Biomedical Engineering Reference
In-Depth Information
the desired result was the production of either a specific intermediate (M or Q), or a pho-
tovoltage. However, the protein is capable of providing much more information to the
researcher, specifically in the form of the photokinetics of both responses. These three
responses, photochromism, photovoltage, and photokinetics, can all be used to facilitate
sensor design.
14.3.2.1 Bacteriorhodopsin and Chemical Sensitivity
Bacteriorhodopsin has been long known to display an extreme sensitivity to its environ-
ment, responding to changes in pH, relative humidity, and ionic strength. The results of
such environmental changes are manifested as modulations of the photocycle kinetics or
the photoelectric effect. Furthermore, a large number of chemicals have well-defined
effects on the protein. One such class of chemical modulators is organic amines, which
prolong the M intermediate. The M state decays as a function of the deprotonation of
amino acid D96, and environmental conditions that favor reduced proton mobility will
prolong this state (e.g., high pH or dehydration). The O state, also characterized by proton
transfer, is similarly sensitive to the presence of environmental additives that alter pH or
reduce proton mobility. Furthermore, Bryl and Yoshihara [88] demonstrated in 2000 that
the BR photoelectric effect can be modulated as a function of the chemical environment
experienced by the protein.
Other classes of chemicals that modulate BR's properties include alcohols (methanol,
ethanol, propanol, and butanol) [88], anesthetics [89-102], and azides [103-107]. The pro-
tein's sensitivity to alcohols is manifested as modulations of the light-induced photocur-
rent, and can be enhanced by chemical modification of the chromophore or genetic
manipulation of the protein—the resulting protein variants exhibit enhanced responses to
various alcohols as compared to the wild type [88]. One plausible mechanism for the alco-
hol-induced sensitivity is local dehydration effects that mimic reduced relative humidity.
Numerous studies have demonstrated the effect of anesthetics on BR, which exhibit sev-
eral general modes of action. Anesthetics modify ion transport at membrane channels and
are of interest in connection with the nerve system. The concentration-induced effects con-
sist of a slight initial blue shift (569-567 nm) to a state with an accelerated M state. Higher
anesthetic concentrations result in a larger blue shift (480 nm), a prolonged M state, and a
loss of both proton-pumping function and the purple membrane lattice structure. Some
anesthetics will produce a 380-nm-absorbing species. At lower anesthetic concentrations,
most effects were found to be reversible. X-ray studies indicate that anesthetics bind at the
lipid protein interface in the center of a BR trimer [91,96]. Other studies have posited dis-
tinct binding sites [93,99,108]. Generally speaking, therefore, chemical agents have several
modes by which they can modulate protein response, including direct interaction with the
protein, or indirect interaction with the lipids proximal to the protein.
Various chemicals have been used in the past to modulate BR's response to light
through modulation of the M and O states. Although these attempts were targeted at pho-
tonic devices (e.g., optical and holographic memories and spatial light modulators), they
are indicative of the BR's ability to respond to and register the state of its environment.
Furthermore, as detailed above, a wide variety of other chemical species have proven to
modulate the protein's response, indicating the potential role BR might play as a chemical
sensor in hybrid protein-semiconductor architectures. Given the ability to interrogate with
light, signals can be extracted that are a reflection of the protein's environment.
Bacteriorhodopsin's potential in hybrid protein-semiconductor devices stems from its
sensitivity to external molecular stimuli, which registers as either a modulation of the pho-
tocycle kinetics or the photovoltaic effect, or a combination of both [89-102,109]. Thus, it
is possible to use either photocycle kinetics [93,95,96] or photovoltaic responsivity
