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The multilamellar liposomes of the racemic 1,2-dipalmitoylphosphatidylcholine (DL-DPPC) existing in the various gel phases were investigated by means of the cross-polarization/magic angle spinning (CP-MAS) 13C-NMR. The intensity and the width of the 13C-NMR signals were found to depend to a large extent on temperature. In the metastable gel phase (L beta) signals from all carbon atoms are apparent, with the signals arising from the glycerol backbone significantly broader than those of the choline function. The signal from C-2 of the glycerol backbone undergoes additional broadening between 298 K and 307 K, and at 307 K its coalescence is observed. In the P beta' phase all carbon atoms give rise to relatively sharp separate resonance lines. In the liquid crystalline phase (L alpha) the signals from the choline function and from the terminal methyl groups of hydrocarbon are not observed in the spectrum. The recorded spectral changes are explained in terms of the rate of motional processes occurring in the lipid bilayer in different phases.

The conformation of the polar headgroup of synthetic D-erythro-stearoylsphingomyelin (1), its L-threo-isomer (2) and phosphorothioyl analogues of 1 (3 and 4) has been studied in detail by high-resolution NMR spectroscopy. In both monomeric and aggregated states the phosphocholine function of 1 adopts the synclinal conformation (alpha 5 torsional angle), in analogy with phosphatidylcholine (Hauser, H., Guyer, W., Pascher, I., Skrabal, P. and Sundell, S. (1980) Biochemistry 19, 366-373). The conformation about the C1-C2 bond (theta 1 angle) of the sphingosine backbone is predominantly -synclinal, analogously to the conformation of the crystalline galactosyl cerebroside (Pascher, I. and Sundell, S. (1977) Chem. Phys. Lipids 20, 175-191). In contrast, the L-threo-isomer displays unrestricted rotation about C1-C2 bond. The possibility of the existence of a hydrogen bond between the 3-hydroxyl function and the bridged oxygen atom of sphingosine responsible for the different conformation of 1 and 2 is discussed. The modification of the phosphate function in 1 with sulfur has no significant effect on the conformation of the resulting analogues. The conformation of all studied compounds about the C-O phosphoester bonds (alpha 1 and alpha 4 torsion angles) is mainly antiperiplanar. Similar to other double-chain phospholipids, sphingomyelin shows a preference towards the antiperiplanar conformation about the C2-C3 bond.