Equilibrium distribution of K+ ions in the hydrophilic spacer of tethered bilayer lipid membranes

Tethered bilayer lipid membranes (tBLMs) have been extensively employed to investigate the function of channel-forming peptides and proteins. They consist of a lipid bilayer tethered to the surface of an Au or Hg electrode through an oligopeptide or polyethyleneoxy hydrophilic spacer''. Upon incorporating an ion-selective channel in the tBLM, an equilibrium distribution of the permeating ion along the hydrophilic spacer is attained at each applied potential. An electrochemical model of tBLMs is developed to calculate the equilibrium concentration profile of the permeating ion along the hydrophilic spacer as a function of the applied potential. The limited spaciousness of the hydrophilic spacer is accounted for by imposing a maximum local volume concentration to the ion. The present approach is general. In this work, it is applied to a particular mercury-supported (tetraethyleneoxy spacer)vertical bar(phospholipid bilayer) tBLM bathed by an aqueous solution of 0.1 M KCl. Penetration of K+ ions into the hydrophilic spacer was realized by incorporating in this tBLM either the ion carrier valinomycin or the ion channel gramicidin. The equilibrium charge of the cation in the hydrophilic spacer at each applied potential was determined by jumping to the given potential from a fixed potential positive enough to exclude the presence of K+ ions in the spacer.