Structure of the complex of β-cyclodextrin with β-naphthyloxyacetic acid in the solid state and in aqueous solution

The structure of the complex of beta -cyclodextrin (cyclomaltoheptaose) with beta -naphthyloxyacetic acid was studied in solid state by X-ray diffraction and in aqueous solution by H-1 NMR spectroscopy. The complex crystallizes in the channel mode, space group C2, with a stoichiometry of 2:1; two beta -cyclodextrin molecules related by a twofold crystal axis form dimers, in the cavity of which one guest molecule is found on average. The above stoichiometry indicates one guest per beta -CD dimer statistically oriented over two positions or two guest molecules in pi-pi intel actions in half of the beta -CD dimers and the rest of the beta -CD dimers empty. In addition, occupancy of 0.5 for the guest per every beta -CD dimer is in accord with the occupancy of the two disordered primary hydroxyls. These two hydroxyl groups, to which the carboxylic oxygen atoms of the guest are hydrogen bonded, point towards the interior of the beta -CD cavity. In aqueous solution, the H-1 NMR spectroscopic study indicated that there is a mixture of complexes with host-guest stoichiometries both 1:1 and 2:1. (C) 2001 Elsevier Science Ltd. All rights reserved.