Spectral Study and Molecular Modeling of the Inclusion Complexes of β-Cyclodextrin with Some Phenoxathiin Derivatives

The inclusion complexes of 3-formyl- (1), 3-acetyl- (2), and 3-carboxyphenoxathiin (3) were investigated by steady-state fluorescence spectroscopy. In the presence of β-cyclodextrin (β-CD), the fluorescence spectra of 1 and 2 point out an enhancement of the emission and a hypsochromic shift of the maxima. In the case of the carboxy derivative the measurements were conducted in buffer solutions, pH = 2.11 and pH = 9.18, to ensure the presence of either the neutral (3a) or dissociated carboxylate (3b) form, as a single predominant species. A different behavior was observed for these two species:  en enhancement of the emission for the neutral form and a quenching for 3b. For all the compounds the best fit to the experimental data was obtained for 1:1 stoichiometry and points out association constants in the range 5000−7000 M-1, for 1, 2, and 3b and 1770 M-1 for 3a. Molecular modeling of the complexes was performed by both molecular mechanics and quantum semiempirical methods. The ligands were introduced in the host cavity, either with the unsubstituted phenyl ring (model A) or with the substituted moiety (model B). For the neutral compounds, about 70% of the binding energy, was due to the van der Waals contribution whereas for 3b the electrostatic interaction was the dominant term. The distance between the center of the cavity and the mass center of the four carbon atoms in the median heteroring of the ligand, points out a deep penetration of the ligands. The flexibility of the sulfur-containing ring could be a positive factor for a better fit of the ligand into the cavity accounting for the large values of the association constants. The calculated heat of formation of the complexes are in the range usually found for the inclusion complexes excepting 3b, for which unexpected large values were predicted. The calculations did not allow us to distinguish between the two possible ways the ligands approach the cavity, the results for model A and model B being similar.