Environment of solubilized molecules in bile salt micelles

Abstract

Bile salt micelles, and mixed bile salt-phospholipid micelles, solubilize cholesterol and dietary lipids. As a first step in a study of the mechanism of solubilization by bile salt micelles, we have studied the solubilization of two fluorescence probe molecules and an electron spin resonance probe molecule. One fluorescence probe, 1-methylanthracene, is free to rotate in the molecular plane when solubilized. The other probe molecules, N-phenylnaphthalen-1-amine and 4-(11-carboxy-1-hexylundec-1-yl)- 2,2,6,6-tetramethylpiperidin-1-yloxy, are greatly restricted in mobility when solubilized. The 1-methylanthracene molecule is planar, and is small enough to fit into the hydrophobic region of a bile salt micelle without protruding. The other probe molecules used do not fit these criteria. We suggest that small planar molecules in general may be free to rotate (about an axis perpendicular to the plane) in bile salt micelles. Thus solubilization of planar molecules would be entropically favoured. In particular, cholesterol is a planar molecule known to be highly solubilized by bile salt micelles. The cholesterol molecule meets the above criteria, and thus we suggest that its solubilization is entropically favoured over non-planar molecules.