Biomedical Engineering Reference
In-Depth Information
to bundles of KC molecules, with the oxygens turned toward the
interior of the bundles. In these hydrophilic pores the inorganic
ions can move back and forth following the a.c. signal, causing an
increase in differential capacitance. The decrease in differential
capacitance from 6.5 to 1.8 PF cm
-2
after the second phase transi-
tion suggests that this transition involves the disruption of the
pores and a return to a random distribution of DOPC and KC mol-
ecules within the mixed monolayer. The permeability of the lipid
film to Tl
+
ions induced by the first phase transition, and its abrupt
decrease induced by the second phase transition, support the for-
mation and subsequent disruption of pores across the lipid mono-
layer.
Ubiquinone-10 (UQ) is an important, ubiquitous biomolecule
that is present in many membranes and acts as a proton and elec-
tron carrier in the respiratory chain of the mitochondrial mem-
brane. It consists of a quinone ring, which can be reduced to the
corresponding quinol, and is provided with a long, rigid isoprenoid
chain imparting a high affinity for lipids. The mechanism of its
reduction inside a phospholipid monolayer supported by mercury
was investigated by carrying out a series of consecutive potential
steps from a fixed initial value
E
i
,
where UQ is still electroinactive,
to progressively more negative potentials,
E
f
, and by measuring the
charge
Q
(
t
,
E
f
) following each step, as a function of both time
t
and
potential
E
f
.
64
The faradaic charge,
Q
f
(
t
,
E
f
), was obtained by sub-
tracting the capacitive charge measured in the absence of UQ from
Q
(
t
,
E
f
). From the linear dependence of
E
f
upon log
t
, at constant
Q
f
(
t
,
E
f
) and pH, and upon pH, at constant
Q
f
(
t
,
E
f
) and
t,
it was
concluded that UQ electroreduction to ubiquinol, UQH
2
, takes
place via a one-electron transfer in quasi equilibrium, followed by
the rate-determining uptake of a proton; this rate determining step
is followed by the rapid uptake of a further electron and a further
proton, yielding UQH
2
. The generic chronocoulometric procedure
employed for UQ was also adopted to study the electrochemical
behavior of dioactadecylviologen
65
and of vitamine K
1
,
66
an essen-
tial constituent for blood coagulation, upon incorporating these
molecules in a self-assembled DOPC monolayer supported by
mercury.
Ion channels are responsible for the flow of hydrophilic ions
across biomembranes along their electrochemical potential gradi-
ent, namely from the membrane side where the electrochemical
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