Biomedical Engineering Reference
In-Depth Information
D96N. Its primary application is nondestructive testing of manufactured components
through microscopic deformation and interferometric analysis [57].
Despite many efforts to extend the M-state lifetime long enough to facilitate extended
holographic or photonic memory applications, neither genetic nor chemical modification
of the protein has managed to extend the M state to more than a few minutes. The “Holy
Grail” in applied BR research has long been a truly bistable system—a two-state optical
system that can be used for long-term memory applications. In 1993, Andreas Popp and
colleagues [58] described a branch off of the main BR photocycle O state that produced a
highly stable blue-shifted intermediate characterized by a 9-
cis
chromophore, the Q state.
The so-called branched photocycle is accessed by exposure of the O state to red light—as
such, it is a sequential two-photon process. First the BR photocycle is initiated with green
light, followed by an exposure to red light once the O state has formed. As a result, a small
percentage of the O state is driven into the branched photocycle, forming the P interme-
diate, a thermal precursor to the Q state. Although Q is stable for better than 10 years
[58,59], the all-
trans
to 9-
cis
isomerization occurs with a very low quantum efficiency
(~10
-4
-10
-3
) due to steric interactions within the binding site [59]. These steric interactions
are introduced upon formation of the 9-
cis
chromophore and are strong enough to drive
the hydrolysis of the Schiff base bond. The result is a trapped, but unbound chromophore
in the binding site. Exposure of the protein to blue light reisomerizes the chromophore to
all-
trans
, thereby regenerating the BR resting state. It is questionable whether the branched
photocycle occurs to any extent in nature, or whether it offers the organism any competi-
tive advantage. However, it has been suggested that the branched photocycle confers
some protection to the organism during periods of high UV light flux [59].
The discovery of a permanent and reversible photochromic BR intermediate has
resulted in three-dimensional architectures for both binary photonic and holographic
memories based on the branched photocycle [22,60,61]. Unlike the M state, the branched
photocycle is a three-state system in which the permanent states (bR and Q) are separated
both energetically and temporally. This unique feature allows the branched photocycle to
be utilized in nondestructive volumetric architectures [22,28-30]. The primary problem
with architectures based on the branched photocycle is the low quantum efficiency with
which the permanent state (Q) is accessed; this typically demands the use of either high-
intensity lasers or prolonged exposure times, and both these requirements effectively pre-
clude commercial viability for the wild-type protein. However, new molecular biological
techniques are producing genetically-engineered variants custom-tailored to device appli-
cations. Some of these techniques will be described below.
14.2.4
The Bacteriorhodopsin Photoelectric Effect
The final characteristic worth mentioning with respect to BR-based device applications is
the protein's photoelectric effect. Already alluded to above, the photoelectric effect is BR's
concerted voltage response to light, and can be separated into three components (see
Figure 14.2). The first is a picosecond-scale shift of electron density toward the Schiff base
bond, concomitant with chromophore isomerization (B1). This is followed by a microsec-
ond-scale response of opposite polarity (B2), which has recently been proposed to result
from the movement of a specific amino acid (R82) toward the chromophore [62]. Finally,
the B3 component originates from the action of the proton pump, and has a millisecond
time-scale. The photoelectric effect has been employed as the basis for several microelec-
tronic device applications, including wavelength-specific light-activated Fourier-effect tran-
sistors (GeAs BR-based MODFET), artificial retina prototypes, and polarization-sensitive
microelectronic devices [63,64]. Introduction of polarization sensitivity is done by selectively
