Ionophore properties of valinomycin in the model bilayer lipid membrane 1. Selectivity towards a cation

The electrochemical impedance spectroscopy (EIS) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS) techniques were employed to study the ionophore properties of valinomycin in a model floating bilayer lipid membrane (fBLM) in perchlorate supporting electrolytes with potassium and sodium cations. Valinomycin decreases the membrane resistance of the fBLM in KClO4 solution by about 180 times, while it has a negligible effect on the membrane resistivity in NaClO4 solution. The IR spectra indicate that valinomycin forms a complex with K+, but not with Na+. The valinomycin-K+ complex adopts a small tilt angle with respect to the electrode surface normal and is well interdigitated between the acyl chains of the bilayer. The EIS and PM-IRRAS results indicate that valinomycin forms complexes with K+ and transports K+ across the lipid bilayer. This transport is potential independent and hence has a passive character.