Free-standing lipidic mesophase film hosting gramicidin A -Membrane ion transport study

Abstract

The free-standing lipid mesophase film within an H-cell is proposed as a suitable device for electrochemical impedance spectroscopy and voltammetric or chronoamperometry studies of ion transport through a lipidic membrane. Unlike the commonly used electrodes covered with lipid films, it has an aqueous solution on both sides of the membrane and, therefore, can serve as a more realistic biological membrane model. This approach was used to investigate the incorporation of ion channel peptides, exemplified by the potassium channel gramicidin A, into the liquid-crystalline cubic phase membrane. The lipidic mesophase samples with and without gramicidin peptide were characterized using SAXS and electrochemical methods. The improvement of conductivity of the lipidic cubic structure in the presence of gramicidin was ascribed to gramicidin A molecules spanning the lipidic walls to connect the aqueous channels of the liquid-crystalline structure and enhance the transport of ions through the membrane. The voltammetric and EIS experiments also revealed a clear inhibitory effect of Ca²⁺ as a divalent cation on gramicidin, which is known to be a specific monovalent-ion channel. Based on the results of this study focused on gramicidin reconstituted in the flexible lipid matrix, it is suggested as a suitable lipid platform for studying other ion-transport or catalytic membrane proteins and peptides, as well as their activators and inhibitors.