2H-NMR Study of Model Membranes: Investigating the Effect of Sterol Structure on the Chain Ordering and Phase Behavior of Lipid Mixtures

We employed deuterium nuclear magnetic resonance spectroscopy (2H-NMR) to investigate the effect of sterol structure on lipid membrane organization. Cholesterol is the major sterol component of mammalian cell plasma membranes. It strongly affects the properties of phospholipid membranes. For example, incorporating cholesterol in liquid crystalline membranes increases lipid acyl chain order, and induces the liquid ordered phase which is considered to have biological importance. We first measured the chain ordering in pure bilayers of 1-cholesterylhemisuccinoyl-2-palmitoyl(d31)-sn-glycero-3-phosphocholine (CholPC), a sterol-modified phospholipid with a cholesterol moiety covalently attached to the phospholipid glycerol backbone in place of one of the lipid acyl chains. We then compared CholPC’s chain ordering with that of 1-palmitoyl-2-palmitoyl-d31-sn-glycero-3-phosphocholine (DPPC-d31)/cholesterol and found that constrainded cholesterol’s ability to order adjacent acyl chains is greatly reduced. Several sterols, broadly similar in structure to cholesterol but with specific chemical modifications, are prevalent in plant or fungal cell plasma membranes. We used 2H-NMR to study the influence of sterol structure on its effectiveness in modifying the acyl chain order of a 1-palmitoyl(d31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) membrane. Spectra of POPC-d31 multilamellar vesicles containing campesterol, β-sitosterol, brassicasterol or stigmasterol were taken at 25oC for sterol concentrations up to 45 mol% and compared to previous observations obtained using cholesterol, 7-dehydrocholesterol (7-DHC) or ergosterol. Among the sterol structural modifications we compared, the C22 double bond reduced the sterol’s ordering ability the most, followed by a C24 ethyl or methyl substituent. Finally we used 2H-NMR to study the effect of sterol structure on the propensity of sterols to induce phase separation in equimolar DPPC/POPC/sterol membranes containing 7-DHC, brassicasterol or stigmasterol. The results were compared to previous observations obtained for membranes containing cholesterol or ergosterol, which highlighted the significance of sterol structure on phase separation promoting properties. Such comparative studies are prerequisites to establishing the underlying principles of sterol/phospholipid interactions.