Transfer of protonated anesthetics across the water o-nitrophenyl octyl ether interface: Effect of the ion structure on the transfer kinetics and pharmacological activity
Abstract
Voltammetric and impedance measurements were used to study the transfer of protonated forms of local anesthetics (procaine, tetracaine, cocaine, prilocaine, bupivacaine, lidocaine, dibucaine) between water (W) and o-nitrophenyl octyl ether (o-NPOE). The ion structure was shown to have little effect on the ion transport properties in the bulk aqueous phase, as characterized by the diffusion coefficient, D-w=(7-10) x 10(-6) cm(2) s(-1), as well as in the bulk o-NPOE phase. as characterized by the diffusion coefficient D-0=(5-6) x 10(-7) cm(2) s(-1): and at the Wl o-NPOE interface, as characterized by the apparent standard rate constant, k(0)(s)=0.013 cm s(-1).
While ion transport in the bulk of a phase is influenced mainly by the ion size and medium viscosity (Stokes law), the rate of interfacial ion transfer comprises the contributions of the ion distribution in the electrical double layer and of the activated transfer step, which compensate largely for each other. The pharmacological activity of local anesthetics correlates with the standard potential differences of their protonated forms at the Wlo-NPOE interface.
This activity can be related to the ion partition between the body fluid and the membrane, or to the rate of membrane ion transfer at a constant potential difference.