Biomedical Engineering Reference
In-Depth Information
consequence of an asymmetric chemical gradient across the membrane. Rather, it arises
from the asymmetrical orientation of the photopigment in the membrane.
Both bR and rhodopsin contain a covalently bound vitamin A aldehyde as the chro-
mophore, and both consist of seven segments of transmembrane
-helices with the
C
-ter-
minus located on the cytoplasmic side and the
N
-terminus on the opposite side (see [15]).
Both pigments exhibit similar AC photoelectric signals in their native membranes. We
therefore use analogous terminology for the signal components. The ERP consists of a
faster
R
1
component that is temperature-insensitive and a slower
R
2
component that can
be reversibly inhibited by low temperature (0ºC) [33]. The analogous AC signal compo-
nents elicited by a brief light pulse from a reconstituted purple membrane are named
B
1
and
B
2
, which are differentiated on the basis of temperature sensitivity as in the ERP [55].
15.4.1
Mechanistic Aspects of Signal Generation
In the literature, the ERP has been interpreted as the electrical manifestation of light-induced
intramolecular
charge displacement in rhodopsin and the molecular mechanism is usually
referred to as the dipole mechanism, based on the following reasoning. Rhodopsin contains
several charged amino-acid residues and maintains a fixed orientation in the visual mem-
brane. Photoactivation of rhodopsin leads to a sequence of conformational changes and con-
current changes of absorption spectra known as the photobleaching sequence. The
concurrent charge shifts can, in principle, generate a transient electric dipole in the direction
perpendicular to the membrane surface. Such charge separation may or may not span the
entire membrane thickness and may or may not lead to a transmembrane net charge trans-
port. There is no evidence that photoactivation of rhodopsin leads to a net charge transport
(see Section 15.5). Presumably, all separated charges subsequently recombine upon recovery
of rhodopsin from the photoexcitation. Such a process is depicted as the
oriented dipole
(OD)
mechanism shown in Figure 15.5 [52]. Thus, the AC photoelectric signal is a manifestation
of charge separation during the formation of a transient array of electric dipoles and the sub-
sequent charge recombination following its recovery. This process is tantamount to the
charging and discharging of a capacitance,
C
p
. By applying the Gouy-Chapman diffuse dou-
ble layer theory and chemical kinetic analysis to this simple model, an equivalent circuit can
be derived for the process of charge separation and recombination [35,52].
The OD mechanism is however not the only possible mechanism to generate an AC
photosignal. In the aforementioned Mg porphyrin membrane, there is no electric dipole
formation during photoactivation because Mg porphyrin molecules are free to diffuse
(translationally and rotationally) and, therefore, they cannot maintain a fixed orientation
in the membrane. Yet the AC photoelectric signal so generated exhibits all major charac-
teristics of the ERP [35]. Indeed, an interfacial electron (or proton) transfer constitutes
another kind of charge separation (across a membrane-water interface). In the case of the
Mg porphyrin membrane, when an electron is transferred from the membrane-bound por-
phyrin molecule to the aqueous acceptor ferrocyanide, the membrane-bound porphyrin
molecule becomes a monocation and a pair of charges becomes separated across the inter-
face. In the case of rhodopsin, there is evidence suggesting the presence of an interfacial
proton-transfer reaction; so far there is no evidence suggesting the presence of an interfa-
cial electron-transfer reaction. The
R
2
component of the ERP has been known to appear at
the time when the photointermediate metarhodopsin I is converted to metarhodopsin II
[33]. This latter reaction is an acid-base reaction and an aqueous proton is transferred to
the membrane-bound pigment, that is, the proton donor is a hydronium ion, which is
present in the adjacent aqueous phase. Thus, when a proton is transferred from the aque-
ous phase to the membrane phase, a “counterion,” which takes the form of molecular
water, must be left behind in the adjacent aqueous phase, leading to charge separation
