Abstract
Membrane extrusion vesicles of egg yolk lecithin were prepared and their diameters and lamellarity were determined from dynamic light scattering measurement and the fluorescent probe method, respectively. The fluorescent probe lipid used was dimyristoylphosphatidylethanolamine 4-nitrobenz-2-oxa-1, 3-diazole (NBD-DMPE). The mean radius of the vesicles sharply decreased with the number of extrusions and became ca. 60 nm at more than ten cycles of extrusion through a 0.1 mm pore-size membrane filter. For a unilamellar vesicle with 60 nm radius and bilayer thickness of 3 nm, the external lipid fraction should be 0.53. The experimentally obtained value was 0.52±0.03 at a molar ratio of NBD-DMPE in total lipid less than 0.3%. At higher NBD-DMPE (0.3%1.0%), the external marker lipid fraction was 0.550.61 immediately following preparation and subsequently decreased slowly to ca. 0.5. It thus follows that 1) vesicles after ten cycles of extrusion are unilamellar, 2) NBD-DMPE becomes distributed in the outer layer of the lecithin bilayer during extrusion, and 3) flip-flop motion of NBD-DMPE occurs after extrusion, leading to even distribution of the marker lipid in the vesicles.
