Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 234

pare favorably with those of conventional BLMs. Ion-channel ac-
tivity of gramicidin and melittin incorporated in such a tBLM and
ion-selectivity of gramicidin were verified by EIS. 34 However,
here too, the lateral mobility of the lipid bilayer is hindered by the
hydrophobic group of the thiolipid molecules, which provides a
fraction of the proximal leaflet of the lipid bilayer.
The ability of the volume enclosed between the gold electrode
and the lipid bilayer moiety of (thiolipid-spacer)-based tBLMs to
accommodate water molecules and inorganic ions was estimated
from the level of conductance induced by the incorporation of a
given amount of the ionophore valinomycin. This ionic reservoir
was simulated by a capacitance C s and the lipid bilayer by an
R m C m mesh. By using a number of thiolipids and spacers of varia-
ble length, Cornell and coworkers 34 found that the conductance,
1/R m , of the lipid bilayer increases with an increase in the length of
the thiolipid and with a decrease in that of the spacer. In a subse-
quent paper, the same authors used a long thiolipid terminated with
a benzyl disulfide anchor in order to increase the separation among
the thiolipid molecules, and mixed it with different proportions of
the short mercaptoacetic acid disulfide. The capacitance, C s , of the
ionic reservoir of the resulting tBLMs was regarded as consisting
of the capacitance, C H , of tightly bound inorganic ions in series
with the capacitance, C d , of diffuse layer ions, in analogy with the
Helmholtz layer and diffuse layer capacitances at a bare gold elec-
trode. 200 The separation between the C H and C d capacitances was
estimated from the dependence of the overall capacitance C s of the
ionic reservoir on the bulk ionic concentration and on the applied
potential. As a matter of fact, the situation within the ionic reser-
voir is somewhat different from that at a bare electrode because the
inorganic ions are prevented from coming in direct contact with
the electrode surface, due to the presence of the short spacer and of
the benzyl ring.
Relatively bulky membrane proteins with extramembrane do-
mains can be accommodated in the bilayer if the areas covered by
the short thiolated spacer are sufficiently large. Differences in
chain length 201 and chemistry 202 between thiolipids and spacers
may hopefully favor the formation of phase-demixed domains on
the nanoscale, facilitating protein incorporation. This seems to be
the case with mixed monolayers of cholesteryl-based thiolipids
and short thioalcohol spacers on gold. 203 Reductive desorption