Thermodynamic Comparison of the Interactions of Cholesterol with Unsaturated Phospholipid and Sphingomyelins

A comparative analysis of the interaction of cholesterol (Chol) with palmitoyl-oleoyl-phosphatidylcholine (POPC) and sphingomyelins (SM) was performed in largely homogeneous, fluid-phase membranes at 50°C. To this end, three independent assays for isothermal titration calorimetry were applied to POPC/SM/Chol mixtures. Cholesterol is solubilized by randomly methylated-β-cyclodextrin and the uptake of Chol into (or release from) large unilamellar vesicles is measured. The affinity of Chol to a POPC/SM (1:1) membrane with 30 mol % Chol is approximately two times higher than to POPC alone; extrapolation to pure SM yields an affinity ratio of RK ∼ 5. Bringing Chol in contact with SM is highly exothermic (−7 kJ/mol for POPC/SM (1:1), and −13 kJ/mol extrapolated to pure SM, both compared to POPC). No pronounced differences were observed between egg, bovine brain, and palmitoyl SM. With decreasing Chol content, RK increases and ΔH becomes more exothermic, suggesting a trend toward superlattice formation. That SM/Chol-interactions are enthalpically favorable implies that the preference of Chol for SM increases upon cooling and can induce domain formation below a certain temperature. The enthalpy gain is partially compensated by a loss in entropy in accordance with the concept of Chol-induced chain ordering, which improves intermolecular interactions (van der Waals, H-bond) but reduces conformational and motional freedom. The ability of cyclodextrin to extract sphingomyelin from membranes is twofold-weaker than for POPC.