Crystal structure of anhydrous cholesterol

Abstract

CHOLESTEROL, the most abundant steroid in the animal kingdom, is found mainly as a component in cell membranes and lipoproteins. The presence of the hydrophilic C(3) hydroxyl group in this otherwise hydrophobic molecule allows cholesterol to occupy a position at polar–nonpolar interfaces. Crystals of 3-hydroxy steroids and their hydrates show a tendency towards double layer structures with an end-for-end arrangement of approximately parallel molecules¹. As this molecular organisation is generally similar to that in membranes, it can be expected that some of the principles of intermolecular association in membranes might be revealed in the molecular packing in the crystal structures. Another common feature in these structures is the presence of more than one molecule in the crystallographic asymmetric unit. While this is a complicating factor in structure analysis, it clearly increases the amount of information regarding preferred conformation and intermolecular association to be gained from a structure determination. Craven² has recently reported the remarkable crystal structure of one form of cholesterol monohydrate. The arrangement of the eight crystallographically independent molecules in the triclinic cell shows unusual local pseudo-symmetry. One pair of molecules is related to another by a non-crystallographic b/2 translation, while a third pair, oriented end-for-end relative to the first, is related to the fourth by a similar a/2 translation. Non-crystallographic twofold rotation symmetry relates members of the first and the third pairs. The overall structure is a stacking of bilayers of 33.9 Å total thickness. We report here that anhydrous cholesterol crystals are also triclinic, with eight independent molecules per cell. This structure offers additional insight into the packing of cholesterol molecules, and a comparison with the monohydrate structure.