Differential Interaction of Cholesterol with Phosphatidylcholine on the Inner and Outer Surfaces of Lipid Bilayer Vesicles

The separate identification of the -N(+)-(CH(3))(3) groups located on the outside and inside of phospholipid bilayers observed in proton magnetic resonance spectroscopy upon addition of low concentrations of praseodymium ion was exploited to investigate the effects of incorporated cholesterol. Only about 7% of the phosphate groups on the outside of the bilayer belong to a class of strong binding or shifting sites. Upon addition of up to about 30% cholesterol to egg lecithin bilayers, no changes in chemical shift or ratio of areas of peaks due to outer and inner -N(+)(CH(3))(3) groups appear. At about 30% incorporated cholesterol, an abrupt decrease occurs in the chemical-shift difference between -N(+)(CH(3))(3) groups located on the outer and inner bilayer surfaces, and an abrupt increase occurs in the ratio of the areas of the two peaks. For L-alpha-dipalmitoyl lecithin bilayers, an abrupt change in chemical-shift difference occurring between 10 and 20% cholesterol is not accompanied by a change in the relative number of -N(+)(CH(3))(3) groups located on the outer and inner surfaces. These results are interpreted as due to the homogeneous distribution of up to 30% cholesterol in egg lecithin bilayers. Above 30%, cholesterol is asymmetrically distributed in favor of the inner layer. In egg lecithin with a variety of polyunsaturated side chains, the side chains with the greater number of double bonds are preferentially displaced by high concentrations of cholesterol, which accounts for the increase in the ratio of outer to inner -N(+)(CH(3))(3) groups. Such preferential displacement by cholesterol cannot occur with the saturated L-alpha-dipalmitoyl lecithin. It is suggested that modified phospholipid vesicles of low radii of curvature may provide high concentrations of "active sites" present in membranes.