The crystal structure of cholesteryl iodide

The X-ray analysis of cholesteryl iodide is an example of the use that can be made of the presence of a heavy atom, here iodine, in working out the structure of an organic molecule containing a number of asymmetric centres. The method of analysis employed has been to calculate the electron density within the unit cell, using the observed F values and, at first, phase angles deriyed from the contributions of the iodine atoms alone. The electron density pattern so deduced necessarily has a centre of symmetry, since the positions of the two iodine atoms in the unit cell are related to one another by a centre of symmetry. It therefore shows, somewhat approximately, the positions of all the atoms in the molecule superimposed on those of a spurious mirror image molecule. Selection of the atomic positions belonging to one molecule was made through a consideration of normal carbon-carbon bond lengths and valency angles. The analysis was greatly assisted by the comparatively simple crystallo­ graphic character of the compound. Cholesteryl iodide crystallizes in two forms, A and JB, of closely related crystal structure, both monoclinic, P2V with n = 2. Fourier projections on (010) were derived for both forms and show clearly the outlines of the sterol molecules and their general arrangement. The resolution of the atomic positions is best in the case of B and three-dimensional analysis was therefore attempted on this form only. The crystal structure derived confirms the general view of sterol crystallography put forward by Bernal and also the present accepted chemical structure of the sterol skeleton. The accuracy with which the positions of the individual atoms are fixed is not great, but it appears sufficient to establish their mode of linking and to provide new evidence on their stereochemical relations.