Bilayer asymmetry in lysophosphatidylcholine/cholesterol (1:1) vesicles. A phosphorus-31 NMR study

Prolonged sonication (3 h) of equimolar amounts of lysophosphatidylcholine (lysoPC) and cholesterol (chol) produces small unilamellar vesicles. Phosphorus-31 NMR (32.20 MHz) of the vesicles gave rise to a single peak (40.5 ppm) which was split upon addition of lanthanide ions. An additional, more intense signal appeared downfield near 51.0 ppm due to 2.4 mM Pr3+, upfield near 34.3 ppm due to 5 mM Yb3+. The more intense signals responsive to paramagnetic ions were assigned to lysoPC located in the outer vesicle leaflet; the signal not shifted by the ions was assigned to inside lysoPC. Based on peak intensities, an outside-to-inside lysoPC ratio (Ro/i) of 6.5-6.6 was determined. Essentially the same Ro/i values (6.6-6.8) were obtained when Pr3+ was present only in the vesicle interior or when Pr3+ was on the inside and Pr3+ and Yb3+ were on the outside. Ion leakage did not occur. Our data demonstrate that lysoPC/chol (1:1) vesicles are drastically asymmetric and that lysoPC shows a distinct preference for the outer bilayer leaflet.