Spectroelectrochemistry electron transfer kinetics

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. ... 

The OTTLE method is very useful for slow electron transfer kinetics such as those of biological redox processes, in which a mediator is frequently used in an indirect coulometric titration [6, 36] and metal complexes with slow heterogeneous kinetics. An example of the latter is a copper complex in which the Cu(II/I) couple very often shows a broad quasi-reversible wave due to slow heterogeneous kinetics. In this case, spectroelectrochemistry is a useful method for obtaining accurate E° values [37]. 

Vitamin B 2 (cyanocob(III)alamin) is an example of a quasi-reversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B 2 is a single two-electron process at = -0.93 V vs SCE to the Co(I) redox state (Figure lOA). However, thin-layer spectroelectrochemistry using a... 

When spectroelectrochemistry is used as a tool in reaction kinetics, it is important to know accurately the rate of generation of reactive intermediates, that is, the accurate potential of the working electrode. This requirement becomes a particular problem when an OTE is the preferred electrode because of the ohmic drop in the electrode itself and the nonuniform current distributions often encountered. For the OTTLEs in particular, the accurate modeling of the diffusion in the cell also leads to rather complicated mathematical equations [346]. The most profitable way of operation is therefore to use a potential-step procedure where the potential is stepped to a value at which the heterogeneous electron transfer reaction proceeds at the diffusion-controlled rate. In transmission spectroscopy the absorbance, AB(t), of the initial electrode product B, in the absence of chemical follow-up reactions, is given by Eq. (99) [347,348], where b is the extinction coefficient of B. 

Since the establishment of spectroelectrochemistry very little effort has been devoted to the direct electrochemistry of redox proteins. Although many thermodynamic and kinetic parameters can be determined by UV-VIS spectroelectrochemistry, the electrochemical reaction mechanisms for redox proteins are not well understood. New techniques md new theoretical treatments are needed to address this issue. Moreover, most attention has been placed on relatively simple electron transfer proteins to date no one has reported the direct electrochemistry of a more complex system (e.g., a redox enzyme system) which unequivocally undergoes electron transfer to (or from) its active site. Considerable experimental work is needed to develop more fully spectroelectrochemical methods for biological systems. 

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