Molecular interaction of β-CD with 3-carboxy-1-[(2-phenyl-1,3-thiazol-4-yl) methyl]pyridin-1-ium iodide analyzed by isothermal titration calorimetry and NMR spectroscopy

Assessment of interaction between beta-cyclodextrin and 3-carboxy-1-[(2-phenyl-1,3-thiazol-4-yl) methyl]pyridin-1-ium iodide (3CPTMPI) in aqueous solution were investigated by isothermal titration calorimetry (ITC) and 1D and 2D H-1 NMR spectroscopy at 298 K. Thermodynamic analysis using ITC revealed that the association constant of beta-cyclodextrin and 3CPTMPI is 441.6 M-1 with favorable enthalpy and entropy changes. These thermodynamic parameters indicate that the binding is dominated by hydrophobic interactions, which is in agreement with inclusion complex formation. The details of beta-CD/3CPTMPI molecular interaction was analyzed by H-1 2D NMR allowing the proposition of an inclusion model for 3CPTMPI into beta-CD. Rotating frame NOE spectroscopy (ROESY) was used to certain the solution geometry host-guest complex. The results reveal that the 3CPTMPI molecule penetrates into beta-CD cavity with both aromatic and thiazol rings. For this type of inclusion complex, the association constant K obtained by H-1 NMR and ITC are in good agreement and both methods sustain a 1:1 stoichiometry.