X-ray scattering of vesicles of N-acyl sphingomyelins. Determination of bilayer thickness.

A series of N-acyl sphingomyelins (C16:0, C18:0, C20:0, C22:0, and C24:0) have been synthesized and single bilayer vesicles formed by sonication and ultracentrifugation. X-ray scattering data have been collected from the sphingomyelin vesicles at 50 degrees C in the melted-chain state. The x-ray scattering data have been transformed to the corresponding Patterson functions and Fourier electron density profiles; analysis of these functions has provided the intrabilayer phosphate-phosphate separation dp-p, a measure of the lipid bilayer thickness. The bilayer thickness increases linearly with increasing chain length (increment 1.3-1.4 A) and the intercept, 14.3-15.0 A, suggests a contribution of 7.0-7.5 A for each phosphorylcholine group to the bilayer thickness. The electron-density profiles have features suggestive of chain interdigitation when the length of the N-acyl chain (C20:0, C22:0, and C24:0) exceeds significantly the length of the invariant sphingosine chain.