Conformation and packing of unsaturated chains in cholesteryl linolelaidate at 123 K.

At 123 K, crystals of cholesteryl trans-9-trans-12-octadecadienoate (cholesteryl linolelaidate, C47H76O2) are monoclinic, space group P2(1) with cell dimensions a = 13.03(3), b = 8.76(2), c = 17.90(4) A, beta = 89.7(2) degrees, having two molecules per unit cell. The crystal structure has been determined from 2041 X-ray intensities with sin theta/lambda less than 0.48 A-1, of which 922 gave I greater than 2 sigma(I). The hydrogen atoms were found in a difference Fourier synthesis. Block diagonal least squares refinement assuming isotropic thermal parameters has converged with Rw = 0.13. The molecule is fully extended (length 43.3 A), except for a symmetric bowing in the linolelaidate chain segment which contains the two unconjugated trans ethylenic bonds. The torsion angles at the four C--C bonds adjacent to the C=C bonds are all in the preferred (+/-)-skew range. Chain packing is efficient, without having a regular subcell structure. There is a similarity with the overall conformation of the oleate chains in crystals of cholesteryl oleate. Although chemically disparate, the oleate and linolelaidate chains have similar crystal environments.