A thermodynamic analysis of the partitioning of cholesterol and related compounds between trioleoylglycerol and egg phosphatidylcholine bilayers.

The free energy of transfer of a number of alcohols, including cholesterol, from a bulk isotropic lipid phase, trioleoylglycerol (TG), to an anisotropic lipid phase, egg phosphatidylcholine (PC), was determined. n-Alkane-1-ols partitioned preferentially into the bilayer phase; for example, the free energy of transfer of octanol-1 from TG to PC was about -1.0 kcal/mol. This preference declined with increasing number of carbons at a rate of 40 cal/mol of CH2. Cholesterol had a much stronger preference for the bilayer with a free energy of -1.3 kcal/mol, compared to an extrapolated value of -0.2 kcal/mol for a normal alkane-1-ol with the same number of carbon atoms. Thus, the excess free energy of -1.1 kcal/mol represents the favourable interaction of the cholesterol skeleton with the bilayer phase. This conclusion was confirmed by comparing cholesterol 3-hemisuccinate to oleic acid. Substituting TG for water as the standard state has eliminated the large hydrophobic effect and has permitted us to identify for the first time the subtle binding increment of the steroid ring system.