Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 251

R : ; the capacitance C s simulates the ionic reservoir and the R m C m
mesh simulates the lipid bilayer. 238 The capacitance C m and re-
sistance R m of the bilayer amount to about 6 PF cm -2 and 800 k:
cm 2 . This high capacitance and low resistance denote a loosely
packed lipid bilayer, partly ascribable to the presence of a high
protein content. As ferri-cytochrome c is added in the presence of
oxygen, at potentials negative of 270 mV/NHE it starts to be elec-
troreduced to ferro-cytochrome c . This triggers an electrocatalytic
process whereby, in the enzymatic cavity of COX, ferro-
cytochrome c is oxidized by oxygen to ferri-cytochrome c , which
can be continuously electroreduced to ferro-cytochrome c . This
gives rise to a reduction current in the negative potential region.
When the COX is oriented with the cytochrome c binding side
pointing toward the electrode surface, the primary electron accep-
tor, Cu A , is also oriented toward the electrode. In this case, the
cyclic voltammogram in the absence of oxygen shows a single
reduction peak at about -274 mV/NHE, due to the electroreduction
of the enzyme, and a corresponding oxidation peak at about -209
mV/NHE. 239.240 The peak currents increase linearly with the scan
rate, denoting a surface confined process. Moreover, scan rates < 1
V s -1 leave the peak potentials unaltered, indicating that the elec-
tron transfer is reversible (i.e., in quasi-equilibrium) at these scan
rates. The fact that the reduction and oxidation peak potentials do
not coincide was ascribed to some purely chemical protonation
step within the protein. The midpoint potential is shifted by about -
450 mV with respect to the standard potential of +230 mV/NHE,
determined for the Cu A redox center in isolated COX. This was
explained by assuming that the first transient electron acceptor in
the reduction process is not the Cu A center, but rather the Ni(II)
ion of the Ni(II)/Ni(I) redox couple of the Ni-NTA complex. At
scan rates < 1 V s -1 , oxidation and reduction peaks are slightly
asymmetric. A further increase in scan rate enhances this asym-
metry, leading to the appearance of a second peak at scan rates
> 20 V s -1 , as shown in Fig. 24 . The resulting complex voltammo-
gram of the four-redox-site COX protein was deconvoluted into
four Gaussian components. Kinetic parameters of the four one-
electron transfer steps, one for each redox center, were extracted
from the plots of the four deconvoluted peak potentials against the
logarithm of the scan rate (the so-called trumpet plot).