The acid/base properties of procaine, dibucaine, and dyclonine in aqueous solution studied by Fourier transform infrared spectrometry

Abstract

The primary effect of local anesthetic agents in producing a nerve conduction block is to decrease the permeability of the nerve membrane to sodium ions. According to the specific receptor hypothesis, the most favored theory today, the anesthetic must first diffuse through the nerve sheath and then bind to a receptor site located in the nerve membrane. It is well known that the pK or dissociation constant of a specific local anesthetic is directly related to its nerve blocking action. Fourier transform infrared spectrometry was used to analyze the characteristics of procaine, dibucaine, and dyclonine, all local anesthetics. A pK study was performed using aqueous and mixed solvent solutions of varying hydrophobicity to investigate the solvent effects on the pK of these local anesthetic agents. By comparing and contrasting the infrared spectra of these anesthetic agents in aqueous, 25%, 50% and 75% acetonitrile solutions, the effects of varying solvents and pH could be studied. Procaine, dibucaine, and dyclonine were analyzed in an approximate concentration of 0.04 M at a pH range of 2-12. Common absorption bands were observed as the solvents and pH were changed. From their absorption spectra, the pK values of procaine, dibucaine and dyclonine in water were experimentally determined using two different methods of calculation. These experimental values, listed in Table 3, compare favorably with reported literature values for aqueous solutions. Addition of acetonitrile to prepare aqueous solutions containing 25, 50 and 75 volume percent acetonitrile caused the pK values to vary, neither steadily increasing nor steadily decreasing but remaining in the range of 8 to 9.