Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 224

Finally, the last semicircle has the same resistance as the electro-
lyte solution and a very low capacitance, of the order of 1 nF cm -2 ,
and was ascribed to the electrolyte solution adjacent to the tBLM.
The same tBLM was used to incorporate the peptide melittin,
whose D-helical monomers aggregate in the lipid bilayer to form
an ion channel. 190 The impedance spectrum was again fitted to an
equivalent circuit consisting of four RC meshes, and the capaci-
tance C m and resistance R m of the mesh ascribed to the lipid bilayer
moiety were examined as a function of the applied potential and of
the lipid composition of the bilayer. The bilayer conductance,
1/ C m , was found to increase abruptly above the background level
at a sufficiently negative applied potential, attaining a maximum
value, in agreement with the voltage-gated nature of the melittin
channel. In fact, it is known that the melittin molecules lie flat on
the surface of biomembranes and penetrate them only when the
transmembrane potential on the opposite side of the biomembrane
becomes sufficiently negative. The conductance increases with the
composition of the distal monolayer of the tBLM in the order
DOPC:cholesterol (60:40) < DOPC < DOPC:sphingomyelin:cho-
lesterol (59:15:26) < DOPS. When using the ternary mixture,
which may form lipid rafts, a fifth RC mesh of high capacitance
and low resistance is clearly visible in the M plot; it disappears at
the same applied potential at which the conductance increases ab-
ruptly. This behavior was tentatively ascribed to the formation of a
melittin layer on top of the lipid rafts of the distal mixed lipid
monolayer: at the applied potential at which the melittin molecules
penetrate the lipid bilayer, this melittin layer is disrupted. The
sigmoidal charge vs. time curves following a potential step from a
value at which melittin channels are not formed to one at which
they are formed was interpreted on the basis of a generic kinetic
model. 191 This model accounts for the potential-independent dis-
ruption of melittin clusters adsorbed flat on the lipid bilayer, in-
duced by the potential-dependent penetration of the resulting mon-
omers into the lipid bilayer; the potential-independent aggregation
of the monomers inside the bilayer with channel formation is then
treated on the basis of a mechanism of nucleation and growth. This
explains the initial induction period responsible for the sigmoidal
shape of charge vs. time curves.
The channel-forming protein OmpF porin from Escherichia
coli was incorporated in a mercury-supported DPTL/DPhyPC bi-