Nuclear magnetic resonance study of lecithin-cholesterol interactions

Abstract

The interactions of cholesterol with lecithin in excess water have been examined in sonicated dispersions using high resolution nuclear magnetic resonance spectroscopy, and in unsonicated dispersions using wide line nuclear magnetic resonance. Detailed spectroscopic and molecular interpretations are derived by comparison of the lecithin spectra before and after the addition of cholesterol. When dispersed in excess water, lecithin and cholesterol form an equimolar complex which has a lifetime greater than 30 msec. Apolar interactions between the steroid molecule and the first ten methylene groups of each lecithin chain severely restrict the motions of this region of the chains, leaving their terminal methyl ends relatively free. The lecithin phosphate group and cholesterol hydroxyl appear to be juxtaposed and may be hydrogen bonded. In a codispersion where the cholesterol is present in less than equimolar amounts, the complex is not distributed evenly within the bilayers, but forms clusters. Earlier observations on the chain-melting behaviour of lecithin-cholesterol codispersions are correlated with the existence of the complex and with its temperature dependence. The biological significance of the rigidity profiles across lecithin and mixed lecithin-cholesterol bilayers and of the possibility of heterogeneous distributions of complex within the plane of a bilayer is discussed.