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 ((2)H-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 C(2) and C(3) 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 (2)H-NMR spectra revealed phase separation in bilayers composed of sphingomyelin and monounsaturated phosphatidylcholine, in broad agreement with existing phase diagrams.

Alpha-synuclein has a high affinity for packing defects in a bilayer membrane: a thermodynamics study.

A number of neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are characterized by the intracellular deposition of fibrillar aggregates that contain a high proportion of alpha-synuclein (alphaS). The interaction with the membrane-water interface strongly modulates folding and aggregation of the protein. The present study investigates the lipid binding and the coil-helix transition of alphaS, using titration calorimetry, differential scanning calorimetry, and circular dichroism spectroscopy. Titration of the protein with small unilamellar vesicles composed of zwitterionic phospholipids below the chain melting temperature of the lipids yielded exceptionally large exothermic heat values. The sigmoidal titration curves were evaluated in terms of a simple model that assumes saturable binding sites at the vesicle surface. The cumulative heat release and the ellipticity were linearly correlated as a result of simultaneous binding and helix folding. There was no heat release and folding of alphaS in the presence of large unilamellar vesicles, indicating that a small radius of curvature is necessary for the alphaS-membrane interaction. The heat release and the negative heat capacity of the protein-vesicle interaction could not be attributed to the coil-helix transition of the protein alone. We speculate that binding and helix folding of alphaS depends on the presence of defect structures in the membrane-water interface, which in turn results in lipid ordering in the highly curved vesicular membranes. This will be discussed with regard to a possible role of the protein for the stabilization of synaptic vesicle membranes.

Hydration and lateral organization in phospholipid bilayers containing sphingomyelin: a 2H-NMR study.

Interfacial properties of lipid bilayers were studied by (2)H nuclear magnetic resonance spectroscopy, with emphasis on a comparison between phosphatidylcholine and sphingomyelin. Spectral resolution and sensitivity was improved by macroscopic membrane alignment. The motionally averaged quadrupolar interaction of interlamellar deuterium oxide was employed to probe the interfacial polarity of the membranes. The D(2)O quadrupolar splittings indicated that the sphingomyelin lipid-water interface is less polar above the phase transition temperature T(m) than below T(m). The opposite behavior was found in phosphatidylcholine bilayers. Macroscopically aligned sphingomyelin bilayers also furnished (2)H-signals from the amide residue and from the hydroxyl group of the sphingosine moiety. The rate of water-hydroxyl deuteron exchange could be measured, whereas the exchange of the amide deuteron was too slow for the inversion-transfer technique employed, suggesting that the amide residue is involved in intermolecular hydrogen bonding. Order parameter profiles in mixtures of sphingomyelin and chain-perdeuterated phosphatidylcholine revealed an ordering effect as a result of the highly saturated chains of the sphingolipids. The temperature dependence of the (2)H quadrupolar splittings was indicative of lateral phase separation in the mixed systems. The results are discussed with regard to interfacial structure and lateral organization in sphingomyelin-containing biomembranes.