Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 323

num electrodes immersed in the buffer-filled compartments of the
setup. In the following, we will present two different characteristic
photocurrent traces that were obtained during the course of the ex-
periments. The recorded photocurrent upon illumination of the bR-
containing membrane, shown in Fig. 21 A (scenario A), is consid-
erably different from that shown in Fig. 21 C (scenario B). The
photocurrent depicted in Fig. 21 A rises to a constant value upon
switching the light on, and decreases to zero upon switching the
light off. However, for the photocurrent shown in Fig. 21 C , first a
transient current is observed upon illumination, followed by a
small stationary current density of about 0.15 nA cm -2 . Upon
switching the light off, a second transient current is observed. If 2
μM of the protonophor CCCP is added, the initial maximum cur-
rent is increased, as well as the steady state current.
To elucidate why, in principle, two different current traces are
monitored, impedance spectra were recorded, providing additional
information about the electric properties of the bilayers. In both
cases, the impedance data revealed that insulating pore-suspending
bilayers were formed after the addition of bR-free liposomes. A
resistance at low frequencies of more than 10 5 ȍ is obtained.
However, the impedance spectrum shown in Fig. 21 D exhibits
two dispersions, which is an indication of incomplete vesicle fu-
sion (see Fig. 5 B ), while that depicted in Fig. 21 B shows only
one dispersion. In the same manner, after incubation with bR-
containing liposomes, the impedance spectra differ considerably.
The absolute value of the impedance depicted in Fig. 21 D still
features two dispersions and indicates only a slight drop in the
overall resistance, whereas the impedance shown in Fig. 21 B
monitored after incubation with bR-containing liposomes has
dropped by one order of magnitude.
From these results together with the theoretical considerations
(Section V.1), we conclude that pore-suspending membranes
doped with bR molecules have been formed in scenario A. The
proteins are integrated into the pore-suspending bilayer, thus
pumping the protons into the underlying aqueous phase of the
pores. In this case, model B ( Fig. 17 B ) is valid. The simulation
based on model B and the experimental data are strikingly similar.
The finite increasing and decreasing current in the experiment
( Fig. 21 A ) is a result of the filter function used to filter the data.
The photocurrent shown in Fig. 21 C (scenario B) is interpreted as