A Cholesterol Isopropanol Gel
In the course of measuring solubilities of cholesterol in pure and mixed solvents we have observed the formation of a transparent gel in the cholesterol+isopropanol system. Because of the historical importance of liquid crystals of cholesteryl esters1 and the biological importance of cholesterol, we are reporting here some observations on the properties of this gel. Liquid crystals of cholesterol with fatty alkanols (C12-C18) are known2,3, but have not been reported for smaller alcohols. The transparent gel of cholesterol and isopropanol seems to be stable at 4 °C for concentrations of 4.6-5.3% (w/w) cholesterol. At higher concentrations and temperatures (up to 10.2% at 22 °C), the transparent gel can be observed for a day or more, then white 'snowflakes' appear throughout the gel and grow slowly into a translucent or opaque mass. On warming, the transparent gels liquify, and the translucent gels seem to go directly to the liquid state. The transparent gel is unstable to vigorous shaking; samples with less than 5.5% cholesterol liquify then return to the transparent gel state on standing at 4 °C. Samples with larger concentrations form the translucent or opaque state on shaking and do not return to the transparent state over a period of several weeks. Observations of the transparent gel under a polarising microscope at room temperature showed the sample to be mostly isotropic with some small points of optical activity.
W. E. Acree and G. L. Bertrand, "A Cholesterol Isopropanol Gel," Nature, Natural Publishing Group, Jan 1977.
The definitive version is available at http://dx.doi.org/10.1038/269450a0
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© 1977 Natural Publishing Group, All rights reserved.