Thermal Stability of Biological Lipids I: Differential Scanning Calorimetry of Mixtures of Cholesteryl Myristate and Cholesteryl Palmitate

Differential scanning calorimetry was used to study phase transitions in binary mixtures of mesomorphic (liquid crystalline) saturated cholesteryl esters. The solid to mesomorphic phase transition temperatures for binary mixtures of cholesteryl myristate and cholesteryl palmitate are lower than the phase transition temperature for either pure compound. An apparent eutectic phase formation was found for all compositions of the two components. The mesomorphic phase consists initially of a smectic phase, which converts to a cholesteric phase as the temperature is increased. The cholesteric phase exists over a narrow temperature range, above which the system becomes an isotropic liquid. The phase transition temperatures of the smectic to cholesteric and the cholesteric to isotropic phase transitions are virtually linear functions of the composition of the mixtures. The ∆H values for each phase transition were determined. The inajor portion of the overall ∆H for the solid to isotropic phase transitions for these mixtures occurs at the solid to mesomorphic phase transition. The behavior of these mixtures is of interest since mesomorphic mixtures of these cholesteryl esters along with other cholesteryl esters are components of the lipid deposits of atheromatous lesions.