Redox mediators chemical structures

Since mainly the E° of the mediator dictates at what potential the heterogenous electron transfer occurs, the oxidation of NADH can now take place at a much lower potential. The different mediator structures used to produce CMEs for NADH oxidation at a decreased overpotential are summarized in Table I. As is seen in the table, not only chemically modified electrodes based on only immobilized redox mediators have been used for this purpose, but also electrodes based on the combination of redox mediators and NADH oxidizing enzymes (diaphorase and NADH dehydrogenase) as well as electrodes made of the conducting radical salts of tetrathiafulvalinium-7,7,8,8-tetracyanoquinodimethan (TTF-TCNQ) and W-methyl-phenazin-5-ium-7,7,8,8-tetracyanoquinodimethan (NMP-TCNQ). 

Figure 6.8 Chemical structures of some common redox mediators (a) dimethyl ferrocene (b) tetrathiafulvalene (c) tetracyanoquinodimethane (d) Meldola Blue.

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

Fig. 8 (a) Chemical and stylized representation of the strategy of redox-mediated molecular brake passing from sulhde to sulfoxide and sulfone (b) an example of oxygen-flipped rotary switch (c) its stylized representation (d) X-ray structure of a bisarylanthracene peroxide (H atoms were omitted for clarity) (e) control of the frequency of molecular motions in rotaxanes of which annulus (macrocycle) contains a photoisomerizable dianthrylethane group (see text for details)... 

What are the most important properties of redox mediators suitable for biosensors. First of all, the electrochemistry has to be reversible and they need to be stable in the oxidized and reduced forms. No side reactions should occur. The redox potential needs to be compatible with the enzymatic reaction. It is helpful if the basic structure of the redox mediator also allows for chemical modifications... 

In addition to self-generated shuttles, synthetic compounds with similar chemical structures also show the same function. Changing the molecular structure of phenazine-type redox mediators by artidcial synthesis can signid-cantiy induence microbial extracellular electron transfer. Immobilization... 

Figure 1.54 Chemical structure of various forms (oxidation states) of PANI and proton-mediated interconversion between them via redox chemical reactions.

Composite sol-gel electrodes containing GOx and coimmobilized redox mediators have been applied for the preparation of glucose biosensors [111-115]. In these procedures, the redox mediator is either added during the gellification process (resulting in its physical entrapment in the silicate structure) [112] or is chemically bound to the silicate network (e.g. Ai-(3-trimethoxysilylpropyl) ferrocenylacetamide (18) may be used as a functionalized comonomer) [114,... 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

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