Evaluation of Thermal Transitions in Some Cholesteryl Esters of Saturated Aliphatic Acids

Abstract

The heats and temperatures of transition for fourteen cholesteryl esters of saturated aliphatic acids have been evaluated using a Perkin-Elmer Differential Scanning Calorimeter, DSC-IB. Most esters tested were first recrystallized from n-pentyl alchol. Among several recrystallization solvents tested, n-pentyl alchol was most effective in removing impurities and eliminating ester transitions caused by these impurities. Ester purity was determined by a recently reported technique which is based on an analysis of the complete DSC recorder scan for transitions. The transition data in this study indicate that the lowest molecular weight saturated aliphatic ester of cholesterol to show mesophase behavior is the propionate. For the formate or acetate less pure samples may be supercooled in the melt so that mesophases may be observed. New transition data for the odd esters of cholesterol from undecanoate to nonadecanoate have also been obtained. Cholesteryl eicosanoate was examined and found to have only one transition, both on heating and cooling, which may indicate that the highest members of this series do not display mesophase behavior in the pure state. A small but definite odd-even effect in the sum of the transition entropies for each compound has been established for cholesteryl esters from nonanoate through nonadecanoate. Similarly, there is a small but definite odd-even effect for the temperatures of the crystal-mesophase transition for esters from tridecanoate through nonadecanoate.