Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 225

layer, and the resulting impedance spectrum in aqueous KCl was
fitted by an equivalent circuit consisting of four RC meshes. 183 The
dependence of the resulting circuit elements upon the applied po-
tential was interpreted on the basis of an approximate approach
based on a model of the electrified interface and on the kinetics of
the translocation of potassium and chloride ions across the lipid
bilayer. Incorporation of OmpF porin increased the conductivity of
the lipid bilayer moiety of the tBLM over a narrow potential range
straddling the zero value of the potential difference across the lipid
bilayer moiety.
Gramicidin is a peculiar channel-forming peptide whose heli-
cal structure differs from the D-helix of common peptides and
membrane proteins by the fact that it has a lumen large enough to
allow the passage of simple desolvated monovalent cations. Since
its length is about one half that of a biomembrane, it spans the
membrane by forming a dimeric channel. The C-terminuses of the
two gramicidin monomers composing a dimer contain three tryp-
tophan residues that form hydrogen bonds with the polar heads of
the lipid bilayer. Consequently, the corresponding N-terminuses
are directed toward each other, in the middle of the lipid bilayer,
just as the dipole moments of the two monomeric units. A trans-
membrane potential different from zero is, therefore, expected to
favor electrostatically one monomeric orientation at the expense of
the other, thus destabilizing the dimer. Nonetheless, the stationary
current due to the flow of potassium ions along gramicidin chan-
nels incorporated in a conventional BLM increases with an in-
crease in the transmembrane potential, exhibiting an almost quad-
ratic dependence. 192 This behavior was confirmed by incorporating
gramicidin in a mercury-supported DPTL/DPhyPC bilayer and by
carrying out a series of potential steps from a potential positive
enough to repel potassium ions from the TEO spacer to progres-
sively more negative potentials and by recording the resulting
charge vs. potential curves. 2 The slope of the initial portion of the-
se curves measures the stationary current due to the ionic flux.
Plotting this current against the transmembrane potential yields a
curve in good agreement with that obtained with conventional
BLMs. To explain this behavior, it was assumed that the rate con-
stant for dimer formation increases in parallel with an increase in
the ionic flux. In fact, when the time elapsed between the passage
of two consecutive cations through the junction between the two