Journal of Lipid Research, Vol 28, 993-1005, Copyright © 1987 by Lipid Research, Inc.
Cholesteryl esters of saturated fatty acids: cosolubility and fractionation of binary mixtures
DL Dorset
Electron Diffraction Department, Medical Foundation of Buffalo, Inc., NY 14203.
Factors affecting the solid state miscibility of saturated chain
cholesteryl esters were determined from electron diffraction and
differential scanning calorimetric measurements on a homologous series
which included two types of crystal packing. Electron diffraction patterns
from solution- and epitaxially crystallized microcrystals gave measured
unit cell constants consistent with the bilayer crystal form for myristate,
pentadecanoate, palmitate, and stearate esters. Cholesteryl undecanoate
crystallized as the monolayer I structure and cholesteryl laurate was
polymorphic, packing in either monolayer I or bilayer forms. No evidence
was found for the monolayer II form of the laurate claimed in earlier work.
It is clear that solid solution formation follows general rules formulated
earlier by Kitaigorodskii for molecular crystals. A symmetry criterion must
be satisfied first of all, i.e., two compounds that solidify in greatly
different crystal structures will not form continuous solid solutions
(e.g., cholesteryl undecanoate/cholesteryl myristate). Within a given
crystal structure type, solid solution is permitted when the molecular
volumes are similar. (For example, cholesteryl myristate forms an ideal
solid solution with cholesteryl pentadecanoate, a nonideal solution with
cholesteryl palmitate, and a eutectic of solid solutions with cholesteryl
stearate.) For the polymorphic cholesteryl laurate, solid solutions of
either the monolayer I structure (e.g., with cholesteryl undecanoate) or
bilayer structure (e.g., with cholesteryl myristate) are permitted.
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