Journal of Lipid Research, Vol 35, 584-591, Copyright © 1994 by Lipid Research, Inc.
Structural determination and packing analysis of a cholesteryl caprate/cholesteryl laurate solid solution
MP McCourt, P Strong, W Pangborn and DL Dorset
Electron Diffraction Department, Medical Foundation of Buffalo, Inc., NY 14203.
This paper describes the X-ray crystal structure analysis of a cholesteryl
ester solid solution, cholesteryl decanoate/cholesteryl laurate, grown from
a bulk concentration with molar ratio 0.56/0.43. The unit cell is
monoclinic with a = 12.969, b = 9.048, c = 31.137 A, and beta = 91.12
degrees and the space group P2(1) with Z = 4 (two molecules per asymmetric
unit). The cell constants closely represent an average value of crystal
parameters for the two pure components (hence, nearly corresponding to
Vegard's law). Although the overall monolayer 1 lamellar packing is
superficially similar to the earlier-studied cholesteryl
undecanoate/cholesteryl laurate solid solution, a more partitioned
distribution of acyl chains, i.e., a microfractionation corresponding to
the observed nonideal phase behavior, is suggested. The behavior is similar
to that found for n-paraffin binaries cooled below a binodal phase
boundary. Although it cannot be detected conclusively in this determination
(due to high thermal motion of terminal acyl chain atoms), the
non-stoichiometric combination of components also requires some partial
occupancy of atomic sites on the chain termini. This structural arrangement
is contrasted with the alternative expression found earlier for the more
ideal undecanoate/laurate solid solution, i.e., random co-packing leading
to fractional atomic occupancy in an average laurate structure. The final
weighted R factor for 4578 reflections is 0.138.
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