Double electron transfer rate

The decrease of the concentration of the electroactive species with increasing potential has to be attributed to double layer effects. As first pointed out by Frumkin [58], in dilute solutions the electron transfer rate is affected by variations of the potential in the double layer in two ways. The potential in the outer Helmholtz plane, fa, is due to the extension of the double layer not identical to the potential in the solution (at the end of the double layer), so that the effective driving force of the reaction is DL — fa. Furthermore, the concentration of ionic reactants in the reaction plane, c, is influenced by electrostatic effects and differs from the concentration just outside the double layer, c0, by a Boltzmann term ... 

A doubly metallated 15 base-pair double helix containing ruthenium and rhodium at each end of the strands [106] showed the efficiency of DNA for coupling electron donors and acceptors over a very long range, greater than 40 A. The DNA double helix was found to behave like a piece of molecular wire with fast electron-transfer rates (>1010 s l) for the photoinduced electron transfer between the metallointercalators [107-109] and semiempirical Hartree-Fock calculations of HAB for DNA mediated electron transfer [110] were described. 

In the linked systems studied by Wegewis and Verhoeven [14], electron-transfer rates were estimated by the same methods—quenching of the acceptor fluorescence and linewidth measurements. The rates were found to vary over many orders of magnitude in the weakly coupled systems they are 10 -10 s (for the reduced forms lr-3r), increasing to lO" s for the double bond conjugated forms 1-3, and... 

This paper attempts to model and define the conditions under which platinum Eh measurements are likely to reflect the true electrical potential of aqueous solutions. The double layer at the surface of the electrode is modeled as a fixed capacitor (C jj), and the rate at which an electrode equilibrates with a solution (i.e. the rate at which C jj is charged) is assumed to be proportional to the electrical current at this interface. The current across the electrode/solution interface can be calculated from classical electrochemical theory, in which the current is linearly proportional to the concentration and electron-transfer rate constant of the aqueous species, and is exponentially proportional to the potential across the interface. 

TABLE 13.7.1 Typical Experimental Results Showing Corrections of Heterogeneous Electron-transfer Rate Data for Double-Layer Effects... 

Fig. 2.18 An equivalent circuit representing an electrode/solution interface. The electrode surface is covered by a monolayer of a redox-active species. e ac potential across the faradaic unit of equivalent circuit, Ca double-layer capacitance, Rs -uncompensated solution resistance, Zf impedance representing solely the electron transfer reaction process of the monolayer, )> ac current due to the faradaic process, Z, total impedance of the whole system, ks. heterogeneous electron transfer rate constant of the monolayer of electroactive species, R charge transfer resistance, Q capacitance associated with the redox reaction of the adsorbed species.

The composition of the double layer influences the electron transfer rate (see Sect. 1.3.1.5). Some ions and molecules specifically adsorbed at the eleetrode surface enhance the rate of the electrode process. In such a situation, we talk about heterogeneous electrocatalysis. On the other hand, there are numerous compounds that, after adsorption, decrease the elecdon tfansfer rate and therefore are simply... 

Saji T, Yamada T, Aoyagui S (1975) Electron-transfer rate constants for redox systems of Fe(III)/Fe(II) complexes with 2,2 -bipyridine and/or cyanide ion as measured by the galvanostatic double pulse method. J Electroanal Chem Interfacial Electrochem 61 147-153... 

Recently, collaboration between the author, A-E. Nassar, and N. Nakashima resulted in the preparation of stable films of calf thymus double-stranded DNA and proteins on electrodes [55]. Direct electron transfer was achieved for myoglobin or hemoglobin in DNA films on pyrolytic graphite (PG) electrodes. As with the surfactant films, enhanced electron transfer rates were achieved compared to bare PG electrodes with proteins in solution. DNA films also extracted proteins from solution. Mb appears to diffuse through pure DNA films such faster than Hb. Conformational changes in both DNA and protein upon binding are likely within these films. DNA-protein films may find applications in electrochemical and spectroscopic studies of DNA-protein and DNA-enzyme-sub-strate interactions, and as biosensors for proteins. 

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