Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 223

actions between the alkyl chains of the thiolipid and those of the
lipid, this simple procedure gives rise to a lipid bilayer anchored to
the mercury surface via the hydrophilic spacer moiety of the thio-
lipid. By avoiding the use of vesicles, this procedure excludes any
artifacts due to partially fused vesicles. These advantageous fea-
tures make the incorporation of membrane proteins in mercury-
supported thiolipid-based tBLMs easier and safer than in solid-
supported tBLMs.
A tBLM obtained by self-assembling a DPhyPC monolayer on
top of a DPTL-coated mercury electrode was investigated by im-
pedance spectroscopy over a frequency range from 1u10 -2 to 1u10 5
Hz and over a potential range of 0.8 V, both in the absence and in
the presence of the ion carrier valinomycin. 3 The impedance spec-
tra obtained upon incorporating valinomycin in this tBLM from
aqueous 0.1 M KCl are particularly rich of characteristic features,
especially when reported in a Z Z ' versus Z Z ” plot (the M plot; cf.
Section III). This plot shows four partially fused semicircles (see
Fig. 5 ), which can be straightforwardly fitted by an equivalent cir-
cuit consisting of a series of four RC meshes. Each semicircle cor-
responds to a RC mesh; the diameter of the semicircle measures
the reciprocal, 1/ C , of the capacitance of the RC mesh, while the
angular frequency Z at the maximum of the semicircle measures
the reciprocal of its time constant, W= RC . The angular frequency
increases in the direction of increasing Z Z ”. The R and C values
relative to the four RC meshes vary appreciably with the applied
potential. To this end, a generic approximate approach was devel-
oped, which applies the concepts of impedance spectroscopy to a
model of the electrified interface and to the kinetics of potassium
ion transport assisted by valinomycin across the tBLM. 3 This per-
mits the four RC meshes to be ascribed to four different slabs
composing the tBLM. Proceeding in the positive direction of the
abscissas, the first semicircle has the highest resistance and was
ascribed to the lipoic acid residue, in direct contact with the elec-
trode surface. The second semicircle has a capacitance of about 7
PF cm -2 , close to that of a monolayer of tetraethyleneoxythiol self-
assembled on mercury, and was ascribed to the TEO moiety. The
third semicircle in the absence of valinomycin has a capacitance
close to 1 PF cm -2 , a value typical of a conventional solvent-free
BLM, and was reasonably ascribed to the lipid bilayer moiety.