Abstract
Lamellar neutron diffraction was measured from hydrated multilayers of a mixture of egg lecithin and cholesterol oriented on glass substrates. One-dimensional Fourier profiles of the coherent scattering amplitude density across the bilayer repeat units were calculated for multilayers hydrated with both H2O and D2O using phase assignments which were directly determined. The D2O−H2O difference Fourier profiles show that the amounts of water across the repeat units are greatest between the polar groups of adjacent bilayers, and are flat in the hydrocarbon region within the experimental accuracy of ± 0·3 water molecules/lecithin molecule. The observed distribution of water can only be accounted for if the Zwitterionic dipole of the phosphatidyl choline group is oriented in the plane of the bilayer. The position of the hydroxyl group of the cholesterol molecule was determined by deuterium labelling of the adjacent carbon atom and use of a direct scaling method for comparing data from labelled and unlabelled samples. The deuterium label was found to be at the position of the glyceryl-fatty acid ester bonds, and the cholesterol molecule is therefore immersed in the hydrocarbon region of the bilayer.
Published Version
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