Molecular modelling and 1H-NMR: Ultimate tools for the investigation of tolbutamide: β-cyclodextrin and tolbutamide: Hydroxypropyl-β-cyclodextrin complexes

Abstract

A structural study of the inclusion compound of tolbutamide (TBM) with β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) was attempted by means of 1H-nuclear magnetic resonance (1H-NMR) experiments and computer molecular modelling. To establish the stoichiometry and stability constant of the β-CD:TBM complex, the continuous variation method was used. The presence of true inclusion complexes between TBM and β-CD or HP-β-CD in solution was clearly evidenced by the 1H-NMR technique. Changes in chemical shifts of H-3 and H-5 protons, located inside the CD cavity, associated with variations in the chemical shifts of TBM aromatic protons provided clear evidence of inclusion complexation, suggesting that the phenyl moiety of the drug molecule was included in the hydrophobic cavity of CDs. This view was further supported by the observation of intermolecular NOEs between TBM and β-CD and by the aid of a molecular modelling program, which established the most probable structure of the complex. The molecular graphic computation confirmed that the minimum energy, positioning TBM relative to β-CD, occurs when the aromatic ring of TBM is included within the β-CD cavity by its wider side, leaving the aliphatic chain externally, which is in good agreement with the results of 1H-NMR studies.