Nuclear magnetic resonance study of sphingomyelin bilayers.

Bilayers of D-erthro-(N-stearoylsphingosyl)-1-phosphocholine (C18-SPM), previously characterized by differential scanning calorimetry [Bruzik, K. S., & Tsai, M.-D. (1987) Biochemistry 26, 5364-5368] in various phases, were studied by means of wide-line 31P, 2H, high-resolution 13C CP-MAS, and 1H MAS NMR. The fully relaxed gel phase of C18-SPM at temperatures below 306 K displayed 31P NMR spectra characteristic of the rigid phase with frozen rotation of the phosphocholine head group. Three other gel phases existing in the temperature range 306-318 K displayed spectra with incompletely averaged axially symmetric powder line shapes and were difficult to differentiate on the basis of their 31P NMR spectra. The gel-to-gel transition at 306 K was found to be fully reversible. The main phase transition at 318 K resulted in the formation of the liquid-crystalline phase for which spectra with axially symmetric line shapes of uniform width were obtained, regardless of the nature of the starting gel phase. 13C CP-MAS NMR spectra revealed significant differences in the molecular dynamics of sphingomyelin in various phases. All carbon atoms of the polar head group in the liquid-crystalline phase gave rise to a separate resonance lines. Numerous carbon atom signals were doubled in the stable phase, demonstrating the existence of two slowly interconverting conformers.