Applications in Redox Protein Electrochemistry

The electrochemistry of redox proteins is characterized by a strong dependence on the nature of the electrode surface. Extensive studies by Hill etal. (cited in Refs. 31-33) show that provided the electrode surface is modified to be compatible with the redox protein, direct electrochemistry can be rapid. Their studies have emphasized the importance of the orientation of the protein at the electrode surface so that the distance over which the electron must transfer is not excessive. This is important because redox sites in these proteins are generally located toward one side of the protein, and the exponential dependence of the electron transmission coefficient/ei on distance means that the rate of electron transfer drops rapidly as distance increases (Fig. 9.11). Most of this work has used low molecular weight modifiers adsorbed at the electrode surface, although similar effects should be possible at polymer-modified electrode surfaces. 

To date studies using electroactive polymers with redox proteins have been much less numerous despite the fact that it should be possible to extend the general principles elucidated by Hill and colleagues using adsorbed monolayers at electrode surfaces to design suitable electroactive polymers for this application. For example studies with poly(5-carboxyindole), a conducting polymer formed by electropolymerization of 5-carboxyindole have shown that it can be used for the direct 

FIGURE 9.11. Effect of distance on the rate of electron transfer. The diagram is drawn schematically to represent cytochrome c, where the redox center is located close to the one side of the protein. Depending on the orientation of the protein at the electrode surface, the distance over which the electron must transfer, and hence the relative rate of electron transfer kreu varies. 

FIGURE 9.12. Proposed structure for poly(5-carboxyindole). (From Ref. 35.) Note added in proof Subsequent work by Mackintosh and Mount [J.G. Mackintosh and A.R. Mount, J. Chem. Soc., Faraday Trans. 90, 1121 (1994)] has shown that the polymerization of 5-carboxyindole leads to the formation of a polymer of asynunetric trimers of 5-carboxyindole rather than the linear polymer postulated by Bartlett et al in their original work. 

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