Biomedical Engineering Reference
In-Depth Information
1.
Theoretical Description of Light-Induced
bR-Photocurrents
In this Section, we will first give a theoretical description of the
photocurrent observed upon illumination of bR containing lipid
membranes. Different membrane setups will be elucidated and the
characteristic current traces as a function of time will be simulated.
The theoretical approach is based on the work of Herrmann and
Rayfield,
92
who described the photocurrent of light activated bR
reconstituted in vesicles, partially fused to BLMs. Absorbed light
drives a pump current
I
p
, carried by the transported protons, which
generates a voltage
V
p
across the membrane. According to
Herrmann and Rayfield,
92
I
p
is a linear function of the voltage
V
p
:
V
§
·
p
I
I
1
(8)
¨
¸
¨
¸
p
p,0
V
©
¹
I
p,0
is the initial pump current at
V
p
= 0,
V
*
is an intrinsic con-
stant. Only with
V
*
>>
V
p
,
I
p
is a linear function of
V
p
.
V
p
strongly
depends on the electrochemical properties of the environment,
which is electrically coupled to the bR-containing membranes.
Figure 16
sketches the general setup (A) together with an equiva-
lent circuit (B), in which the upper part represents a bR-containing
membrane acting as a current source, while the bottom part needs
to be defined by the entire electrochemical system. We will discuss
two different scenarios for part X:
Model A: Attachment of purple membranes on nano-BLMs;
Model B: Insertion of bR in pore-spanning membranes.
(
i
)
Purple Membranes Attached to Nano-BLMs
In the first scenario, a
sandwich
-like structure is assumed, in
which purple membranes (PM) are attached to a lipid bilayer
(model A). Simulations of the photocurrents induced by bR in the-
se purple membranes are based on the equivalent circuit shown in
Bamberg et al.
93
for purple membranes adsorbed to classical
BLMs.
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