Electron-transfer relays

Without illumination, the reaction proceeds slowly, but by no means it is negligible 8% of phenylpinac-olin is formed, with no regard to the prolonged duration. The addition of ferrous chloride in amounts of 40% to molar equivalent of Phi results in incisive acceleration of this reaction. The disappearance of Phi is observed within 20 min, replaced by 74% of the substitution product, PhCH2COCMe3, and ca. 10% of the disubstitution prodnct, Ph2CHCOCMe3. The authors cite diverse data, theorizing that iron(II), associated with the enolate ion, acts as an electron-transfer relay between the enolate and phenyl iodide (Scheme 5.25). 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

Fig. 7.15 Oxidase enzyme wired with electron transfer relays (adapted from Heller, 1999)...

An electron transfer relay may also operate via the electronic coupling term, HDA. Mixing between the electron-donor and acceptor orbitals provides the physical basis for electron coupling. In view of the exponential decay of atomic wavefunctions and their corresponding linear combinations, it is not surprising that Hda exhibits an exponential dependence on the distance between the electron-acceptor (A) and donor (D) [197],... 

Fig. 1.12. Schematb drawing of the glucose oxidase molecule, showing the electron-transfer distances involved in the various steps of moving an electron from its two flavin adenine dinucleotide/reduced flavin adenine dinucleotide (FAD/FADHg) centers to a metal electrode. Left The enzyme before modification. Right The modified enzyme, after chemical attachment of an array of electron transfer relays ( R ). (Reprinted from Y. Degani and A. Heller, J. Phys. Chem. 91 1286, 1987.)...

Enzyme biosensors containing pol3mieric electron transfer systems have been studied for more than a decade. One of the earlier systems was first reported by Degani and Heller [1,2] using electron transfer relays to improve electrochemical assay of substrates. Soon after Okamoto, Skotheim, Hale and co-workers reported various flexible polymeric electron transfer systems appUed to amperometric enz5une biosensors [3-16], Heller and co-workers further developed a concept of wired amperometric enzyme electrodes [17—27] to increase sensor accuracy and stability. 

Electron-transfer relay systems binding electron transfer relays to enzyme proteins... 

Unfortunately, the response of sensors containing glucose oxidase and ferrocene modified electron-transfer relay system began to deviate fi om linearity below 10 mM of glucose concentration. Additionally, sensor sensitivity toward oxygen and oxidation of interferent species at operating potential has yet to be solved. 

When ferrocene-containing polysiloxane proved to be an efficient electron-transfer relay system, further modification of this type redox pol3uner was investigated to develop optimal enzyme biosensors. Attempts were made to synthesize redox polymers with different mediators and/or different polymer backbones and/or different side chains through which mediators are attached to the polymer backbone. Resulting redox poisoners were tested to construct different types of enzyme sensors. 

Iron can be said to be the most chemically versatile of all the elements used by nature. It is essential for dioxygen uptake and transport in the vast majority of living systems, is ubiquitous in electron transfer relays and oxygen metabolism, is used widely as the catalytic center in enzymes catalyzing chemical changes as diverse as dinitrogen fixation, nitrate reduction, and isopenicillin-N-synthase, and is vital for DNA... 

Degani Y, Heller A (1988) Direct electrical communication between chemically modified enzymes and metal electrodes. 2. Methods for bonding electron-transfer relays to glucose oxidase and D-amino-acid oxidase. J Am Chem Soc 110 2615-2620... 

Degani, Y., Heller, A., Direct Electrical Communication between Chemically Modified Enzymes and Metal Electrodes. 2. Methods for Bonding Electron-Transfer Relays to Glucose Oxidase and D-Amino-Acid Oxidase , J. Am. Chem. Soc. 110 (1988) 2615-2620. 

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