Biomedical Engineering Reference
In-Depth Information
15.3
Electron as a Charge Carrier: An Artificial Light-Driven Electron Pump
We shall begin the analysis with a minimalist approach by devising a simple model mem-
brane system with well-defined photochemical reactions that exhibits both the AC and the
DC photoelectric effects [34-36]. The choice of such a simple system is necessary to avoid
ambiguity in molecular and mechanistic interpretation. This requirement is fulfilled by
redox reactions of lipid-soluble magnesium porphyrins. Magnesium octaethylporphyrin or
long-chain alkyl esters of magnesium mesoporphyrin IX were used as the membrane-bound
light-sensitive redox component, whereas potassium ferricyanide, K
3
Fe(CN)
6
, and ferro-
cyanide, K
4
Fe(CN)
6
, were used as the water-soluble electron acceptor and donor, respec-
tively. The redox reaction in the dark is a reversible one-electron-transfer reaction [37,38]:
3
4
PFe(CN)
P Fe(CN)
,
6
6
where P and P
+
are the ground-state neutral magnesium porphyrin and the corresponding
oxidized monocation, respectively. The oxidized pigment monocation is thermodynami-
cally stable. The photochemistry is also simple and devoid of side reactions:
P
*
+ Fe(CN)
3−
6
P
+
+ Fe(CN)
4
6
h
P + Fe(CN)
3
6
where P
*
is neutral magnesium porphyrin in the excited state.
In the model membrane, the magnesium porphyrin molecules are confined to the mem-
brane phase because of hydrophobicity conferred by esterifying magnesium porphyrins
with long alkyl chain alcohols (up to 20 carbons). The electron-transfer (redox) reactions
between the membrane-bound magnesium porphyrin and the aqueous-borne ferri-
cyanide/ferrocyanide can only take place across a membrane-water interface (interfacial
electron-transfer reactions). Thus, there are two interfacial redox (electron-transfer) reac-
tions, one at each interface. Since magnesium porphyrin molecules are mobile in the mem-
brane phase, vectorial electron transport is made possible by imposing an aqueous redox
gradient across the membrane. The transmembrane diffusion of P and P
allows the two
interfacial electron-transfer reactions to exchange reactants and products, that is, the two
interfacial reactions are coupled by transmembrane diffusion of P and P
.
This model system constitutes a rudimentary photosynthetic system; light pumps elec-
trons unidirectionally across the membrane. Although this simple system is not as efficient
as natural photosynthetic systems, it captures the essence of a minimalist scheme for
photosynthesis and serves to conceptually link chlorophyll-based photosynthetic mem-
branes to the purple membrane of
Halobacteria
.
In a typical experiment, one of the two aqueous phases (called the oxidant side) contains
predominantly ferricyanide ions (e.g., 100 mM potassium ferricyanide with or without a
much smaller amount of potassium ferrocyanide ranging from 0.5 to 3 mM) whereas the
other aqueous phase (the reductant side) contains predominantly potassium ferrocyanide
(with or without a trace amount of ferricyanide) (Figure 15.1). The asymmetry in the redox
gradient across the membrane allows more P
to be produced at the oxidant side than at
