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).
Besides that it could be shown that reduced neutral red is also capable as the sole source of reducing power for growth and metabolism of H2 consuming bacteria cultures.(Park, D. H. & Zeikus, J. G. 2000)
The chemical structure is shown in figure 1.

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

Bromophenol Blue

Bromphenolblue is a triarylmethane dye that is similar to neutral red and also capable to function as mediator.

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

Cytochromes

Cytochromes are hemoproteins which act as a part of electron transport in the respiratory chain. Depending on the characteristics of the cytochrome it can be assigned to one of three main groups, called cytochrome a, b and c.
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 as 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.
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 electrondonor as well as an acceptor by changing the redox state of the iron atom of their porphyrin complex.
The heme group of cytochrome c is shwon in figure 3.

Figure 3:heme group of cytochrome c

Thus the fact that the heme groups of cytochromes are located in the inner core of the protein is an advantage for reduction/oxidation reactions as well as a problem at the same time. For the avoidance of unwanted reaction with gases like oxygen or carbon monoxide the structure of c-type cytochromes is favourable because it ensures that the heme group is sufficiently protected from these molecules. However it is assumed that this structure could also be a reason why the direct electron transfer between the electrode and the heme group could be limited or blocked because the heme group is poorly accessible. For that reason the electrode material is essential and different ones have to be tested for optimization of direct electron transfer via cytochromes.

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

• 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.