Biomedical Engineering Reference
In-Depth Information
er, their lateral mobility is hindered by the presence of adsorbed or
hemifused vesicles and by the roughness of the metal support (see
Section IV.5). Moreover, the hydration of the polyethyleneoxy
moiety of thiolipids anchored to gold is low, while the incorpora-
tion of proteins with extramembrane domains requires a significant
hydration of the spacer. Only small peptides and ionophores can be
accommodated in the lipid bilayer moiety of polyethyleneoxy-
based tBLMs, via incorporation from their aqueous solutions.
A typical example is the depsipeptide valinomicin, an ion car-
rier that complexes a potassium ion with its carbonyl groups.
Valinomycin cages potassium ions and shuttles them across the
bilayer from the aqueous solution to the hydrophilic polyethylene-
oxy chain, moving back and forth. Upon incorporating valinomy-
cin in the tBLM, the impedance spectrum was fitted to an equiva-
lent circuit consisting of three circuit elements in series: a capaci-
tance
C
s
simulating the spacer, a R
m
C
m
mesh simulating the lipid
bilayer, and a resistance
R
:
, simulating the aqueous solution.
161,169
In a KCl solution, the conductance, 1/
R
m
, of the lipid bilayer moie-
ty was found to increase proportionally to the bulk valinomycin
concentration,
c
v
. The lipid bilayer capacitance,
C
m
, remained
practically constant with varying
c
v
using the DPTL thiolipid
169
,
but increased linearly with
c
v
using a different thiolipid.
161
The
high selectivity of valinomycin toward potassium ion with respect
to sodium ion was verified. The impedance spectra of a gold-
supported DPTL/DPhyPC tBLM incorporating valinomycin were
also interpreted with a SPICE computer program designed to simu-
late processes across membranes with an electrical network;
169
this
amounts to solving the Nernst-Planck equation for field-assisted
diffusion of permeating ions across a membrane with the appropri-
ate boundary conditions. However, in doing so, the potential dif-
ference across the tBLM, measured with respect to an
Ag/AgCl(sat. NaCl) reference electrode, was identified with its
extrathermodynamic absolute potential difference (see also Ref.
170). A gold-supported DPTL/DPhyPC tBLM was also used to
incorporate the M2 peptide, which forms a straight D-helix span-
ning lipid bilayers.
171
Bundles of M2 D-helixes form a hydrophilic
pore in the membrane, by turning their polar residues toward the
interior of the pore and their nonpolar residues toward its exterior.
M2 is one of the four membrane-spanning segments composing
each of the five subunits that constitute the nicotinic acetylcholine
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