Abstract:
Ion channels, such as gramicidin A, selectively facilitate the transport of ions across biological and
synthetic membranes. The conductance properties of ion channels are frequently characterized in synthetic
bilayer lipid membranes (BLMs). The instability of BLMs has seriously limited the range of applications
for these structures, and tethered bilayer lipid membranes (tBLMs) have addressed the problem through
tethering many of the membrane components to a solid surface. In the present study, thin gold substrates
have been used to tether thiol- and disulfide-terminated membrane components to form a tBLM electrode
to provide a reservoir for ions. This study reports on the ion selectivity and apparent permeability of
gramicidin channels in such tethered bilayer membranes. The investigations using electrical impedance
spectroscopy indicated that the magnitude of ionic conductance varies substantially in reservoirs with
different chemical structures. This study addressed the effect of changing ionic concentration, the effect
of changing the species in the bulk solution above the membrane, and the influence of the chemical structure
of the reservoir tethers. The effect of two-dimensional packing on membrane conductance was also
investigated. The present observations suggested that (a) the reservoir region resistivity has a major
influence on the overall conductivity of the membrane and in some instances can dominate conduction,
(b) the conduction behavior is nonlinear and exhibits saturation with increasing electrolyte concentration,
and (c) that ion pairing in the reduced dielectric ( 50) reservoir region is the likely basis for the latter
effect. The inferred limiting ionic mobilities of alkali chloride species in the membrane reservoir regions
were 3-4 orders of magnitude less than in aqueous solution, indicating that the reservoirs resembled
hydrated polymer gels.
