High-sensitivity scanning calorimetric study of mixtures of cholesterol with dimyristoyl- and dipalmitoylphosphatidylcholines

A highly sensitive and stable scanning microcalorimeter is employed in a reinvestigation of the effect of cholesterol on multilamellar suspensions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). Below 20 mol % cholesterol the DPPC mixtures give heat-capacity curves each of which can be resolved into a narrow and a broad peak, suggesting the coexistence of two immiscible solid phases; above 20 mol % only the broad peak is observed and this disappears at about 50 mol %. The DMPC mixtures show a more complicated behavior; from about 13.5 to 20 mol % cholesterol the observed curves appear to be the sum of three component peaks. As with the DPPC mixtures, only a single broad peak is observed above 20 mol % cholesterol, and this broad peak becomes undetectable above about 50 mol %. These results are discussed.