Biomedical Engineering Reference
In-Depth Information
molecules. Whether the conformation is extended or coiled de-
pends both on the interfacial electric field and on the nature of the
metal support. TBLMs fabricated with polyethyleneoxy-base thio-
lipids exhibit capacitances in the range from 0.5 to 0.7 PF cm
-2
and
resistances in the range from 5 to 10 M: cm
2
, which are compara-
ble with the values found for conventional BLMs; however,
somewhat higher capacitance values (1 PF cm
-2
) have been report-
ed for long spacers.
161
A particularly convenient tBLM of this
type, called DPTL, was synthesized in the Max Planck Institute of
Polymer Science in Mainz.
1
It consists of a hydrophilic tetra-
ethyleneoxy chain covalently linked to a lipoic acid residue for
anchoring to the metal at one end, and bound via ether linkages to
two phytanyl chains at the other end (see
Fig. 1A
). Its cross-
sectional area is of about 55 Å
2
.
Thiolipopeptides
consist of an oligopeptide chain terminated
at one end with a sulfhydryl group and covalently linked at the
other end to the polar head of a phospholipid. They are considered
to assume a helical structure, rather than a E-sheet structure.
162
In
this case, their cross-sectional area amounts to about 75 Å
2
. They
are usually obtained by tethering to a gold electrode a
thiopeptide
consisting of an oligopeptide chain terminated with a sulfhydryl
group at one end and with a carboxyl group at the other end. This
thiopeptide monolayer is then coupled
in situ
with dimyristoyl-
phosphatidylethanolamine (DMPE).
163
TBLMs fabricated with
thiolipopeptides have been employed for the incorporation of a
number of integral proteins; they exhibit high capacitances (from 2
to 10 PF cm
-2
) and low resistances, of the order of 10
4
: cm
2
,
which are more than three orders of magnitude lower than those of
conventional BLMs.
164-166
The low resistance is ascribed to a not
perfectly homogeneous coverage of the thiolipid monolayer by the
distal lipid monolayer; this coverage is estimated at about 70% of
full coverage and makes the tBLMs insufficiently insulating.
163
This
is indicative of the presence of an appreciable number of pin-
holes and other defects in the bilayer, as well as of a certain disor-
der of the hydrocarbon chains. It was suggested that the thioli-
popeptide monolayer actually consists of a mixture of the thioli-
popeptide and of the corresponding uncoupled thiopeptide. This
conclusion was based on the observation that gold-supported
tBLMs formed from a presynthesized thiolipopeptide did not per-
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