Ferrocene derivatives oxidation-reduction potentials

An alternative biosensor system has been developed by Hart et al. [44] which involves the use of the NAD+-dependent GDH enzyme. The first step of the reaction scheme involves the enzymatic reduction of NAD+ to NADH, which is bought about by the action of GDH on glucose. The analytical signal arises from the electrocatalytic oxidation of NADH back to NAD+ in the presence of the electrocatalyst Meldola s Blue (MB), at a potential of only 0Y. Biosensors utilising this mediator have been reviewed elsewhere [1,17]. Razumiene et al. [45] employed a similar system using both GDH and alcohol dehydrogenase with the cofactor pyrroloquinoline quinone (PQQ), the oxidation of which was mediated by a ferrocene derivative. 

The deprotonated form, 1-FcAq, shows reversible two-step 1 e reduction at = -1.26 and -1.71 V versus ferrocenium/ferrocene (Fc /Fc) derived from the anthraquinone moiety, and reversible 1 e" oxidation at = 0.22 V due to the fer-rocenyl moiety in BU4NCIO4-CH2CI2 (Table 3.4). The first reduction potential shifts dramatically in the positive direction to E = -0.06 V, and the oxidation potential shifts moderately in the positive direction to E = 0.33 V in the protonation product, [1-FvAqH], whereas the second reduction potential is little changed. These results correspond to the structural changes in both ferrocenyl and anthraquinone moieties by protonation. 

The ability of the ruthenacycle to assist the interaction between the ferrocenyl moieties was evaluated by the stability of the mixed-valence state of the 2,5-diferrocenyl derivative, 35.79 A reversible reduction wave of the ruthenacycle was observed at —1.40 V vs. Fc+/Fc, and two quasi-reversible waves of the ferrocene moieties appeared at —0.01 and 0.23 V vs. Fc+/Fc in the cyclic voltammogram (Figure 9.11). The separation of the oxidation waves by the ferrocenyl moieties in 35 indicated the existence of an electronic interaction between the two ferrocene moieties. The difference in potential of the two waves, 0.24 V, was smaller than... 

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