Asymmetric disposition of detergents within vesicle bilayer and its effect on ion permeation through the membrane

Abstract

Phospholipid vesicles were prepared by detergent removal using hydrophobic porous beads, Amberlite XAD-2, or dialysis from detergent-phospholipid mixed micelles. The liposomes formed were found to be mostly unilammellar vesicles. The vesicle diameter was estimated, by both quasi-elastic light-scattering and gel-exclusion chromatography on Sephacryl S-1000, to be 80 nm for the vesicles formed by removal of octaethylene glycol monododecyl ether by the bead method. The effect of detergents within a bilayer on ion permeation was demonstrated. When the content of octaethylene glycol monododecyl ether reached a molar ratio of 0.2, the intrinsic ion selectivity of the phospholipid membrane between anion and cation was diminished. The ion permeability measured for vesicles with detergent incorporated into initially detergent-free vesicles was about 10-times greater than that for vesicles with detergent remaining following the process of detergent removal. This observation was explained by the different disposition of the detergent in the bilayer, that is, when vesicles were formed by the removal of detergent from mixed micelles, the residual detergent became distributed in both the outer and inner leaflets, and when the detergent was incorporated into initially detergent-free vesicles, the detergent became distributed only in the outer leaflet within the experimental time limits. This idea was supported by the NMR studies. It was also found that, as a detergent, octaethylene glycol monododecyl ether has a stronger effect on ion permeation than octyl glucoside.