Biomedical Engineering Reference
In-Depth Information
that induce pore conductance. Addition of melittin to a gold-
supported alkanethiol/phospholipid bilayer increases the faradaic
current for ferricyanide electroreduction, rendering it practically
identical with that observed across the sole alkanethiol monolay-
er;
78,84
this suggests that melittin may permeate the fluid lipid
monolayer, but not the underlying alkanethiol monolayer.
The integral protein cytochrome
c
oxidase (COX) spans the
inner mitochondrial membrane and is the terminal component of
the respiratory electron-transport chain. It catalyzes the redox reac-
tion between the small peripheral protein cytochrome
c
in its re-
duced form, i.e., ferro-cytochrome
c
, and oxygen, with formation
of ferri-cytochrome
c
and water; it also pumps protons from the
matrix to the intermembrane space, where it interacts with ferro-
cytochrome
c
. COX was reported to retain its functional redox
activity after incorporation in a gold-supported hybrid octadecan-
ethiol/lipid bilayer from its detergent solution.
85
Thus, the cyclic
voltammogram of COX in this hybrid bilayer yields an oxidation
and a reduction peak whose peak potentials are practically equidis-
tant from the known standard potential of the cytochrome
a
3
redox
site of this enzyme. However, this result was obtained by choosing
thiol concentrations and experimental conditions allowing the for-
mation of large uncovered areas of the gold electrode, where COX
could partition into the hybrid bilayer during the dialysis proce-
dure. This system was subsequently improved by monitoring the
exact amount of octadecanethiol required for the formation of a
thiol submonolayer, using a quartz crystal microbalance.
86
Moreo-
ver, gold was replaced by silver as the substrate. The Ag-S bond
being stronger than the Au-S bond was considered to increase the
stability of the membrane. In addition, the fact that the alkyl chains
of the tethered thiol molecules are less tilted on silver than on gold
was believed to favor a closer approach of the enzyme to the elec-
trode surface. The cyclic voltammogram of COX incorporated in
the silver-supported hybrid bilayer shows an oxidation peak much
higher than the reduction one. This was ascribed to a nonfaradaic
charge flowing during the positive voltage scan, as a consequence
of a conformational transition allowing ion movement into and out
of the hybrid bilayer.
Alkanethiol-coated electrodes may be usefully employed for
adsorbing membrane fragments whose proteins retain their normal
activity. Thus, erythrocyte ghosts, i.e., fragments of the plasma
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