On the nature of dilute aqueous cholesterol suspensions.

Cholesterol is commonly supposed to form a micelle in aqueous solution. We have reassessed this finding using a variety of techniques. Centrifugation sedimented cholesterol at all concentrations above 10(-8) M. Density gradient analysis of more concentrated solutions revealed two bands whose densities corresponded to crystalline anhydrous cholesterol and crystalline cholesterol monohydrate. The monohydrate was characterized by light microscopy and phase transition. This band is also claimed to contain micelles, but no enhancement of perylene or diphenylhexatriene fluorescence could be detected. Some crystals of monohydrate could pass through 0.7-microns filters but not through 1.5-nm filters. Crystals of monohydrate were detected on the latter filters when solutions with cholesterol concentrations as low as 2.6 X 10(-8) M were filtered. Thus, under all conditions where micelles might be expected, we have detected micro-crystals of cholesterol monohydrate and we can find no independent evidence to support the existence of a cholesterol micelle.