Solid electrodes, studies with cytochrome

In a recent report, it was demonstrated that adsorption of 4,4 -bipyridine on platinum led to quasi-reversible rates of electron transfer with cytochrome c as evidenced by cyclic voltammetry. However, the concentration of 4,4 -bipyridine required to produce this electrochemical response was five times that which is required at gold electrodes. This difference was ascribed to the difference in the tendency of 4,4 -bipyridine to adsorb at gold and platinum electrodes. These results indicate that the use of 4,4 -bipyridine may be applicable to other solid electrodes as well for the study of cytochrome c electron transfer reactions. 

At this point, the direct electrochemistry of cytochrome c at a host of solid electrodes had become well controlled, stable, and quasi-reversible. This group began to then use this platform to study properties of cytochrome c using direct electrochemical methods, often conpled with optical probes. Stndies of the temperature dependence of the formal redaction potential and the heterogeneons electron transfer kinetics were subseqnently reported, and reaction center entropy valnes were shown to agree well with earlier reports in work by Kent B. KoUer... 

Some microorganisms can directly transfer electrons to the electrode via a physical contact of the cell membrane or a membrane organelle with the anode. No diffusional redox species are involved in this electron transfer process. As illustrated in Figure 2.6a, the direct electron transfer requires the microorganisms to possess (1) membrane-bound protein relays which transfer electrons from the inside of the bacterial cell to its outside, and (2) an outer membrane (OM) redox protein which accepts the electrons and delivers them to an external, solid electron acceptor (a metal oxide or an MFC anode). The most studied OM redox proteins are c-type cytochromes, which are involved in metal-reducing microorganisms such as Geobacter, Rhodqferax and Shewanella. These bacteria often have to rely on solid terminal electron acceptors like iron(lll) oxides in their natural environments. 

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