Neutral Red

Neutral red (3-Amino-7-dimethylamino-2-methylphenazine hydrochloride) is a phenazine-based dye which is normally used as pH-indicator due to the fact that it changes its colour from red (pH 6.8) to yellow (pH 8.0).
It could be shown that reduced neutral red is also capable as the sole source of reducing power for growth and metabolism of H₂Park, D. H. & Zeikus, J. G. 2000)
The chemical structure is shown in figure 1.

Figure 1: Chemical structure of the triphenylmethane dye neutral red.

Bromphenol Blue

Bromphenol blue is a triarylmethane dye that is similar to neutral red and is normally used as a biological stain or as a pH indicator.(NCBI:PubChem Compound) Having a redox potential of -739 mV vs. SCE it is also capable to function as a mediator.(Strehlitz et al., 1994)
Figure 2 shows the chemical structure of bromphenol blue.

Figure 2: Chemical structure of the triphenylmethane dye bromphenol blue.

Cytochromes

Apart from the synthetic mediators like neutral red or bromphenol blue there are naturally occuring mediators like cytochromes. Cytochromes are hemoproteins which are part of electron transport in the respiratory chain and therefore they play a role in direct electron transfer from an electrode to the cells. Depending on its characteristics cytochromes can be assigned to one of three main groups, cytochrome a, b and c.(Spektrum, 2001)
Members of the cytochrome a class are the reduction/oxidation units of the cytochrome-oxidase which catalyzes the electron transport from cytochrome c to oxygen, the final electron acceptor. B-type cytochromes are known to have the lowest redoxpotential of the respiratory chain and are located between ubichinon and cytochrome c.(Spektrum, 2001)
However, c-type cytochromes are the best studied and most abundant group of cytochromes. For metal-reducing bacteria it has been already shown that several of these c-type cytochromes are essential for electron transfer between an electrode and the cell. Therefore they can function as an electron donor as well as an electron acceptor by changing the redox state of their heme group. The heme group consists of an iron atom surrounded by porphyrin rings and is shown in Figure 3.(Qiao et al., 2010)

Figure 3:heme group of cytochrome c

These heme groups are located in the inner core of the protein which is an advantage for reduction/oxidation reactions as well as a problem at the same time. To avoid side reactions with gases like oxygen or carbon monoxide the position of the heme group is favourable, because it ensures that it is sufficiently protected from an unwanted reactant. However, it is assumed that the location in the inner core of the protein and the resulting poor accessibility could also be a reason for a limited or blocked direct electron transfer between the electrode and the heme. For that reason the electrode material is key factor and different materials have to be tested to optimize the direct electron transfer via cytochromes.(Qiao et al., 2010)
One of the best known organisms that are able to accept electrons from an electrode is Geobacter sulfurreducens. It has been demonstrated that current-consuming biofilms of this organism highly express a periplasmic key-type cytochrome c with one heme group. The deletion of the corresponding gene GSU 3274 leads to a total inhibition of electron uptake.(Strycharz et al., 2011)
This indicates the importance of this c-type cytochrome for current-consuming biofilms and caused us to consider it as an potential alternative to realize enhanced electron uptake in E. coli.

References

• Park, D. H. & Zeikus, J. G. (2000) Electricity generation in microbial fuel cells using neutral red as an electronophore. In: Applied and Environmental Microbiology, 66 (4), pp. 1292 - 1297.

• Azariah, A. N., Berchmans, S., Yegnaraman (1998) Electrochemical behaviour of neutral red. Bulletin of Electrochemistry. , 14 (10), pp. 309-314

• Fultz, M. L., Durst, R. A. (1982): Mediator compounds for the electrochemical study of biological redox systems: a compilation Analytica Chimica Acta. ,140, pp. 1-18

• Park, D. H.,Laivenieks, M., Guettler, M. V., Jain, M. K. & Zeikus, J. G. (1999) Microbial utilization of electrically reduced neutral red as the sole electron donor for growth and metabolic production. In: Appl. Environ. Microbiol., 65 (7), pp. 2912 - 2917.

• NCBI:PubChem Compound: Bromphenol Blue

• Strehlitz, B., Gründig, B., Vorlop, K.-D., Bartholmes, P., Kotte, H., Stottmeister, U. (1994) Artificial electron donors for nitrate and nitrite reductases usable as mediators in amperometric biosensors. In: Fresenius' Journal of Analytical Chemistry, 349, pp. 676-678.

• Spektrum Akademischer Verlag (2001): Kompaktlexikon der Biologie - Cytochrome

• Qiao, Y., Bao, S. & Li, C. M. (2010): Electrocatalysis in microbial fuel cells—from electrode material to direct electrochemistry. In: Energy Environ. Sci. 3 (5), pp. 544 - 553.

• Stycharz, S. M., Glaven, R. H., Coppi, M. V., Gannon, S. M., Perpetua, L. A., Liu, A., Nevin, K. P. &amp. Lovley, D. R. (2011):Gene expression and deletion analysis of mechanisms for electron transfer from electrodes to Geobacter sulfurreducens. In: Bioelectrochemistry 80, pp. 142 - 150.
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