Journal of Lipid Research, Vol 30, 893-898, Copyright © 1989 by Lipid Research, Inc.
Crystal structure of form II of cholesteryl palmitelaidate at 295 K
RC Srivastava and BM Craven
Department of Crystallography, University of Pittsburgh, PA 15260.
Form II for cholesteryl palmitelaidate (trans-9-hexadecenoate) (C43H74O2)
is monoclinic P2(1) with a = 12.745(3), b = 9.006(2), c = 18.153(4) A, beta
= 96.63 (2) degrees, Z = 2. The X-ray crystal structure of form II has been
determined from 2506 reflections of which 2126 gave (F greater than 2
sigma). The data up to sin theta/lambda = 0.44A-1 (Dmin = 1.14 A) were
measured with CuK alpha radiation from a sealed tube. These were
supplemented up to sin theta/lambda = 0.52 A-1 (Dmin = 0.96 A) by
measurements on the same crystal using a rotating anode X-ray source. The
electron density was diffuse in the ester chain and the atoms of the
cholesteryl tail were found to be disordered. The tail and the chain atoms
were refined by restrained least squares methods to give R = 0.087 and Rw =
0.10 for reflections with F greater than 2 sigma. Crystal forms I and II
represent two standard structure types already characterized for fatty acid
esters of cholesterol. In form II, the ester chain is almost fully extended
as is also the case for one of the two independent molecules (A) in form I.
In form II, the chains pack loosely together for most of their length. M.s.
amplitudes of thermal vibration for the chain C-atoms are almost uniform
along the entire chain (approximately 0.25 A2 at 295 K). In form I, the
proximal part of the A chain is surrounded by rigid cholesteryl groups. In
this region, C-atom m.s. amplitudes are much reduced (approximately 0.10
A2) but they increase to about 0.5 A2 at the distal end of the chain where
packing is very loose.
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