Interfacial Effects Dominate Ion Permeation through Membrane Channels in Low Ionic Strength Solutions

Abstract

A number of protein channels and solid-state nanopores share the importance of electrostatic interactions between the permeating ions and the nanochannel. Ion transport at the nanoscale occurs under confinement conditions so that interfacial effects such as access resistance (AR) become relevant. Here we show that interpreting correctly electrophysiological measurements in terms of channel ion selective properties requires the consideration of interfacial effects, up to the point that they dominate protein channel conductance in diluted solutions. We measure AR in a large ion channel, the bacterial porin OmpF, by means of single-channel conductance measurements in salt solutions with varying concentrations of high molecular weight PEG, sterically excluded from the pore. Comparison of experiments done in charged and neutral planar bilayers shows that lipid surface charges modify the ion distribution and determine the value of AR, indicating that lipid molecules are more than passive scaffolds even in the case of large transmembrane proteins. We also found that AR might reach up to 80% of the total channel conductance in diluted solutions, where current recordings reflect essentially the AR of the system and depend marginally on the pore ion concentrations and selective properties. These findings are of key importance for several low aspect ratio biological channels that perform their physiological function in a low ionic strength and macromolecule crowded environment, just the two conditions enhancing the AR contribution.