Ion transport across a bilayer lipid membrane facilitated by gramicidin A – Effect of counter anions on the cation transport

The facilitated ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) containing gramicidin A (GA) as an ionophore was investigated by cyclic voltammetry. Cyclic voltammograms for the ion transport across the BLM were symmetrical about the origin (0V, 0A) and the magnitude of the ion transfer current increased with an increase in the absolute value of the applied potential. The magnitude of the ion transfer current at a definite potential in the voltammograms depended on the hydrophobicity of not only cation species but also anion species, and the ion transfer current was proportional to the squared value of the GA concentration in the BLM. Taking into account the conjugated ion transfers at the W1∣BLM and BLM∣W2 interfaces and the distribution of ion pairs, the positive current that flowed from W1 to W2 across the BLM was attributable to both the transfer of the complex-forming cation from W1 to W2 and the transfer of the anion, which was distributed in the BLM as a counter ion, from W2 to W1. The negative current was then attributed to the opposite reaction.