Stopped-flow electron transfer

Cyclic voltammograms of 48 recorded in ACN solutions containing as much as fivefold excess of pyridine are almost identical to those obtained without this base89. In both cases the product of the anodic reaction is the dimer 49. Identical electrochemical responses in the presence and in the absence of pyridine imply that there are no detectable interactions between the cation radical 48 and the pyridine molecules. However, diverse transformation of the decay profiles of 48 depending on the concentration of pyridine was observed using an electron transfer stopped-flow technique. The last observation was ascribed to the interaction between 48+ and pyridine, in which the proton is extracted by the molecule of the base89. One can therefore assume that this is an example of the process for which the voltammetric measurements at conventional scan rates cannot give full information about the pathway of the electrochemical reaction. 

A comprehensive series of oxidation-reduction potential measurements have shown the FAD moiety to have the following one-electron couples PFl/PFIH = = —290 mV and PFIH 7PFIH2 = —365 mV while the FMN moiety exhibits the following PFl/PFl- = -110 mV and PFIH /PFIH = -270 mV. The FMN and FAD smiquinones were found to both be the neutral form as judged from absorption and ESR spectral data. The overlap of oxidized/semiquinone potential of the FAD moiety withkhe semiquinone/hydroquinone couple of the FMN moiety demonstrates the thermodynamic facilitation of flavin-flavin electron transfer via a one-electron mechanism. Stopped-flow kinetic data are also consistent with this view in... 

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

Researchers studying the stepwise kinetics of nitrogenase electron transfer using stopped-flow kinetic techniques have presented other scenarios. One hypothesis presents kinetic evidence that dissociation of Fe-protein from MoFe-protein is not necessary for re-reduction of Fe-protein by flavodoxins.13 These authors state that the possibility of ADP-ATP exchange while Fe-protein and MoFe-protein are complexed with each other cannot be excluded and that dissociation of the complex during catalysis may not be obligatory when flavodoxin is the Fe-protein reductant. This leads to the hypothesis that MgATP binds to the preformed Fe-protein/... 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

M. Fabian and co-workers have studied the protein s role in internal electron transfer to the catalytic center of cytochrome c oxidase using stopped-flow kinetics. Mitochondrial cytochrome c oxidase, CcO, an enzyme that catalyzes the oxidation of ferrocytochrome c by dioxygen, is discussed more fully in Section 7.8. In the overall process, O2 is reduced to water, requiring the addition of four electrons and four protons to the enzyme s catalytic center. Electrons enter CcO from the cytosolic side, while protons enter from the matrix side of the inner mitochondrial membrane. This redox reaction. 

In a stopped-flow study on cytochrome cdi from P. aeruginosa, the ferrous d heme-NO species was formed despite electron transfer from the c to the d heme being relatively slow, rate constant approximately 1 s , for the relatively short distance between the two hemes (32). Such a distance would normally predict much faster electron transfer. The relatively slow interheme electron transfer rate has been observed on a number of occasions, and before the structure of the protein was known was thought to reflect the relatively large interheme separation distance and/or relative orientation of the two hemes (30). The crystal structures provide no evidence for either of these proposals there is nothing unusual about the relative orientation of the c and di heme groups. 

Blocking electron transfer by any one of these inhibitors stops electron flow from substrate to oxygen because the reactions of the electron transport chain are tightly coupled like meshed gears. 

Although direct complex formation is observed kinetically (stopped flow) and spectrophotometrically, where X = Br or Cl, the reaction with I results in an oxidation of the halide. The reactions are rapid and there is the question of inner- or outer-sphere electron transfer, for the [14]aneN4 complex. However, further studies (140) using ligand substituted (dimethyl) complexes reveal that for the rac-Me2[14]aneN4 isomer, two processes are observed, k = 2.9 x 104 M-1 sec-1 and a subsequent redox step, krci = 5.5 x 103 M-1 sec-1, both of which are iodide dependent. The mechanism proposed involves the formation of an octahedral complex which further reacts with a second mole of I- in the redox step ... 

Electron transfer reactions of the organonickel species with [Co(dmgH)2] have been described (197). Reaction rates are very high (k > 2 x 106 M l sec-1) and cannot be measured by the stopped-flow technique. However, the blue organocobalt(I) products have been characterized by H NMR methods. Rapid reactions have also been observed (193) in the oxidations of [NilMe6[14]4,II-dieneN4]+ and [Ni Me6[14]aneN4]+ by [M(bipy)3]3+ (M = Co, Cr, or Fe), [Ru(NH3)6]3+, and [Co(en)3]3+. 

B. Serra, A.J. Reviejo, C. Parrado and J.M. Pingarron, Graphite-Teflon composite bienzyme electrodes for the determination of L-lactate application to food samples, Biosens. Bioelectron., 14(5) (1999) 505-513. A.A.J. Torriero, E. Salinas, F. Battaglini and J. Raba, Milk lactate determination with a rotating bioreactor based on an electron transfer mediated by osmium complexes incorporating a continuous-flow/ stopped-flow system, Anal. Chim. Acta, 498(1-2) (2003) 155-163. 

A.A.J. Torriero, E. Salinas, F. Battaglini and J. Raba, Milk lactate determination with a rotating bioreactor based on an electron transfer mediated by osmium complexes incorporating a continuous-flow/stopped-flow system, Anal. Chim. Acta, 498 (2003) 155-163. 

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