Thermal behavior of synthetic sphingomyelin-cholesterol dispersions

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Abstract
The thermotropic behavior of aqueous dispersions of palmitoylsphingomyelin-cholesterol and lignoceryl-sphingomyelin-cholesterol mixtures was examined by high sensitivity differential scanning calorimetry. When less than 25 mol% cholesterol is mixed with either sphingomyelin, the calorimetric endotherm is composed of a sharp and a broad component. The sharp-component enthalpy change decreases as the mol percent cholesterol increases, with the extrapolated zero enthalpy point being 25-30 mol%. With palmitoylsphingomyelin, the temperature of maximum heat capacity of the sharp component decreases monotonically with increasing cholesterol content, while the lignocerylsphingomyelin sharp-component maximum remains constant until more than 20 mol% sterol is present. The broad-component enthalpy change maximizes at 3-4 kcal/mol between 10-20 mol% cholesterol and decreases as the ratio of cholesterol is increased or decreased from this range for both sphingomyelins. The results are compared with those from a previous study on dipalmitoylphosphatidylcholine-cholesterol mixtures and are interpreted as evidence for the coexistence of cholesterol-rich and cholesterol-poor phases.