Valinomycin-mediated transport of alkali cations through solid supported membranes

Abstract

Highly flexible lipid bilayers were immobilized on gold surfaces via self-assembly of thiolipids on gold in order to obtain an appropriate matrix for the carrier valinomycin. The bilayers were created by chemisorption of thiolipids to form the first hydrophobic monolayer on the gold substrate and subsequent fusion of unilamellar 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine vesicles resulted in the second physisorbed monolayer. The synthesis of the thiolipid is based on the functionalization of phosphatidylethanolamine with a linker consisting of succinic acid hooked on a thiolated tetraethyleneglycol serving as the hydrophilic linker responsible for the flexibility of the monolayer and the anchor group. The solid supported membranes were characterized by X-ray photoelectron spectroscopy and impedance spectroscopy. The latter technique revealed that the bilayers form a considerable barrier against ions in solution. The capacitance and resistivity of the prepared bilayers amount to C m=1.0±0.2 μF/cm 2 and R m=11 000±1000 Ω cm 2 in 10 mM Tris, 50 mM N(CH 3) 4Cl, pH 7.0. Ion transport of sodium and potassium ions through the bilayers in the absence and presence of valinomycin was investigated by impedance spectroscopy in the frequency range of 10 −1–10 6 s −1. Valinomycin was dissolved in dimethyl sulfoxide and added to the bilayer. Data evaluation was performed using a modified model established by de Levie for carrier-mediated ion transport through free-standing lipid bilayers making use of the continuum equation. The membrane resistance showed the expected linear relation to the reciprocal of the valinomycin concentration in solution. The conductivity of the membrane in the presence of valinomycin corrected for the conductivity in the absence of the carrier vs. the concentration of alkali ions in solution showed a tenfold larger slope for potassium than for sodium ions.