Interfacial Effects of Ion Channels in Lipid Membranes: Mean-Field Computation from 3D Atomic Structures Versus Analytical Estimates

Abstract

Recent experimental studies have stressed the importance of interfacial effects in pores of nanometer dimensions, particularly in protein channels with low aspect ratio. On the one hand, the well known concept of Access Resistance (AR) or Converge Resistance is essential in the description of ionic transport across these biological channels. In fact, AR may become a dominant contribution to the total channel resistance, as demonstrated experimentally for a bacterial porin, under low ion concentration or macromolecule crowding in the surrounding solutions, two conditions that are often met in the cell environment. On the other hand, charged polar head groups of the lipid membrane may have a strong influence on the electric potential and the ionic concentration in the vicinity of the channel-solution interface. Charged residues within the protein located near the pore mouth can also play a role, although to a lesser extent than AR and membrane surface charges. These three factors are obviously coupled and are also strongly dependent on the channel aperture size, 3D structure and channel-lipid assembling. We perform mean-field calculations based on the channel atomic structure and compare them with estimations obtained from analytical expressions and AR experimental measurements. Our results indicate that the inverse relationship between AR and both bulk conductivity and pore radius should be modified by additional channel-dependent effects. Our study tests the limits of the available description of ion transport at the nanoscale and provides tools for the characterization of protein channel function in physiologically relevant confined environments.