Electrochemical Analysis on the Biomimetic Coupling Reaction Between Ion Transport and Electron Transport through an Artificial Liquid Membrane

Abstract

Introduction In the energy-transducing membrane as mitochondria, some ion transports are coupled with electron transports. In the biological membrane, the ion transport region and the electron transport region are located separately on the membrane. Under this condition, in order to maintain the electroneutrality inside and outside the membrane, same amounts of ions and electrons should be transported through a membrane under the established membrane potential. The coupling of the ion transport and the electron transport in different regions of the membrane would be different from that in same region of the membrane such as a liquid membrane where ions are distributed with electrons across the aqueous/membrane interface to keep electroneutrality in an aqueous phase and membrane phase [1]. In the present study, an artificial membrane system with separated charge transport sites is constructed, and novel coupling mechanism between ion transport and electron transport is demonstrated. Experimental Two liquid membrane systems, W1/O1/W2 and W1’/O2/W2, were connected through a common phase of W2. The former is the ion transport system and the latter is the electron transport system. The organic solvent used in O1 and O2 was nitrobenzene. W1 and O1 contained the transported ion (K+ or tetraethyl ammonium ion, TEA+) as hydrophilic and hydrophobic salts, respectively. W1’ and W2 contained [Fe(CN)6]4- and [Fe(CN)6]3-, respectively. O2 contained 7,7,8,8-tetracyanoquinodimethane, TCNQ, and TCNQ·-. When W1 and W1’ were connected through Ag/AgCl electrodes, K+ or TEA+, was transported from W1 to W2, and an electron was transported from W1’ to W2. That is, the electric current accompanied with the coupling between the ion transport and the electron transport flowed through an outer circuit from W1’ to W1. The electric current was measured by using a galvanometer, and simultaneously the membrane potential between W1 and W2 and that between W1’ and W2 were recorded using electrometers. Furthermore, the voltammogram for ion transport and that for electron transport were recorded independently in W1/O1/W2 system and W1’/O2/W2 system. Results and Discussion The membrane potential for the coupling reaction was predicted from the viewpoint of the conservation of electroneutrality in all phases of W1, W1’, O1, O2 and W2, which was determined as a potential where positive current for ion transport is equal to negative current for electron transport in comparison of voltammograms recorded in W1/O1/W2 and W1’/O2/W2 systems. When two liquid membrane systems were connected, the membrane potentials of W1-W2 and W1’-W2 approached each other after connecting W1 and W1’, and the electric current decreased gradually. The finally attained membrane potential agreed with the potential expected from the voltammogram for ion transport and that for electron transport. Further, when the concentration of [Fe(CN)6]3- in W2 changed from 1 x 10-4 mol dm-3 to 5 x 10-4 mol dm-3, the membrane potential attained in the coupling reaction shifted positively and the current decreased, which agreed with the prediction obtained from voltammograms. These results indicate that the coupling reaction in the separated ion transfer site and electron transfer site proceeds under the identical membrane potential in both charge transport region to keep electroneutrality in all phases. In addition, the possibility of the violation of electroneutrality is discussed in the transient period immediately after connecting W1 and W1’. Especially, the difference in capacitance component between two charge transport systems influenced the disturbance of balance in Faradaic component. [1] H. Ohde, K. Maeda, O. Shirai, Y. Yoshida and S. Kihara, J. Electroanal. Chem., 438, 139-145 (1997).