Conformational studies of sphingolipids by NMR spectroscopy. I. Dihydrosphingomyelin.

The conformational features of dihydrosphingomyelin (DHSM), the major phospholipid of human lens membranes, were investigated by 1H and 31P nuclear magnetic resonance spectroscopy. Several postulates emerge from the observed trends: (a) in partially hydrated samples of DHSM in CDCl3 above 13 mM, at which lipid-lipid interactions prevail, the amide proton is mostly involved in intermolecular H-bonds that link neighboring phospholipids through bridging water molecules. In the absence of water, the NH group is involved in an intramolecular H-bond that restricts the mobility of the phosphate group. (b) In the monomeric form of the lipid molecule, the amide proton of the major conformer is bound intramolecularly with one of the anionic and/or ester oxygens of the phosphate group. A minor conformer may also be present in which the NH proton participates in an intramolecular H-bond linking to the OH group of the sphingoid base. (c) Complete hydration leads to an extension of the head group as water molecules bind to the phosphate and NH groups via H-bonds, thus disrupting the intramolecular H-bonds prevalent at low concentrations.

Conformational studies of sphingolipids by NMR spectroscopy. II. Sphingomyelin.

Sphingomyelin (SM) is the most prevalent sphingolipid in the majority of mammalian membranes. Proton and 31P nuclear magnetic resonance spectral data were acquired to establish the nature of intra- and intermolecular H-bonds in the monomeric and aggregated forms of SM and to assess possible differences between this lipid and dihydrosphingomyelin (DHSM), which lacks the double bond between carbons 4 and 5 of the sphingoid base. The spectral trends suggest the formation of an intramolecular H-bond between the OH group of the sphingosine moiety and the phosphate ester oxygen of the head group. The narrower linewidth and the downfield shift of the resonance corresponding to OH proton in SM suggest that this H-bond is stronger in SM than in DHSM. The NH group appears to be involved predominantly in intramolecular H-bonding in the monomer. As the concentration of SM increases and the molecules come in closer proximity, these intramolecular bonds are partially disrupted and the NH group becomes involved in lipid-water interactions. The difference between the SM and DHSM appears to be not in the nature of these interactions but rather in the degree to which these intermolecular interactions prevail. As SM molecules cannot come as close together as DHSM molecules can, both the NH and OH moieties remain, on average, more intramolecularly bonded as compared to DHSM.