A solid-state NMR study of phospholipid-cholesterol interactions: Sphingomyelin-cholesterol binary systems

We used solid-state NMR techniques to probe the interactions of cholesterol (Chol) with bovine brain sphingomyelin (SM) and for comparison of the interactions of Chol with dipalmitoylphosphatidylcholine (DPPC), which has a similar gel-to-liquid crystalline transition temperature. H-1-, P-31-, and C-13-MASNMR yielded high-resolution spectra from multilamellar dispersions of unlabeled brain SM and Chol for analysis of chemical shifts and linewidths. In addition, H-2-NMR spectra of oriented lipid membranes with specific deuterium labels gave information about membrane ordering and mobility. Chol disrupted the gel-phase of pure SM and increased acyl chain ordering in the liquid crystalline phase. As inferred from C-13 chemical shifts, the boundaries between the ordered and disordered liquid crystalline phases (L-alpha(o) and L-alpha(d)) were similar for SM and DPPC. The solubility limit of Chol in SM was similar to50 mol %, the same value as previously reported for DPPC membranes. We found no evidence for specific H-bonding between Chol and the amide group of SM. The order parameters of a probe molecule, d31-sn1-DPPC, in SM were slightly higher than in DPPC for all carbons except the terminal groups at 30 mol % but were not significantly different at 5 and 60 mol % Chol. These studies show a general similarity with some subtle differences in the way Chol interacts with DPPC and SM. In the environment of a typical biomembrane, the higher proportion of saturated fatty acyl chains in SM compared to other phospholipids may be the most significant factor influencing interactions with Chol.