Structure and Lipid Interaction of N-Palmitoylsphingomyelin in Bilayer Membranes as Revealed by 2H-NMR Spectroscopy

Selectively deuterated N-palmitoyl sphingomyelins were studied by deuterium nuclear magnetic resonance spectroscopy (2H-NMR) to elucidate the backbone conformation as well as the interaction of the sphingolipids with glycerophospholipids. Macroscopic alignment of the lipid bilayers provided good spectral resolution and permitted the convenient control of bilayer hydration. Selective deuteration at the acyl chain carbons C2 and C3 revealed that the N-acyl chain performs a bend, similar to the sn-2 chain of the phosphatidylcholines. Profiles of C-D bond order parameters were derived from the segmental quadrupolar splittings for sphingomyelin alone and for sphingomyelin-phosphatidycholine mixtures. In the liquid-crystalline state, the N-acyl chain of sphingomyelin alone revealed significantly more configurational order than the chains of homologous disaturated or monounsaturated phosphatidylcholines. The average chain order parameters and the relative width of the order parameter distribution were correlated over a range of bilayer compositions. The temperature dependence of the 2H-NMR spectra revealed phase separation in bilayers composed of sphingomyelin and monounsaturated phosphatidylcholine, in broad agreement with existing phase diagrams.