establish, the chain-flexibility gradient and homogeneity of thes e bilayer assemblies. The apparent fusogenic propensities of these bipolar tetraether lipids were investigated in the presence of Na2SO4 with fluoresc ence-resonance energy-transfer fusion assay. Small unilamellar vesicles for med from 1 and three representative biscationic lipids were also studied wi th fluorescence anisotropy and H-1 NMR spectroscopic techniques in the abse nce and the presence of varying amounts of cholesterol.