Relay units

It is a relaying unit to receive actual parameters from the field and relay out corrective signals... 

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

Albeit the substantial progress in the bioelectrochemical activation of enzymes, one could identify two important future challenges in the field (i) The active relay units wiring the redox centers of the enzymes with the electrodes could be generated by photoinduced electron transfer. This could pave the way to the photochemical wiring of enzymes and to the development of photobiofuel cells, (ii) DNA scaffolds provide unique templates for the ordered self-assembly of molecular or biomolecular units through dictated hybridization. The ordering of relay units and enzymes, or of relay units photosystems, on DNA templates associated with electrodes may yield attractive new supramolecular nanostructures for bioelectronics and optobioelec tronic s. 

Murray, C. L. Shabany, H. Gokel, G. W., (2000) The central relay unit in hydraphile channels as a model for the water-and-ion capsule of channel proteins Chem. Commun. 2371-2372. 

The chemical modification of redox enzymes with electron relay groups permits the mediated electron transfer and the electrical wiring of the proteins [83-85] (Figure 5A). The covalent attachment of electron-relay units at the protein periphery, as well as inner sites, yields short inter-relay electron-transfer distances. Electron hopping or tunneling between the periphery and the active site allows electrical communication between the redox enzyme and its environment. The simplest systems of this kind involve electron relay-functionalized enzymes diffusionally communicating with electrodes [83], but more complex assemblies including immobilized enzymes have also been reported. 

Iron relay units provide a route for electron hopping between the electrode and the active redox center of the protein, and thus contribute to the electrical contacting of the enzyme layer with the electrode [88],... 

Enzyme entrapment in polymers functionalized with redox relay units... 

Figure 39. Electrical communication between an enzyme redox center and a photoexcited species attaining light-induced biocatalyzed transformations (A) direct electrical wiring of the protein by its chemical modification with tethered electron-relay units (B) electrical communication by the immobilization of the protein into a redox-functionalized polymer matrix.

Major polymer applications automotive lighting, ignition and braking systems, carburetor parts, fuel components, chip carriers, phone jacks, IC card connectors, transistor encapsulation, tape recorder head moimts, relay components, motor fans, coil bobbins, sockets, relay units, food choppers, steam hair drier parts, lamp sockets, microwave oven components, pump housings, impeller diffusers, oil well valves, halogen lamp sockets... 

The need to improve the electrical communication between redox proteins and electrodes, and the understanding that the structural orientation at the molecular level of redox proteins and electroactive relay units on the conductive surfaces is a key element to facilitate ET, introduced tremendous research efforts to nano-engineer enzyme electrodes with improved ET functionalities. The present chapter addresses recent advances in the assembly of structurally aligned enzyme layers on electrodes by means of surface reconstitution and surface crosslinking of structurally oriented enzyme/cofactor complexes on electrodes. The ET properties of the nano-structured interfaces is discussed, as well as the possible application of the systems in bioelectronic devices such as biosensors, biofuel cell elements or optical and electrical switches. 

By one method a relay-cofactor dyad is assembled on the electrode, and the respective apo-protein is reconstituted on the surface to yield an aligned protein that is linked to the conductive surface by the relay component. The second method involves the synthesis of the relay-cofactor unit and the reconstitution of the apo-protein in solution. The specific immobilization of the enzyme on the electrode by the relay unit provides the structurally organized enzyme electrodes. While the first method is technically easier, the second methodology that involves tedious synthetic and separation steps, permits the fundamental structural characterization of the reconstituted protein. In the two configurations, the redox enzymes are anticipated to be electrically contacted with the electrode by means of the relay, a conductive... 

Electrodes functionalized with monolayers of enzyme cofactors (e.g. NAD+-monolayers) demonstrate the ability to form stable affinity complexes with their respective enzymes [301]. These interfacial complexes can be further cross-linked to produce integrated bioelectrocatalytic matrices consisting of the relay-units, the cofactor, and the enzyme molecules. Electrically contacted biocatalytic electrodes of NAD+-dependent enzymes have been... 

Bioelectronics is another apphcation area, in which rotaxanes, particularly redox-active rotaxanes, could make a significant impact Enzyme electrodes are altered in these apphcations by direct electron transfer between the electrode surface and the redox enzyme. Electronic communication between the surface and the redox enzyme centers is hindered, because a separation exists. This impediment can be circumvented by aligning the enzyme with the electrode and utilizing the redox relay units as go-betweens. The aforementioned concept has been exploited to associate an apoprotein, apo-gjucose oxidase (apo-GOx), onto relay-functionalized materials including flavin adenine dinucleotide (FAD) monolayers, nanoparticles, and carbon nanotubes [85-88]. Katz etal. used reversible redox-active rotaxane shuttles in the bioelectrocatalyzed oxidation of glucose [80]. 

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