Crystal and molecular structure of isomorphous cholesteryl chloride and cholesteryl bromide

Abstract

Cholesteryl chloride and the isomorphous cholesteryl bromide crystallize in the monoclinic space group P21 with two molecules in the unit cell. The cell dimensions are a=10.82, b=7.61, c=21.46 ± 0.02 Å, b.beta=131.° ± 1° for the chloride and a=11.10. b=7.56, c=21.83 b.delta 0.02 Å, b.beta=133.9 ± 1° for the bromide. The crystal structure has been solved by the heavy atom method and refined by least-squares procedure using visually estimated photographic data. The terminal group of atoms C(25), C(26) and C(27) are disordered. The rings A and C in the molecule assume slightly distorted chair conformations. Ring B has a 8b.beta, 9agr halfchair conformation and ring D has a 13b.beta, 14agr halfchair conformation. The ring junctions B-C and C-D are trans whereas the junction A-B is quasi trans. The chain at C(17) is in a fully extended configuration.

The crystal packing is smectic-like, with the molecules arranged in distinct layers. The neighboring molecules in a layer are oriented antiparallel to each other. The unit cells of the cholesteryl halides are two molecules thick, one molecule wide and one molecule long. The direction of thickness makes an angle of 35° with the a-axis. The width is at 8° to the b-axis and the length is at 5° to the c-axis. The modified Hodgkin notation is Ma35, b8°C8°C5° 211.