Structure of cycloheptaamylose inclusion-complexes: crystal structure of substituted benzoic acid and phenol derivatives

Cycloheptaamylose has been crystallized with 2,5-diiodobenzoic acid as guest. The X-ray crystal structure at 1.2-Å resolution with space group C2 and cell dimensions a  19.192 (13), b  24.759 (20), c  15.739 (13) Å, and β  109.6 (3)° was solved by using rotation-translation functions. Complexes of other meta-substituted guests were found to be isomorphous, and were solved by using the phases of the cycloamylose of the 2,5-diiodobenzoic acid complex. The complex with 2-bromo-5-tert-butylphenol having a  19.235 (11), b  24.662 (17), c  16.018 (11) Å, and β  108.9 (2)° was determined at 1.0 Å resolution, and the complexes with m-bromobenzoic acid, m-iodobenzoic acid, m-iodophenol, m-toluic acid, and 2-bromo-4-tert-butylphenol were determined at 2.0-Å resolution. In all cases, the guest molecule was disordered. However, by using information from all the structures, it may be concluded that the functionally important carboxylic acid group lies in the primary-hydroxyl end of the cycloheptaamylose molecule. As studies in solution have shown that the hydrogen-bonding groups of guest molecules interact with the secondary-hydroxyl end of the cycloheptaamylose molecule, it is concluded that the structure seen in the crystals here does not correspond to a catalytically active species. Cyclo-heptaamylose exists as a dimer in the crystal by means of extensive hydrogen bonding across the secondary-hydroxyl ends of two cycloheptaamylose molecules. A continuous channel throughout the crystal is achieved by the stacking of these dimer units.