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

Abstract

Gramicidin D was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer, and its behavior was investigated in aqueous 0.1 M KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy. The impedance spectra, recorded from 0.1 to 1 × 10 5 Hz over a potential range of 0.7 V, were fitted to a series of RC meshes, which were related to the different substructural elements of the biomimetic membrane. These impedance spectra were compared with those obtained by incorporating valinomycin, under otherwise identical conditions. The potential dependence of the stationary currents reported on bilayer lipid membranes by Bamberg and Läuger (Bamberg, E.; Läuger, P. J. Membrane Biol. 1973, 11, 177-194) as well as those extracted from potential-step chronocoulometric measurements was interpreted by relating the increase in gramicidin dimerization to a progressive increase in single-file K + flux along the dimeric channels. An analogous approach was adopted in explaining the difference between the impedance spectra obtained with gramicidin D and those obtained with valinomycin. It is concluded that gramicidin has a low tendency to form dimers in the absence of ionic flux.

4 Electrochemistry of Biomimetic Membranes

“…One RC mesh simulates the electrode, regarded as a combination of the Si space-charge region, the SiO 2 oxide and the thin water layer interposed between the semiconductor surface and the lipid bilayer; the other RC mesh simulates the lipid bilayer. The resistance and capacitance of the electrode amount to about 19 M cm 2 and 2.2 F cm -2 , respectively, those of the lipid bilayer to 0.98 M cm 2 and 0.75 F cm -2 . Values of the two capacitances are close, but the resistance of the electrode is much greater than that of the lipid bilayer, due to the 1.3-1.4 nm thick silicon oxide.…”

“…However, the corresponding specific resistances are much less than those attained with identical tBLMs anchored to macroscopic gold electrodes of areas of the order of 110 -2 cm 2 . In fact, the specific resistance of the latter tBLMs amounts to about 15 M cm 2 . If a 100100 m 2 micro-tBLM had such a specific resistance, it should have a resistance of 150 G.…”

“…This tBLM incorporates gramicidin, a linear neutral pentadecapeptide that spans lipid bilayers by forming a Nterminus-to-N-terminus dimer. 2 The elements of the equivalent circuit are influenced by the movement of K + ions across the lipid bilayer, induced by the gramicidin channel. The spectrum is displayed on a Bode plot, namely a plot of log|Z| and phase angle  against log f, where |Z| is the magnitude of the impedance.…”

“…Plot of log|Z| (solid circles) and  (solid triangles) against log f (Bode plot) for a mercury-supported DPTL|DPhyPC bilayer incorporating gramicidin from its 110 -7 M solution in aqueous 0.1 M KCl at -0.600 V vs. Ag|AgCl(0.1M KCl). 2 The solid curve is the best fit of the equivalent circuit shown in the figure to the impedance spectrum, with Cs=0.25 F, Rm=106 k, Cm=14 nF, and R=146 . Drop area = 2.210 -2 cm 2 .…”

“…The impedance spectrum of the tBLM was fitted by a parallel combination of a resistance and a constant phase element (instead of a capacitance), in series with the electrolyte resistance, R  . The lipid bilayer had a resistance of about 17 M cm 2 and a capacitance of 10 F cm -2 . The resistance is similar to that of gold-supported tBLMs, but the capacitance is one order of magnitude higher.…”