Ion Fluxes to Channel Arrays in Monolayers. Computing the Variable Permeability from Currents

Abstract

Chronoamperometric currents arising from Tl + reduction at a Hg electrode covered with a phospholipid monolayer containing gramicidin monomer ion channels can be quantitatively understood as an interfacial permeation process with a variable local permeability. Any interfacial behavior (described by the permeability) can be combined with the properties of semi-infinite diffusion to yield a simple expression that allows: (a) the computation of the current corresponding to any given variation of the permeability with time and (b) the recovery of the permeability through a semi-integration of the currents. As a result, the diffusion process is accounted for, and one can focus on the obtained permeability, whose variation can, in a further step, be ascribed to phenomena such as a decaying number of active channels, a change in their translocation efficiency, etc. Analysis of the experimental data for the Tl + permeation through gramicidin channels with the derived expression confirms the validity (as a good first approximation within the range of concentrations explored) of considering a simple first-order relaxation process for the gramicidin channel conversion between the conducting and the nonconducting forms coupled with firstorder heterogeneous kinetics for the channel crossing.

Phase phenomena in supported lipid films under varying electric potential

“…This way we found that with 3 tt in the range 1.4,.,2.0 k B T the monolayer is thermodynamically stable, where the observed zero-tension phase gradually transforms from the dense liquid (for lower values of 3 tt ) into gel-and liquid-crystal phases (for higher values). To exemplify, the average projected area per lipid is varying in the range, a xy ¼ 21,., 15 A 2 , which is reasonable for single-tail lipids 29,54 considering that the effective monomer size of 4…”

“…2,13,14,34 There exist also plausible theoretical (phenomenological) approaches to the problem. 10,[15][16][17] It remains, however, largely undetermined what type of transition is reflected by each peak in the C(V ) profile, 34 because such phenomenological hypotheses based on indirect observations cannot be easily verified in a supporting experiment that could reveal the resulting molecular (re-)arrangements. A better understanding and theoretical description in such cases requires further research, which can also be based on modelling with the aid of numerical methods.…”

“…Such experiments are based on measuring conductivity, charge capacity and ion-permeability of the adsorbed material. The aim usually is either (i) studying the interfacial reactions and structural transformations in response to varying electric field, 3,[10][11][12][13][14][15][16][17] or (ii) using the electrochemical phenomena in molecular sensing devices, e.g. bio-and taste sensors.…”

Characterization of ion transport in thin films using electrochemical impedance spectroscopyI. Principles and theory

“…A number of electrochemical methods used to study diffusion may be applied to characterize transport in 0376 such films [15][16][17]. This approach has been used for studying diffusion in solids [17], Nafion [18], electropolymerized thin films [19,20] and in self-assembled lipid bilayers [21] and surfactant monolayers [22]. The present publication generalizes the approaches proposed earlier and adds some aspects important for relatively permeable and not entirely homogeneous thin films, such as the ones employed in polyamide RO/NF composites.…”

“…Although EIS experiments are run in a quasisteady-state mode whereby oscillations of voltage and current become reproducible, in fact it is an inherently non-steady-state technique that allows simultaneous analysis of many charge transfer phenomena on different time scales. The method gained much popularity in studies of coatings and corrosion on metals [23][24][25], self-assembled layers [21,22,26], polymer degradation [27], analytical chemistry [28], etc. A whole separate branch of EIS is studies of ion channels in biological and synthetic lipid membranes [21,29,30].…”