Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 197

ground noise remains low enough to allow the monitoring of sin-
gle channel currents, provided R m is maintained above 1 G:. From
a statistical analysis, the lifetime of nano-BLMs with resistance
higher than 1 G: was estimated at 1.5 days. Single channel cur-
rents of gramicidin, alamethicin 110 (see Fig. 14 ) and OmpF porin 111
were recorded, and the corresponding current amplitude histo-
grams were reported.
OmpF porin is an integral membrane protein located in the
outer membrane of Escherichia coli. It is a non-specific transport
channel that allows the diffusion of small, polar molecules (600-
700 Da in size) through the cell outer membrane, where it forms a
trimer. The characteristic conductance states of the three OmpF
monomers could be observed with the nano-BLM. Fast openings
and closures of a single monomer, superimposed on the slow ki-
netics of the channel, was also recorded. Moreover, the blockade
of the ion flow as a result of interaction of the antibiotic ampicillin
with OmpF porin was verified. These experiments show that single
channel currents of bulky proteins, such as OmpF porin, whose
cross-sectional area amounts to about 80 nm 2 , can be monitored
using nano-BLMs with highly ordered pores of 60 nm in diameter.
These nano-BLMs were also used to record the photocurrents gen-
erated by purple membrane fragments adsorbed on the lipid bi-
layers spanning the micropores. 112 Purple membranes from Halo-
bacterium salinarium are very rich in the proton pump bacteri-
orhodopsin, which pumps protons from the cytoplasmic to the ex-
tracellular side of the bacterium membrane upon photoactivation
by green light. The purple membrane fragments were added to the
compartment exposing the gold-coated side of the alumina mem-
brane. After their adsorption on the lipid bilayers spanning the
nanopores and their illumination, the purple membrane fragments
with the extracellular side facing the lipid bilayers cause protons to
translocate through the proton pump. This movement generates a
current transient that decays with a characteristic decay time; the
current tends to a nonzero stationary value, revealing a small per-
meability of the nano-BLM to protons. Addition of the lipophilic
proton ionophore CCCP increases the level of the stationary cur-
rent by facilitating the diffusion of protons pumped by bacteri-
orhodopsin across the lipid bilayer.
Silicon nitride membranes with 800 nm pores arranged in reg-
ular arrays were hydrophobically functionalized, and bilayers were