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

Department of Biophysical Chemistry, Biocenter of the University of Basel, Basel, Switzerland

Correspondence: Address reprint requests to H. Heerklotz, Tel.: 41-61-267-2180; E-mail: heerklotz{at}gmx.net.

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-ß-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 R_K 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, R_K increases and H 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.

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