Proparacaine complexation with β-cyclodextrin and p-sulfonic acid calix[6]arene, as evaluated by varied 1 H-NMR approaches

Abstract

This study focused on the use of NMR techniques as a tool for the investigation of complex formation between proparacaine and cyclodextrins (CDs) or p-sulfonic acid calix[6]arene. The pH dependence of the complexation of proparacaine with β-CD and p-sulfonic acid calix[6]arene was studied and binding constants were determined by 1H NMR spectroscopy [diffusion-ordered spectroscopy (DOSY)] for the charged and uncharged forms of the local anesthetic in β-CD and p-sulfonic acid calix[6]arene. The stoichiometries of the complexes was determined and rotating frame Overhauser enhancement spectroscopy (ROESY) 1D experiments revealed details of the molecular insertion of proparacaine into the β-CD and p-sulfonic acid calix[6]arene cavities. The results unambiguously demonstrate that pH is an important factor for the development of supramolecular architectures based on β-CD and p-sulfonic acid calix[6]arene as the host molecules. Such host–guest complexes were investigated in view of their potential use as new therapeutic formulations, designed to increase the bioavailability and/or to decrease the systemic toxicity of proparacaine in anesthesia procedures.