Fluorimetric and molecular mechanics study of the inclusion complex of 2-quinoxalinyl-phenoxathiin with β-cyclodextrin

The complex formed by the inclusion of the polarity-sensitive fluorescent probe 2-[2′-quinoxalinyl]-phenoxathiin (QP) into β-cyclodextrin (β-CD) was investigated by steady-state fluorescence spectroscopy in order to confirm the previously stated intramolecular charge transfer nature of the first excited singlet state of QP. A decrease in the emission intensity in the presence of β-CD was observed and explained on this basis. The 1:1 stoichiometry of the inclusion complex and its association constant of 2,223 M⁻¹ were computed. The QP–β-CD complex was further studied by molecular mechanics (MM+ force field), in order to determine its structure and the type of interactions between QP and β-CD. All possible ways QP could penetrate the β-CD cavity were considered and several structures were generated and optimized. The interaction, binding (van der Waals and electrostatic contributions) and perturbation energies were also calculated. The results have showed that the β-CD cavity incorporates the central part of QP and that complexation is mainly due to van der Waals host–guest interactions.