Biomedical Engineering Reference
In-Depth Information
R
e
−
e
−
P
+
A
−
P
+
A
−
+
B
−
+
B
−
e
−
e
−
P
+
A
−
A
−
P
+
L
L
B
−
B
−
+
+
e
−
e
−
A
−
A
−
P
+
P
+
~7 nm
Electrolyte
Bilayer
Electrolyte
(A)
2 s
h
2 s
h
h
I
0
I
0
(B)
Time
(C)
Time
FIGURE 15.19
An experimental prototype showing switching of ionic currents based on light-induced surface potentials. (A)
The BLM contained magnesium octaethylporphyrin, which is lipid-soluble (3.6 mM). The aqueous phases con-
tained the electron acceptor methyl viologen in equal concentrations on both sides (20 mM). Tetraphenylborate
ions,
B
-
, are partitioned into the membrane at the region of polar head groups of lipid. Photoactivation of mag-
nesium octaethylporphyrin generates two symmetrical positive surface potentials, which increase the surface
concentration of
B
-
but decrease the concentration of tetraphenylphosphonium ions (positively charged). (B) The
ionic current of 1
s laser pulse. (C) The
ionic current of 5 mM of tetraphenylphosphonium ions decreases 25% upon illumination. In both (B) and (C), the
spike on the left is the capacitative transient upon imposition of an applied voltage of
M tetraphenylborate ions increases 50% upon illumination with a 1
50 mV. Light pulses are
indicated by arrows. (From Drain, C. M., Christensen, B., Mauzerall, D. (1989). Photogating of ionic currents
across a lipid bilayer.
Proc. Natl. Acad. Sci. USA
86:6959-6962.)
of arrestin to photophosphorylated rhodopsin is facilitated by the cluster of multiple neg-
ative charges, which appears as a consequence of phosphorylation of multiple serine or
threonine residues. However, allosteric effect as a result of conformational changes also
play a role (see [90] for more detail).
In contrast, the work of Drain et al. demonstrated that the electrostatic switching mech-
anism inspired by partially reverse-engineering the visual membrane could be imple-
mented in the laboratory in terms of completely different materials. This is the essence of
biomimetic science: using the same principle but different materials to construct a device.
