Biomedical Engineering Reference
In-Depth Information
component is conveniently suppressed by adsorbing purple membranes to the
surface of a Teflon film. If a genuine BLM is used, both
B
2
and
B
2
The
B
2
are expected to be
observable but their differentiation may be problematic for the following reasons. Both
B
2
and
B
2
represent charge movement in the same direction, and therefore both will have the
same polarity. Since both components are related to interfacial proton transfer, both are
expected to be pH-dependent. However, the differentiation between
B
2
and
B
2
can be
achieved by the following experiment [40,60]. It was pointed out in Section 15.4.2 that the
transmembrane diffusion of P and P
in the Mg porphyrin membrane is slow compared
with interfacial electron transfer—the two interfacial reactions are
chemically decoupled
on
the microsecond time scale. We suspected that this might also be true for transmembrane
proton movement in the purple membrane. If so, the interfacial proton-transfer reaction at
either membrane surface will depend solely on the pH of the adjacent aqueous phase but
not on the pH of the opposite aqueous phase (concept of
local reaction conditions
) [40].
An experiment based on this strategy is shown in Figure 15.11 in which purple mem-
branes are reconstituted into a collodion membrane according to a method developed by
Drachev et al. [67]. When both sides of the membrane face neutral pH, both
B
1
and
B
2
are
observable.
B
2
can be reversibly inhibited by lowering the pH of the cytoplasmic side as
expected. When the pH of both the cytoplasmic side and the extracellular side are kept
low, a negative component appears. This negative component is identified as the hypo-
thetical
B
2
component, because it has a different relaxation-time course and because its
pH dependence is opposite to that of the
B
2
component;
B
2
is enhanced by low pH.
15.4.3
DC Photoelectric Effect: The Null-Current Method
The methodology described above for the analysis of AC photoelectric signals is not suit-
able for the detection of the DC photoelectric signal for several reasons. The use of a Teflon
film, which has a high resistance, does not permit a DC current to pass through. A genuine
Cytoplasmic
Extracellular
Signal 1
ph 7
ph 7
Signal 2
ph 0
ph 7
2
Signal 3
ph 0
ph 0
1
2
0
3
−
1
1
−
2
0
200
400
600
800
1000
Time (
µ
s)
FIGURE 15.11
“Differential experiment” showing the separation of the
B
2
and
B
2
'
component. See text for further detail. (From
Hong, F. T., Okajima, T. L. (1987). Rapid light-induced charge displacements in bacteriorhodopsin membranes:
an electrochemical and electrophysiological study. In: Ebrey, T. G., Frauenfelder, H., Honig, B. Nakanishi, K.
(Eds.).
Biophysical Studies of Retinal Proteins
. Urbana-Champaign, IL: University of Illinois Press, pp. 188-198.)
