Biomedical Engineering Reference
In-Depth Information
with the spacer-coated drop, by exploiting the spontaneous tenden-
cy of a lipid film to form a bilayer when interposed between two
hydrophilic phases. The fluidity of the tBLM was tested by record-
ing the cyclic voltammogram for the electroreduction of ubiqui-
none-10 incorporated in the lipid bilayer. The voltage-dependent
ion channel activity of melittin incorporated in the tBLM was veri-
fied by EIS. A similar mercury-supported tBLM was fabricated by
using a thiolated hexapeptide molecule with a high tendency to
form a 3
10
-helical structure as a spacer.
157
This thiopeptide had two
triethylenoxy side chains to impart it a satisfactory hydrophilicity.
The resulting tBLM was characterized by chronocoulometry and
EIS and its suitability as a biomimetic membrane was tested by
incorporating ubiquinone-10 and valinomycin.
(
ii
)
Thiolipid-based tBLMs
A
thiolipid
molecule consists of a hydrophilic polyethylene-
oxy or oligopeptide hydrophilic chain terminated at one end with a
sulfhydryl or disulfide group for anchoring to the support and co-
valently linked at the other end to two alkyl chains simulating the
hydrocarbon tails of a lipid (see
Fig. 1A
). A tethered thiolipid
monolayer exposes a hydrophobic surface to the bulk aqueous
phase and provides one half of the lipid bilayer. The other half is
obtained by forming a lipid monolayer on top of the thiolipid
monolayer, usually by vesicle fusion. Aqueous solutions of lipids,
solubilized with a detergent in the form of micelles, have also been
employed for this purpose, upon diluting the solution with an elec-
trolyte below the critical micelle concentration of the detergent.
158
(
a
)
Gold-supported thiolipid-based tBLMs
Polyethyleneoxy-based thiolipid monolayers and (polyeth-
yleneoxy-based thiolipid)/phospholipid bilayers tethered to gold
have been characterized by SPR, EIS and cyclic voltammetry, and
the synthesis of the thiolipids has been described.
158-160
The cross-
sectional area of a hydrophilic polyethyleneoxy chain is smaller
than that of the two alkyl chains, if it is in its fully extended con-
formation, but not if it is coiled. In the former case it is sufficiently
hydrated to provide a satisfactory ionic reservoir; conversely, in
the latter case it may accommodate only a limited amount of water
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