Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 238

composed of three subunits, designated D, E and J. The GDP/GTP
binding site is associated with the D subunit. Photoactivated rho-
dopsin (R*) interacts with the D subunit of transducin by catalyz-
ing the replacement of bound GDP with GTP, which results in the
dissociation of the D subunit from the E and J subunits. Scanning
the micropatterned film by SPR shows that rhodopsin is preferen-
tially incorporated into the lipid bilayer on top of the mer-
captoundecanoic acid, which is more fluid and has a more water-
rich reservoir than the lipid bilayer consisting of the anchored thio-
lipid with a lipid monolayer on top. On the other hand, the periph-
eral protein transducin cannot distinguish between the two differ-
ent lipid-coated regions and is equally adsorbed on both. The pro-
gressive increase in film thickness due to transducin binding to the
micropatterned film is monitored by SPR, just as the rapid de-
crease in film thickness due to transducin desorption following
rhodopsin photoactivation in the presence of GTP. After interrupt-
ing illumination, a slow increase in film thickness is observed, due
to transducin rebinding.
The OmpF porin, a pore forming protein present in the outer
membrane of Escherichia coli , was incorporated in a tBLM con-
sisting of a mixture of a diethyleneoxy-based thiolipid and of a
sulphanylpropionic acid spacer. 206,207 This ligand-gated channel
acts as a receptor for the antibacterial toxin colicin N, which
blocks the channel by binding to its mouth. The conductance of the
lipid bilayer incorporating OmpF porin, as measured by EIS,
amounts to 1.54 mS cm -2 . Progressive additions of colicin N cause
a gradual decrease in conductance up to a minimum limiting value
of 1.35 mS cm -2 . Setting the conductance value in the absence of
colicin equal to unity and its limiting value in the presence of
colicin equal to zero, the sigmoidal plot of the resulting normalized
conductance against the logarithm of colicin concentration is prac-
tically identical to that obtained at a conventional BLM. However,
the limiting value of the conductance at a conventional BLM is
more than three orders of magnitude less than the initial value,
while at the gold-supported tBLM it is only 15% less. This is due
to the poor electrical properties of the lipid bilayer, which allows
an appreciable nonspecific ionic flux.