Biomedical Engineering Reference
In-Depth Information
most photosynthetic bacteria), the primary reaction is charge separation involving the pro-
duction of electrons and holes. Electron transfers dominate the process of energy conver-
sion along the so-called electron transport chain, except at the membrane-water interfaces
where the moving part is switched from electrons to protons. The net result is a trans-
membrane proton transport in the opposite direction. This conversion is implemented
with the intervention of a special class of mediators known as quinoid compounds. They
form the cofactors for some redox enzymes: NAD
(nicotinamide adenine dinucleotide) in
the mitochondria inner membrane and NADP
(nicotinamide adenine dinucleotide phos-
phate) in the thylakoid membrane of Photosystem I. These enzymes operate in the aque-
ous phase adjacent to the membrane surface. Two additional quinoid compounds,
plastoquinone and ubiquinone, serve as membrane-bound mobile electron carriers in the
thylakoid and the mitochondrial inner membrane, respectively. They are both redox and
acid/base compounds. The prototype reaction of quinoid compounds is shown in the ben-
zoquinone-benzohydroquine reaction:
Protonated
semiquinone
Hydroquinone
Benzoquinone
O
•
O
OH
+ e
−
+ H
+
+ e
−
+ H
+
−
e
−
−
H
+
e
−
−
H
+
−
O
OH
OH
The structure of quinoid compounds mentioned above contains the benzoquinone moi-
ety. Electrons transferred to enzyme NADP
-ferredoxin-oxidoreductase are converted
into mobile protons:
2Fd
2e
NADP
2H
2Fd
NaDPH
H
red
ox
where Fd
red
and Fd
ox
are the reduced and oxidized form of NADP
-ferredoxin-oxidore-
ductase, respectively. Therefore, a solution for DC-current interfacing of an oriented pur-
ple membrane thin film to a metal electrode can be found by choosing a suitable quinoid
compound to make the required conversion. It may not be necessary to search for such a
compound in natural sources because it may be obtained by means of molecular engi-
neering, using the recombinant DNA technology.
15.8
Discussions and Concluding Remarks
The high-technology revolution during the latter half of the 20th century, which made per-
sonal computers standard office equipment and home appliance, was mainly fueled by
the astonishing advances in microelectronics. Miniaturization allowed more and more cir-
cuit elements to be packed into a small integrated circuit (IC). As indicated by the well-
known Moore's law, the number of the device components contained in a single IC grew
exponentially with the passage of time. Will this trend continue so that device size even-
tually reaches the atomic scale? According to experts, the trend is still going strong but it
certainly cannot continue beyond bounds [111,112]. This consideration has ushered in a
multidisciplinary research effort in molecular electronics [113-117]. bR played a prominent
