Interaction between dibucaine and pig erythrocyte membranes as studied by NOESY experiments in 1H-NMR spectroscopy. Which form of dibucaine interacts more strongly, cationic or uncharged?

The interaction between amine local anesthetic dibucaine and pig erythrocyte membranes has been studied by 1H-NMR spectroscopy. Two-dimensional NOESY spectra were observed to obtain the conformations of cationic and uncharged forms of dibucaine. The NMR spectra were measured at pH 7.4, and the temperature was raised (318-348 K) to increase the concentration of the uncharged form of dibucaine, taking the temperature dependence of the pKa value of dibucaine into consideration. The dibucaine in a buffered solution showed the presence of two kinds of distinctly different species; one is assignable to the cationic form and the other to the uncharged form of dibucaine, suggesting that the protonation equilibrium between the two forms is slow in the presently employed experimental condition. The uncharged dibucaine showed well-defined NOE cross-peaks in the NOESY spectra of the solution containing no erythrocyte membranes, suggesting that its conformation is relatively fixed. Interestingly, however, it was only the cationic dibucaine that showed NOE cross-peaks when the solution contained the membranes, and experiments were performed at a much shorter mixing time for the buildup of NOEs, suggesting that it appeared only the cationic form of dibucaine is interacting with the membranes. It was concluded that the uncharged form of dibucaine, which was produced by raising the temperature, formed micelles in a buffered solution. Thus formed micelles didn't interact with membranes owing to the repulsive forces between the structured water surrounding the micelles and those at the surface of the membranes. This conclusion could be a promising reason why the cationic local anesthetics are much more active than their uncharged counterparts in blocking nerve conduction.