Biomedical Engineering Reference
In-Depth Information
The retinal chromophore of bR naturally exists in one of two states: all-
trans
and 13-
cis
(10). In the dark, a bR population contains a mixture of these two retinal states and is
called “dark-adapted”. Under continuous illumination, the 13-
cis
retinal is converted into
the all-
trans
retinal and all chromophores are in the all-
trans
form, which is referred to as
“light-adapted bR”. The absorption of a photon by light-adapted bR initiates isomeriza-
tion of the retinal from an all-
trans
to a 13-
cis
conformation. This is followed by proton
transport from the cytoplasm of the cell to the exterior, thus converting light energy into
chemical energy needed for cell metabolism (11). Proton transport is accompanied by a
complex photocycle, which involves several intermediate states with distinct spectral
absorption maxima. The Schiff-base protonation and the retinal configuration cause the
shift of the absorption bands. Wild-type bR molecules need approximately 10 ms to com-
plete one photocycle and they do not have a refractory period after completing a photo-
cycle under light-saturated conditions (12).
17.1.3
Proton Transfer Mechanism in Reconstituted Bacteriorhodopsin Films
A major challenge for scientists and engineers is to find ways of incorporating biomateri-
als into practical devices. This requires appropriate immobilization techniques that can
orient the protein molecules and prevent them from denaturing. Thin films are considered
to be the most practical architectures for heterostructural systems containing photosensi-
tive materials because they improve photoelectric functionality compared with the bulk
state. Furthermore, thin films provide a basic arrangement that allows biological compo-
nents to be interfaced with microelectronic systems.
Methods recently used for assembling bR thin films onto solid substrates include elec-
trophoretic sedimentation (EPS), Langmuir-Blodgett deposition (LB), self-assembly, elec-
trostatic layer-by-layer adsorption, antibody-mediated monolayer, and immobilization
within polymer gels (13-18). Success of these methods is largely attributed to bR's highly
stable crystalline structure. The photoelectric conversion efficiency of a bR film is
dependent on the uniformity of molecule orientation within the film. Most of the afore-
mentioned assembly methods are able to pattern bR in manners suitable for various
applications. However, it must be noted that different fabrication methods alter the bio-
molecule's environment from that of its natural state, resulting in different phototrans-
ducing mechanisms.
After light-adapted bR is excited by incident light, the absorbed photons electronically
excite the chromophore and drive retinal isomerization around the C
13
C
14
double bond,
transforming the retinal from an all-
trans
into a 13-
cis
configuration (19). Photoisomerization
triggers proton translocation and initiates a series of thermally driven reaction steps that take
place within a millisecond time frame. The protein structure that surrounds the retinal has
crucial impact on the photocycle and the proton transfer mechanism. In the aqueous phase,
proton transport starts with the release of a proton to the extracellular side during the
L
M
transition and terminates with a proton uptake from the cytoplasmic side during the follow-
ing
M
N
transition (20) (Figure 17.2a). The aspartate residues, Asp85 and Asp96, act as pro-
ton acceptor and donor for reversible protonation and deprotonation of the Schiff-base
linkage, respectively. When the pH
7.0, proton release precedes proton uptake. The
sequence is reversed at pH
7.0, whereby proton uptake is followed by proton release (21).
In contrast, absorption of a photon by bR in its dried form initiates twisting of the retinal.
However, this process does not cause proton pumping due to the lack of water molecules in
the extracellular channel. The corresponding photocycle stops at the
M
state before the Schiff
base can be reprotonated (Figure 17.2b) (22). Ultra fast photoinduced charge separation is fol-
lowed by a slow charge recombination as the protein returns to its ground state.
