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

The structure of the complex of beta -cyclodextrin (cyclomaltoheptaose) wit h beta -naphthyloxyacetic acid was studied in solid state by X-ray diffract ion and in aqueous solution by H-1 NMR spectroscopy. The complex crystalliz es in the channel mode, space group C2, with a stoichiometry of 2:1; two be ta -cyclodextrin molecules related by a twofold crystal axis form dimers, i n the cavity of which one guest molecule is found on average. The above sto ichiometry indicates one guest per beta -CD dimer statistically oriented ov er two positions or two guest molecules in pi-pi intel actions in half of t he 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 t he occupancy of the two disordered primary hydroxyls. These two hydroxyl gr oups, to which the carboxylic oxygen atoms of the guest are hydrogen bonded , point towards the interior of the beta -CD cavity. In aqueous solution, t he H-1 NMR spectroscopic study indicated that there is a mixture of complex es with host-guest stoichiometries both 1:1 and 2:1. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.