Biomedical Engineering Reference
In-Depth Information
(as opposed to BR, which is most commonly used to represent the protein, itself). Upon light
absorption, the protein is driven into the K state, which is the only photochemically gener-
ated state in the primary photocycle, and is the first intermediate after chromophore iso-
merization that can be trapped at low temperature (the method first used to characterize BR
intermediates). The bR
K transition is often referred to as the primary event, and is char-
acterized by a high quantum efficiency of ~65%. All subsequent intermediates are thermal,
consisting of the L, M, N, and O intermediates, each being of lower free energy than the last.
The protein's remarkable stability enables an extraordinarily high cyclicity, that is, the aver-
age number of times the protein can photocycle, on the order of 10
6
. Throughout the photo-
cycle, a proton is transported stepwise through a series of acidic amino acid side chains, and
ultimately delivered to the extracellular medium. The mechanism by which this is accom-
plished is beyond the scope of this text, but many review articles are available to the inter-
ested reader [35-41].
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14.2.3
Bacteriorhodopsin Photochromism
Bacteriorhodopsin is a unique protein, and has warranted a large amount of attention from
the perspective of both the basic and applied sciences. It is of extreme interest for bio-
chemical and biophysical studies because it acts as a light-driven engine that converts light
into chemical energy in an extremely efficient manner. The resulting chemical energy is
used to drive the formation of ATP from ADP and inorganic phosphate, and as such served
as an early example of chemiosmosis [42-44]. Eventually, elaborate mechanisms were pro-
posed to explain how protons were translocated through the protein and across the mem-
brane. Much of the early work in genetic engineering was done on BR as a means by which
the roles of individual amino acids in the proton pump could be determined. Ultimately,
BR became one of the first membrane proteins to be characterized by x-ray crystallography;
structures were even determined for individual photocycle intermediates [45-52].
Interest in BR by the applied sciences followed soon after the protein was initially
described, and was based on the ability to manipulate several of the intermediates with
light. It was discovered early on that the bR and M states could be driven back and forth
using light of appropriate wavelength (570 and 410 nm, respectively). The wide spectral
separation between the two states makes them an ideal pair for optical or holographic
memory operations. Furthermore, chemical additives could be used to prolong the M-
state lifetime, resulting in a transient bistable state (i.e., the M-state lifetime could be elon-
gated, but not made permanent, and would decay back to the bR state); the resulting
photochromism could be used as the basis for transient holograms or short-term optical
computer memories [17,20,21,53,54]. The first of these efforts was made in the Soviet
Union, and was denoted Project Rhodopsin; one of the goals of this military research pro-
gram was to develop BR-based films with the protein suspended in a polymer matrix,
which could be used in optical memory applications. Various chemical additives, mostly
amines, were used to extend the M-state lifetime. The resulting product, Biochrome, was
a real-time photonic and holographic material that could be written, read, and erased by
application of the appropriate wavelength [11]. However, as indicated above, it was based
on the bR-M binary pair and was incapable of permanent memory storage at room tem-
perature. However, several device applications were proposed based on M-state pho-
tochromism, including low-temperature optical memories, spatial light modulators, and a
two-photon three-dimensional optical memory [9,17-19,24,55,56]. A commercially viable
device based on the bR-M pair has also been built and marketed by the company MIB
Biochemicals in Marburg, Germany. Developed by Norbert Hampp and colleagues,
Fringemaker-plus
is a real-time holographic interferometer based on the BR mutant
