Potassium Ion Transport by Valinomycin across a Hg-Supported Lipid Bilayer

Abstract

A biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer was investigated in aqueous KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy, both in the absence and in the presence of the ionophore valinomycin. Impedance spectra, recorded from 1 x 10(-2) to 1 x 10(5) Hz over a potential range of 0.8 V, are satisfactorily fitted to a series of four RC meshes, which are straightforwardly related to the different substructural elements of the biomimetic membrane. The frequency-independent resistances and conductances of both the lipid bilayer and the hydrophilic spacer show a maximum when plotted against the applied potential. This behavior is interpreted on the basis of a general approximate approach that applies the concepts of impedance spectroscopy to a model of the electrified interphase and to the kinetics of potassium ion transport assisted by valinomycin across the lipid bilayer.

Gramicidin Conducting Dimers in Lipid Bilayers Are Stabilized by Single-File Ionic Flux along Them

“…This behavior is similar to that observed with an identical tBLM incorporating the ion carrier valinomycin, under otherwise identical conditions. 28 By following the gradual evolution of semicircles 1 and 2 by varying the applied potential E, the corresponding values of the capacity and conductance were determined as a function of E.…”

“…For comparison, in Figure 7, the g j 1 versus E plot for gramicidin is reported together with the corresponding plot for valinomycin, as obtained in ref 28 by incorporating valinomycin from its 1.5 × 10 -7 M solution in aqueous 0.1M KCl in an identical tBLM. In analogy with the behavior of a tBLM incorporating valinomycin, 28 the large semicircle 3 in Figure 4, which is independent of the applied potential, is due to the aqueous solution adjacent to the tBLM; it will not be considered further.…”

“…This points to a low percentage of gramicidin conducting dimers at these potentials. A detailed analysis of the impedance spectra of tBLMs incorporating gramicidin from aqueous 0.1 M KCl was carried out by using an approach analogous to that adopted for the incorporation of valinomycin, 28 OmpF porin, 29 and melittin 30 in the same tBLM. According to this approach, the whole electrified interface is regarded as consisting of four substructures with different dielectric properties, namely, the lipoic acid residue, the TEO hydrophilic spacer, the lipid bilayer moiety, and the aqueous solution adjacent to the tBLM.…”

“…The elements of this equivalent circuit represent ideal lumpedconstant properties, even though the K + ions are distributed in space across the tBLM. This general approach 28 assumes that the charges within the tBLM are located as shown in Figure 1: a free electronic charge density q on the surface of the mercury electrode, a charge density FΓ 1 at the boundary between the lipoic acid residue and the TEO moiety, and a charge density FΓ 2 at the boundary between the TEO moiety and the lipid bilayer moiety. Here, Γ 1 and Γ 2 are surface concentrations of K + .…”

4 Electrochemistry of Biomimetic Membranes

“…A Bode plot richer in features is obtained by incorporating in the same tBLM the ion carrier valinomycin, a hydrophobic depsipeptide that cages a desolvated potassium ion shuttling it across the lipid bilayer. 3 In this case, the  vs. log f plot exhibits an additional hump, as shown in Fig. 3.…”