Crystal structure of cholesteryl butanoate at 123 K

Cholesteryl butanoate has a complex crystal structure that differs from those of the three main structure type for cholesteryl esters. It contains four molecules (C31H52O2) unrelated by crystal symmetry. The molecules are packed in almost planar sheets and have molecular long axes nearly parallel. However, the molecules have different orientations about their long axes and furthermore, in a given sheet, one of the independent molecules is antiparallel to the other three. Viewed down the molecular long axes, each molecule has six nearest neighbors, but the detailed environment is different for the four independent molecules. Thus the molecular arrangement has features that are characteristic of the short-range order present in the cholesteric mesophase. The monotropic transformation from the crystalline to the cholesteric phase occurs at 98 degrees C. The crystal structure has been accurately determined using 12,146 independent X-ray reflections having sin theta/lambda < 0.63 A-1. All hydrogen atoms were located from a difference Fourier and were included in a refinement that gave R(F2) = 0.064. The C-C bond lengths have sigma = 0.003 A and C-C-C bond angles have sigma = 0.2 degrees. Conformations for the steroid ring system are similar but there are differences in the C17 side chains and the butanoate chains of the four independent molecules. Analysis of atomic m.s. displacement tensors using a segmented-body model indicates that there are internal librations involving both the C17 and butanoate chains in all molecules.