Crystal and molecular structure of an enantiomeric gossypol-acetic acid clathrate

Single crystals of gossypol with three molecules of acetic acid (gossypol triacetic acid) were grown from solutions of gossypol acetic acid and acetone. The crystals were unstable in air but could be stabilized for X-ray diffraction analysis by coating the crystal surfaces with a thin layer of mineral oil. The gossypol triacetic acid complex (C₃₀H₃₀O₈·3C₂H₄O₂) forms an orthorhombic crystal system with P2₁2₁2₁ (Z=4) symmetry. Unit cell dimensions were a=9.0208(7) Å, b=17.4884(10) Å, and c=24.358(2), Å yielding a volume of 3842.7(5) ų and a density of 1.2077(2) g/cm³. As with all previously reported crystals of gossypol, the gossypol molecules were of the aldehyde tautomer, and the two planar naphthalene rings were approximately perpendicular. Acetic acid molecules were found to lie in channels within the gossypol matrix. Individual crystals contained only one gossypol enantiomer, but both enantiomers crystallized from solution. Although the crystal habit could not be used to distinguish between the gossypol enantiomers, a fragment of the crystal could be derivatized and analyzed by high-performance liquid chromatography for this purpose. The ability to grow large, nonracemic crystals leads to a simple procedure for separating small quantities of the individual gossypol enantiomers.