Structure and molecular conformation of anhydrous and of aqueous sphingomyelin bilayers determined by infrared and Raman spectroscopy

Abstract

Fourier transform-infrared spectra of anhydrous sphingomyelin and sphingomyelin bilayers in aqueous and deuterium phases have been investigated in detail. Raman spectra of the aqueous gel have also been measured. From these vibrational data, the structure and the molecular conformation of the head group, interface region and the hydrocarbon chains have been deduced in the solid and bilayer phases of the lipid. Vibrational assignments for the prominent fundamental modes associated with secondary structure of the lipid and molecular conformation of sphingosine and acyl chains, peptide unit and phospho-choline moiety are proposed. Vibrationally complex 0H and NH stretching and deformation regions of the peptide unit and of bilayer water molecules have been probed using isotopic deuterium substitution. Infrared data suggest a strong in-termolecular hydrogen bonding for the 3-hydroxyl groups of the sphingosine chains and an unusual intramolecular hydrogen bonding for the amide protons to carbonyl oxygens in the bilayer. These hydrogen-bonding schemes are likely to increase membrane stability through lateral contacts which may play an important role in membrane regulatory functions. Second derivative infrared spectroscopy has been used to explore the structurally sensitive spectral regions such as carbonyl, methylene deformation and phosphate regions of the lipid. This application revealed many weak bands associated with the specific regions of the membrane structure. Temperature-induced cooperative conformational transitions in sphingomyelin biomembrane have been examined in the aqueous phase. Data suggest a two state order-disorder phase transition at 35°C with a transition enthalphy of 6.5 kcal mol −1, agreeing closely with the enthalpies derived for structurally similar systems. The hydrocarbon chains are packed in a pseudo- or distorted hexagonal lattice in the solid and the gel phases. Data also indicate the existence of a hybrid domain in which lipid molecules coexist over a wide temperature range in the vicinity of the transition temperature, but revealed no evidence of interdigitation of hydrocarbon chains proposed earlier. Raman data also suggest that at least 30% of the hydrocarbon chain segments adopt a gauche conformation in the gel phase as well as in the anhydrous solid at room temperature. This indicates a considerable non-rigid environment for the sphingolipid hydrocarbon chains in the anhydrous and gel phases.