ESR studies on the orientation of cholesteryl ester in phosphatidylcholine multilayers

The alignment of cholesteryl esters in multilayer phosphatidylcholine membranes was investigated using two spin-labelled cholesteryl esters: 10 : 3 ester (I) and 1 : 14 ester (II). The nitroxide label of I is aligned in the membrane with a very large angle of tilt (47 degrees +/- 1.5 degrees) with respect to the normal to the membrane surface; II does not show such a tilt. I gives spectra corresponding to immobilized label while II gives nearly isotropic spectra. Ascorbate treatment of the multilayers shows that the labels in I and II are not present at the phosphatidylcholine-water interphase. The data supports a 'horseshoe' configuration for the cholesteryl ester in the bilayer, with both the fatty acid chain and the cholesteryl moiety extending deep into the hydrophobic region of the membrane and with the ester linkage near the surface.