Homogeneous Electron Transfer Kinetic Studies

A chronoamperometric method for evaluating the kinetics of homogeneous electron transfer between an electrochemically generated mediator/reactant and biological molecules was subsequently reported by Ryan et aiy The effects of solution ionic strength and the charge of the electrochemically generated reactant on the electron transfer kinetics with cytochrome c and cytochrome C2 were reported in that work. 

Several reports have evaluated the homogeneous electron transfer kinetics of cytochrome c using potential step spectroelectrochemistry. These reports together with other studies of biological homogeneous electron transfer reaction kinetics are summarized in Table 3. Evaluation of the kinetics of these reactions requires caution in that the small diffusion coefficients of the biological molecules studied relative to those of the electrochemically generated reactants mandates consideration of these parameters in data analysis.  

Homogeneous Electron Transfer Kinetic Studies of Biological Molecules by Electrochemical Techniques 

Biological molecule Electrochemically generated reactant Reference 

cation radical of l,r-dimethyl-4,4 -bipyridinium dichloride BV+, cation radical of l,l -dibenzyl-4,4 -bipyridinium dibromide Diquaf, cation radical of l,l -ethylene-2,2 -bipyridinium dibromide V 7o, cation radical of 1,1 -propylene-2,2 -bipyridinium dibromide. 

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The material of this section will focus on results of homogeneous electron transfer kinetic studies involving reactions between electrochemically generated redox reactants and on direct heterogeneous electron transfer kinetic studies. An overview of the work in this area concludes this chapter. 

Homogeneous cross-reaction electron-transfer kinetic studies suggest that many other Cu(II/I) systems obey Scheme 1. However, few Cu(II/I) systems have been subjected to sufficiently low temperature or rapid-scan CV measurements to demonstrate the presence of rate-limiting conformational changes. 

Table 4. Homogeneous electron transfer kinetics between biological molecules and mediators studied by optically transparent electrode...

The intensive electrochemical studies of polycyclic systems, especially cyclic volta-metry (CV) are now at a stage which justifies naming cyclic voltametry an electrochemical spectroscopy as was suggested by Heinze 65). Early electrochemical studies referred only to the thermodynamic parameters while CV studies provide direct insight into the kinetics of electrode reactions. These include both heterogeneous and homogeneous electron-transfer steps, as well as chemical reactions which are coupled with the electrochemical process. The kinetic analysis enables the determination of reactive intermediates in the same sense as spectroscopic methods do. As already mentioned, electron transfer processes occur in both the electrochemical and metal reduction reactions. 

A vast literature exists on the kinetics and mechanisms of electron-transfer reactions between dissolved metal ions. A recent review was written by Sutin (1986). Physical chemists, however, have dealt so far almost exclusively with reactions in aqueous solution of very low pH or high ligand concentrations. Such studies have shown that the homogeneous electron-transfer between couples such as Fe(III)/Fe(II) proceeds via three distinct steps. First the two reactants diffuse together and form a reactive intermediate called the precursor complex. The electron transfer occurs after an appropriate reorganization of the nuclear configuration. This yields a short-lived product called the successor complex. Finally the successor decomposes to the separated products of the redox reaction ... 

Oxidation-reduction processes involving the Fe -Fe couple have been intensively studied, and the elementary electron exchange, taking place either in solution or at an electrode interface, has served for a long time as a test case for theoretical interpretations of electron transfer kinetics. For reactions involving ground-state ions, the mechanisms of the reactions are now well understood for the homogeneous systems, for example, electron transfer can occur, as demonstrated by Taube, via either a simple outer-sphere electron... 

References to a number of other kinetic studies of the decomposition of Ni(HC02)2 have been given [375]. Erofe evet al. [1026] observed that doping altered the rate of reaction of this solid and, from conductivity data, concluded that the initial step involves electron transfer (HCOO- - HCOO +e-). Fox et al. [118], using particles of homogeneous size, showed that both the reaction rate and the shape of a time curves were sensitive to the mean particle diameter. However, since the reported measurements refer to reactions at different temperatures, it is at least possible that some part of the effects described could be temperature effects. Decomposition of nickel formate in oxygen [60] yielded NiO and C02 only the shapes of the a—time curves were comparable in some respects with those for reaction in vacuum and E = 160 15 kJ mole-1. Criado et al. [1031] used the Prout—Tompkins equation [eqn. (9)] in a non-isothermal kinetic analysis of nickel formate decomposition and obtained E = 100 4 kJ mole-1. 

Consequently, a wealth of information on the energetics of electron transfer for individual redox couples ("half-reactions") can be extracted from measurements of reversible cell potentials and electrochemical rate constant-overpotential relationships, both studied as a function of temperature. Such electrochemical measurements can, therefore, provide information on the contributions of each redox couple to the energetics of the bimolecular homogeneous reactions which is unobtainable from ordinary chemical thermodynamic and kinetic measurements. 

The validity of an electroanalytical measurement is enhanced if it can be simulated mathematically within a reasonable model , that is, one comprising all of the necessary elements, both kinetic and thermodynamic, needed to describe the system studied. Within the chosen model, the simulation is performed by first deciding which of the possible parameters are indeed variables. Then, a series of mathematical equations are formulated in terms of time, current and potential, thereby allowing the other implicit variables (rate constants of heterogeneous electron-transfer or homogeneous reactions in solution) to be obtained. 

In the last two decades, studies on the kinetics of electron transfer (ET) processes have made considerable progress in many chemical and biological fields. Of special interest to us is that the dynamical properties of solvents have remarkable influences on the ET processes that occur either heterogeneously at the electrode or homogeneously in the solution. The theoretical and experimental details of the dynamical solvent effects on ET processes have been reviewed in the literature [6], The following is an outline of the important role of dynamical solvent properties in ET processes. 

Quantitative studies using LSV and CV can be carried out for both heterogeneous charge transfer kinetics and the kinetics of homogeneous chemical reactions coupled to charge transfer at electrodes. These methods should continue to play a major role in the study of electron transfer reactions. 

For complex mechanisms such as ECE or other schemes involving at least two electron transfer steps with interposed chemical reactions, double electrodes offer a unique probe for the determination of kinetic parameters. Convection from upstream to downstream electrodes allows the study of fast homogeneous processes. The general reaction scheme for an ECE mechanism can be written... 

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

During the past four decades the dynamics and mechanisms of electron-transfer processes have been studied via the application of transition-state theory to the kinetics for electrochemical processes. As a result, both the kinetics of the electron-transfer processes (from solid electrode to the solution species) as well as of pre- and post-electron-transfer homogeneous processes can be characterized quantitatively. 

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