Electrochemical Analysis of Ion Channels and Transporters in Pore-Suspending Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 316

was achieved through a glass window in the cis compartment of
the measuring chamber ( Fig. 1 A ) with the light of a halogen lamp
filtered with a 515 nm cut-off filter. Platinised platinum wires im-
mersed into the buffer solution served as electrodes to detect po-
tential differences across the nano-BLMs. Prior to performing pho-
to-induced photocurrent measurements it is essential to check for
photoartefacts, which are intrinsic to the applied system. Thus,
nano-BLMs were illuminated in the absence of PM-fragments. As
illustrated in Fig. 18 A , photoartefacts known from the illumina-
tion of solid supported membranes immobilized on gold elec-
trodes 22,97 were not observed using nano-BLMs on porous alumi-
na.
If PM-fragments are attached to the nano-BLM, a characteris-
tic current trace is observed. When switching the light source on,
the photocurrent is characterized by a positive net current of pro-
tons from the cis to the trans compartment ( Fig. 18 A ). After
reaching a maximum value, the net current declines to zero. Ac-
cording to the theoretical model A, the photocurrent decreases to
zero when the membrane resistance R m of the underlying bilayer is
infinitely large. By EIS measurements prior to the adsorption of
PM-fragments, we were able to elucidate the electrical properties
of the nano-BLMs. R m proved to be in the gigaohm range, which
suggests that the bilayer acts as a perfect insulator to proton cur-
rents. In this case, solely the capacitance of the nano-BLM governs
the course of the photocurrent during illumination, as corroborated
by the simulations depicted in Fig. 17 A . Upon switching the light
source off, a negative transient current is observed. This is a result
of the potential difference built up during the proton pumping ac-
tivity of bR and indicates that the protons diffuse back across the
bilayer. The observed net current indicates that there is a favoured
orientation of the PM fragments adsorbed on the nano-BLM. Since
the adsorption of purple membranes is partially driven by the elec-
trostatic interaction between the negatively charged membrane
fragments and the positively charged detergent octadecylamine in
the nano-BLM, the favoured orientation might be influenced by
the surface charge density of the two leaflets of the PM. The cyto-
plasmic surface exhibits a surface charge density of -0.22 C m -2 ,