Electron transfer hypothesis

Hamilton, W. A. (2003). Microbially influenced corrosion as a model system for the study of metal microbe interactions a unifying electron transfer hypothesis. Biofouling 19, 65-76. 

Habib and Bockris measured the potential differences across bilayer lipid membranes separating two solutions containing redox systems of different redox potentials (Figure 2) under no applied potential condition. They also measured the potential differences when the bilayer membrane was replaced with a Pt membrane, which represents the ideal situation in the presence of electronic condition when the electron transfer hypothesis for the membrane potential is undoubtedly applicable. Correspondingly, they found that the measured membrane potential is, indeed, the difference of the redox potentials on the two sides of the Pt membrane (line I, Figure 5). 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

It is becoming clear that the MgATP hydrolysis is not required to induce protein-protein electron transfer, but its role in nitrogenase function is still undefined. The most likely hypothesis at the moment is that its hydrolysis, on the Fe protein, induces important changes in the MoFe protein, presumably by altering the conformation of the enzyme complex. Nevertheless, the nature of the changes in the MoFe protein remain obscure. 

In the last decade, an intense and successful investigation of this phenomenon has focused on its mechanism. The experimental facts discovered and the debate of their interpretation form large portions of these volumes. The views expressed come both from experimentalists, who have devised clever tests of each new hypothesis, and from theorists, who have applied these findings and refined the powerful theories of electron transfer reactions. Indeed, from a purely scientific view, the cooperative marriage of theory and experiment in this pursuit is a powerful outcome likely to oudast the recent intense interest in this field. 

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/... 

The spatial separation between the components of the electron transport chain and the site of ATP synthesis was incompatible with simple interpretations of the chemical coupling hypothesis. In 1964, Paul Boyer suggested that conformational changes in components in the electron transport system consequent to electron transfer might be coupled to ATP formation, the conformational coupling hypothesis. No evidence for direct association has been forthcoming but conformational changes in the subunits of the FI particle are now included in the current mechanism for oxidative phosphorylation. 

As far as the appearance of separated electron transfer processes is concerned, we must now take into consideration that they can arise either from internal electronic communication, or from the different coordination environment of each Fe(II) subunit. As a matter of fact, theoretical calculations favour the last hypothesis,122 thus pointing out some difference with triferrocene, in which only two different Fe(II) coordinations are present. 

An important question concerning energy trapping is whether its kinetics are limited substantially by (a) exciton diffusion from the antenna to RCs or (b) electron transfer reactions which occur within the RC itself. The former is known as the diffusion limited model while the latter is trap limited. For many years PSII was considered to be diffusion limited, due mainly to the extensive kinetic modelling studies of Butler and coworkers [232,233] in which this hypothesis was assumed. More recently this point of view has been strongly contested by Holzwarth and coworkers [230,234,235] who have convincingly analyzed the main open RC PSII fluorescence decay components (200-300 ps, 500-600 ps for PSII with outer plus inner antenna) in terms of exciton dynamics within a system of first order rate processes. A similar analysis has also been presented to explain the two PSII photovoltage rise components (300 ps, 500 ps)... 

Clearly, the spectroscopic properties of the P clusters in the proteins do not reveal their structural nature. However, extrusion of these clusters from the protein leads to the clear identification of 3-4 Fe S clusters(13.291. Despite the uncertainties inherent in the extrusion procedure (due to possible cluster rearrangement) the extrusion result supports the Dominant Hypothesis, which designates the P centers as Fe S units, albeit highly unusual ones. The P clusters are thought to be involved in electron transfer and storage presumably providing a reservoir of low potential electrons to be used by the M center (FeMo-co) in substrate reduction. 

However, the most severe criticism of the CIEEL hypothesis relates to the chemiexcita-tion efficiency experimentally obtained for the standard CIEEL systems, diphenoyl peroxide (4) and 1,2-dioxetanone (2) . In a study on the electron transfer catalyzed decomposition of l,4-dimethoxy-9,10-diphenylanthracence peroxide (21), Catalan and Wilson obtained very low chemiexcitation quantum yields with various commonly utilized activators (4>s =2 10 EmoH ) and reinvestigated the CL of diphenoyl peroxide (4), determining quantum yields in the same order of magnitude (4>s = (2 1)10 Emol ) as those obtained by 21 (Table 1). We have more recently determined the quantum yields in the rubrene-catalyzed decomposition of dimethyl-1,2-dioxetanone (9) and also found a much lower value than the one initially reported (Table 1) °. Since the diphenoyl peroxide and the 1,2-dioxetanone systems are the two prototype CIEEL systems, the validity of this hypothesis itself might be questioned due to its low efficiency in excited-state formation. ... 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

It must be pointed out that, despite the lack of direct experimental support for this hypothesis, the involvement of the electron transfer in luminol chemiexcitation is related to the chemiexcitation steps proposed for several highly efficient chemi- and biolu-minescent systems, such as activated peroxyoxalate chemiluminescence and the firefly luciferin/luciferase system (Section... 

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