Submitted to: Journal of Chemical Crystallography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2007
Publication Date: 9/25/2007
Citation: Dowd, M.K., Stevens, E. 2007. Crystal and molecular structure of 6,6´-dimethoxy-gossypol:acetic acid (1:1). Journal of Chemical Crystallography. 37:765-770.
Interpretive Summary: Low temperature x-ray diffraction was used to study the structure of a new crystalline derivative of gossypol, dimethoxy-gossypol. Like gossypol, a natural product of the cotton plant, this compound also appears to form complexes with low molecular weight organic molecules. For this compound, the gossypol derivative and acetic acid are in a 1:1 molar ratio and the packing of the molecules is similar to the complex formed between gossypol and acetic acid. The results will be of interest to scientists working to develop products based on gossypol unusual ability to form inclusion complexes with small molecular weight molecules
Technical Abstract: By crystallization from dilute solutions of acetic acid (2-4%) in diethyl ether, acetone, or methyl ethyl ketone, 6,6´-dimethoxy-gossypol forms an inclusion complex with acetic acid in a one-to-one molar ratio. The compound crystallizes in the triclinic P1bar1¯space group and has unit cell dimensions of alpha = 7.5793(10) A, b = 14.7211(19) alpha and c = 14.740(2) Å, alpha = 106.260(3)°, beta = 102.310(3)°, gamma = 95.975(3)°, Z = 2. The structure was solved by direct methods and refined to an R1 value of 0.0394 on 4252 observed reflections. Enantiomeric pairs of dimethoxy-gossypol molecules form centrosymmetic dimers that are characterized by a pair of intermolecular hydrogen bonds and by hydrophobic stacking interactions between a pair of naphthalene rings. The acetic acid molecule accepts a hydrogen bond from a gossypol hydroxyl group and donates into a hydrogen bond with an aldehyde group of an adjacent gossypol molecule. Although there is less hydrogen bonding in this structure than in the gossypol:acetic acid (1:1) structure, the molecular packing of the gossypol compounds is similar. Comparison of the HPLC chromatographic peak areas formed by reaction of the single crystal of 6,6´-dimethoxy-gossypol with R-(-)-2-amino-1-propanol indicates that the previously reported isolation of the compound from cotton roots results in a racemic preparation. By analogy with the results for gossypol and 6,6´-dimethoxy-gossypol, the preparation of 6-methoxy-gossypol from cotton roots also appears to form a racemate.