Team:Bielefeld-CeBiTec/Project/rMFC/Mediators

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rMFC

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.
That indicates that based on its redox-potential it can function as an electron-shuttle from the electrode to the cells getting reduced by an one step two electron transfer.(Azariah, A. N. et al., 1998)
There is also evidence that electrically reduced neutral red can bind to the cell membrane and chemically reduces NAD. Furthermore it is not toxic to the cells and can replace H2 which is the natural electron shuttle for some bacteria species. (Park et al., 1999)
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. 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. 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 redox-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 c-type cytochromes are essential for electron transfer between an electrode and the cell.

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.
  • 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.

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