Phase Diagrams of Systems Containing Cholesterol, Cholesteryl Esters, and Triglycerides.

Abstract

Binary and ternary systems of cholesterol, the 3 cholesteryl esters, linoleate, oleate and stearate, and the 2 triglycerides, triolein and tristearin were studied in order to determine the phase transitions and the conditions for the cholesteric and smectic mesophages. Phase transitions were determined using differential thermal analysis, melting point determination and polarizing microscopy. The cholesterol-cholesteryl ester systems studied are of the eutectic type with limited solid solubility. The mesophases, cholesteric and smectic, are monotropic as to the crystalline state and exist up to .apprx. 75 wt % cholesterol. Ternary systems of cholesterol and 2 cholesteryl esters show the same general features as the binary systems. The melting point of cholesterol is depressed by increasing amounts of triglycerides down to an eutectic point at high concentrations of the triglycerides. The solubility of anhydrous cholesterol and cholesterol monohydrate in triolein was found to be the same. In mixed systems with cholesterol, triglycerides and cholesteryl esters even low concentrations of the triglycerides removed the cholesteric mesophase typical for cholesteryl ester systems. At higher concentrations of triolein the smectic mesophase was also removed. In systems with cholesterol, a cholesteryl ester and tristearin, an apparently smectic mesophase with mosaic texture, was exhibited. Cholesterol, cholesteryl esters and triglycerides belong to the major components of atherosclerotic deposits and serum lipoproteins. From this standpoint, the phase conditions of these lipids help to understand the physicochemical mechanism of the lipid deposition in atherosclerotic arteries.