The molecular framework of steroids was firmly established from the successful X-ray analysis of cholesteryl iodide crystals over fifty years ago. At the time, this was a triumph for the rapidly developing technique of X-ray crystallography. We describe here new analyses of cholesteryl iodide using crystals of forms I and II(A) which are at least 50 years old. Comparison of these two structures reveals subtle differences in the geometry of the steroid skeleton, the attached side chain and the covalently bound iodine. The crystals of both forms occur in the same monoclinic space group, P21, with similar but nevertheless different unit cells, particularly in the b-axis length. As a consequence, there are subtle differences in the packing mechanisms of the two polymorphs. The two forms were originally classified by Bernal and Crowfoot, on the basis of their studies of more than 60 crystals of sterols and sterol derivatives, as “normal, type A - the common form” (corresponding to form II) and “reverse or type B” (corresponding to form I), respectively. Those crystals of form I which have survived for almost 55 years are poor X-ray diffractors at room temperature and have been analysed at liquid N temperature, diffracting well under these conditions. Form II crystals diffract well at room temperature and for this reason the crystals were not cooled. The main differences in geometry occur in the side-chain which is disordered in the room temperature form II crystals, whilst the individual ring conformations in the steroid skeleton are similar, ring A being a strained chair, ring B a 7α/8β half-chair, ring C a symmetrical chair and D a 13β/14α half-chair.
(A) Originally denoted Forms B and A respectively. See Additional Note (A) for explanation.
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Notes
Both choices of unit cell would be considered unsuitable in contemporary analyses in view of the extremely obtuse β angle of 149°, selected at the time presumably on the basis of crystal morphology. It is also worth noting that the c axes of these elongated cells are approximately parallel to the lengths of the steroid molecules in both original crystal structures,12 and b is approximately perpendicular to the steroid plane (see Figure 3a and b) in both structures. Transformation using the matrix [1 0 0; 0 −1 0; 2 0 1] gives the unit cell: a=10.93, b=10.34, c=11.59 Å, β=107.4° for form A [corresponding to form II (Table 1)]; and a=12.57, b=9.04, c=12.95 Å, β=119.5° for form B [corresponding to form I (Table 1)]. The agreement between the cells is highly suggestive of the proposed correspondence (form A = form II, form B = form I). The length of the unique b axis is of course invariant with these transformations.
Whilst the space group is preserved in this case, it is not in fact unusual for a transformation of space group to occur on cooling to this extent.
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Crystallographic data (excluding structure factors) for the structures reported here have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication numbers CCDC 210483 & 210484. Copies of available material can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (email teched@chemcrys.cam.ac.uk).
Dedicated to the pioneering work on the structures of steroids undertaken by D. M. Crowfoot (Dorothy Hodgkin), J. D. Bernal and C.H. Carlisle in the 1930’s and 40’s.
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Palmer, R.A., Palmer, H.T., Frampton, C.S. et al. X-ray structures of fifty year old crystals of cholesterol iodide forms I and II. J Chem Crystallogr 37, 1–14 (2007). https://doi.org/10.1007/s10870-006-9140-2
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DOI: https://doi.org/10.1007/s10870-006-9140-2