Diffusional electron mediators

Scheme 7 Electronic transduction of photo-switchable bioelectrocatalytic functions of proteins, (A) by the tethering of photoisomerizable units to the protein (R is a diffusional electron mediator that electrically contacts the redox...

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

Fig. 12 Photoswitchable bioelectrocatalyzed oxidation of glucose (8 x 10-2 M) by a composite monolayer consisting of COx reconstituted onto FAD units and nitrospiropyran photoi-somerizable units in the presence of 29 as a diffusional electron mediator, (a) In the presence...

It also was found that the direction of the photobiocatalytic switch of the nitrospiropyran-FAD-reconstituted enzyme is controlled by the electrical properties of the electron transfer mediator. With ferrocene dicarboxylic acid as a diffusional electron transfer mediator, the enzyme in the nitrospiropyran-FAD state (10a) was found to correspond to the OFF state bio-catalyst, while the protonated nitromerocyanine state of the enzyme (10b) exhibits ON behavior. In the presence of the protonated 1-[1-(dimethyl-amino )ethyl]ferrocene, the direction of the photobioelectrocatalytic switch is reversed. The nitrospiropyran-enzyme state (10a) is activated toward the electrocatalyzed ox idation of glucose, while the protonated nitromerocyanine enzyme state (10b) is switched OFF, and is inactive for the electrochemical oxidation of glucose. This control of the photoswitch direction of the photoisomerizable reconstituted enzyme was attributed to electrostatic interactions between the diffusional electron mediator and the photoisomefizable unit... 

Figure 3-31. Cyclic voltammograms corresponding to the photoswitchable bioelectrocatalyzed oxidation of glucose, 50 mM, in the presence of ferrocene carboxylic acid, (21), 5x 0 M, as diffusional electron mediator (a) and (c) In the presence of the SP-GOx monolayer electrode generated by the irradiation of the electrode A, > 475 run. (b) and (d) In the presence of the MRlT-GOx monolayer electrode generated by the illumination of the electrode with filtered light 320 nm < A < 380 nm. Inset cychc photoswitchable ON and OFF amperometric responses of the functionalized enzyme monolayer upon the light-induced isomerization of the interface between the SP GOx and MRI I GOx, respectively. Reproduced with permission from ref. 88. Copyright 1997 American Chemical Society.

The bioelectrocatalyzed oxidation of glucose in this system originates from the primary oxidation of the ferrocene carboxylic acid, (21), to the respective ferrocenylium cation that mediates the oxidation of the enzyme s redox center and its activation towards the oxidation of glucose. Photoisomerization of the enzyme monolayer to the MRH-GO state switched-off the bioelectrocatalytic functions of the protein monolayer, and only the electrical response of the diffusional electron mediator was observed, Fig. 3-31, curves (b) and (d). By the cyclic photoisomerization of the enzyme-monolayer electrode between the SP-GOx and MRlT-GOx states, the reversible photoswitching of the enzyme activity between ON and OFF states was demonstrated, Fig. 3-31 (inset). 

In configuration A, ET from the enzyme redox center to the electrode is made possible by addition of a diffusional electron mediator that rapidly shuttles the electron (s) to and from the electrode. In configuration B, redox labels are covalently tethered to the enzyme surface and relay the electrons to and from the electrode. 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [386, 389] (Fig. 35). Diffusional electron mediators (viologen (30) or ferrocene (31) derivatives) were functionalized with photoisomerizable spiropy-ran/merocyanine units. These mediators can be reversibly photoisomerized from... 

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