The thermotropic behavior of aqueous dispersions of stearoylsphingomyelin-cholesterol mixtures was examined by high-sensitivity differential scanning calorimetry. When less than 20 mol % cholesterol was mixed with the sphingomyelin and the samples were held at room temperature for 7-9 days before the initiation of calorimetric measurements, a sharp endotherm at 56-57 °C and a broad endotherm at 35-50 °C were observed. In addition, samples containing 15-20 mol % stearol exhibited a sharp endotherm at 43-45°C. If samples were held at room temperature for less than 2 h before the initiation of calorimetric analysis, the 56-57 °C endotherm was usually not seen. Instead, a combination of broad and sharp endotherms over the range of 35-50 °C was observed. Occasionally, exotherms were also observed within this temperature range. These results, along with those from previous studies, imply that a cholesterol-rich phase coexists with a cholesterol-poor phase in which the sphingomyelin molecules may exist in two distinctly different gel states.
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