Crystal structure of the cyclomaltohexaose (α-cyclodextrin, α-CD) inclusion complex with p-aminobenzoic acid (pABA) has been determined by X-ray diffraction. The host:guest stoichiometry is 1:1. The pABA molecule is included in the cavity with its axis coincident with the axis of α-CD; the benzoic group is inserted in the cavity, while the amino group sticks out from the cavity. Four water molecules are located near the cavity rims and in interstices between the molecules of α-CD participating in a dense network of intermolecular hydrogen bonds. UV-visible spectroscopy was applied to estimate the stability constant (K ) at different temperatures on the basis of the Benesi-Hildebrand equation. This allowed calculation of complexation ΔH° and ΔS° on the basis of the Van't Hoff equation. The results are in good agreement with the values obtained by other methods in the literature. Phase-solubility profiles indicate that the solubility of pABA is significantly increased in the presence of α-CD at different pH values, and it was classified as AL-type, indicating a 1:1 stoichiometric inclusion complex in solution. A theoretical investigation has also been carried out on the α-CD-pABA systems in order to search for other stable complexes. PM6 semi-empirical calculations were made to investigate equilibrium geometries of inclusion complexes formed between α-CD and neutral, anionic, cationic and zwitterionic forms of pABA. Two possible orientations were considered (A, with the carboxylic end inside the cavity and B, with the amino group inside the cavity). Preference between A and B orientations of each α-CD-pABA form results from different H-bond interaction patterns.
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