LOCATION OF CHOLESTEROL IN MODEL MEMBRANES BY MAGIC-ANGLE-SAMPLE-SPINNING NMR

High-resolution magic-angle-sample-spinning C-13-NMR was applied to de termine the specific location of cholesterol in non-perturbed multilam ellar model membranes formed by egg yolk phosphatidylcholine. C-13 spi n-lattice relaxation times of both the phospholipid and cholesterol mo lecules were measured in the absence and in the presence of Gd3+, a pa ramagnetic agent, in order to obtain information on molecular distance s. The effect of Gd3+ on the spin-lattice relaxation times of the lipi d resonances has an explicit distance dependence, allowing it to be us ed to evaluate relative distances on a molecular scale. It has been fo und that cholesterol is placed in such a position that it is not readi ly exposed to the solvent: the hydrophobic steroid ring is oriented pa rallel to the membrane phospholipids, the hydroxyl group is in close v icinity to the phospholipid ester carbonyl groups and the isooctyl sid e chain is deeply buried in the center of the membrane. These data are consistent with an organization such that mixtures of cholesterol and phospholipids present a packing similar to that found in interdigitat ed lipid bilayer systems.