Morphology of gel state phosphatidylethanolamine and phosphatidylcholine liposomes: a negative stain electron microscopic study.

The capture volumes (internal aqueous spaces) of liposomes prepared from a series of saturated phosphatidylcholines (PC) and saturated phosphatidylethanolamines (PE) had previously been found to be a function of lipid structure. PE vesicles have larger internal aqueous spaces than PC vesicles and for lipids with the same head group, capture volume increases with lengthening of the fatty acyl chains. Capture volume is determined by vesicle size, number of lamellae, and interlamellar distance. In this study, liposomes were found from a saturated PC or PE and their morphology studied In the gel state using the technique of negative staining transmission electron microscopy. The measured interlamellar distances were quite similar among these various lipids while the number of lamellae was found to decrease as the fatty acyl chain length increased. In general PEs form fewer lamellae than PCs and in particular mono- and di-methylated dipalmitoyl-PE form only unilamellar vesicles. The number of lamellae then appears to bear a relationship to the size of the capture volume in that liposomes with larger capture volumes have fewer lamellae.