Electron tunneling pathways

Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4]. The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. 

In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 

Winkler JR. Electron tunneling pathways in proteins. Curr Opin Chem Biol 2000 4 192-8. 

Beth, J.N., Beratan, J.N., and Onuchic, J.N. (1992) Mapping electron tunneling pathways an algorithm that finds the minimum length Vmaximum coupling pathway between electron donors and acceptors in proteins, J. Am. Chem. Soc. 114, 4043-4046. 

Beratan, D. N., Nelson Onuchic, J., Winkler, J. R., and Gray, H. B., 1992, Electron-tunneling pathways in proteins. Science 258 1740nl741. 

Fig. 13.16 Alternative electron tunneling pathways from ruthenated His62 to the heme in a mutant yeast cytochrome c. [From Bowler, B. E. Meade.

Onuchic. J. N. Betts. J. N. Bowler, B. E. Gray. H. B. J. Am. Chem. Soc. 1990, 112, 7915-7921. 2 In many ways, the electronic tunneling pathway is analogous to various electric circuits with ditferent resistors The longer the circuit" in these systems, the greater the resistance. Hydrogen bonds tend to increase resistance and open space has the greatest of all. 

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