Oxidation-reduction reactions Intermolecular electron transfer

Intermolecular photo-oxidation-reduction reactions involve a light initiated electron transfer between a complex and any other suitable molecule available in the medium. An oxidized or a reduced form of the complex may be obtained. 

Reaction of Cytochrome cimu with Tris(oxalato)cobalt(III) The cytochrome c protein was also used as reductant in a study of the redox reaction with tris (oxalato)cobalt(III).284 Selection of the anionic cobalt(III) species, [Conl(ox)3]3 was prompted, in part, because it was surmised that it would form a sufficiently stable precursor complex with the positively charged cyt c so that the equilibrium constant for precursor complex formation (K) would be of a magnitude that would permit it to be separated in the kinetic analysis of an intermolecular electron transfer process from the actual electron transfer kinetic step (kET).2S5 The reaction scheme for oxidation of cyt c11 may be outlined ... 

Oxidation or reduction (redox reaction [4], Scheme 4) generate radicals by an intermolecular electron transfer. The Kolbe reaction is a representative example (Scheme 5) [5]. 

A simple reaction of the former molecule with H2 would reduce its two clusters again. The final level of reduction of the Ee-S clusters would then only depend on the effective redox potential in the system. Under 1 per cent H2 at pH 6, where the redox potential is about —295 mV, part of the Fe-S clusters are oxidized. At this pH they only reduce under f 00 per cent H2. At pH 8 or higher, however, the redox potential of both fOO per cent H2 and 1 per cent H2 is low enough to keep all clusters fully reduced. So via intermolecular, one-electron transfer reactions the Fe-S clusters can presumably follow the current potential imposed upon the system by H2 even in the absence of redox mediators. Of course, the presence of such mediators facilitates electron (re)distribution. 

The exact nature of the reaction (oxidative vs. reductive) will depend on the redox properties of I ) and Q. The electron transfer process is a special case of exciplex formation favored in the strongly polar solvents, such as water. The involvement of an exciplex in a photochemical reaction is generally established by studying the effects of known exciplex quenchers such as amines on the exciplex fluorescence and the product formation. The heavy atom effect, due to the presence of substituents such as bromine or iodine intra- or intermolecularly, causes an exciplex to move to the triplet state preferentially, with a quenching of fluorescence. 

Figure 21. Reaction scheme describing the stepwise reduction of P. aeruginosa and P. stutzeri NiR. Symbols Circle Heme-c Square Heme-di. Empty symbols represent an oxidized site, filled ones a reduced site. Each protein subunit includes one heme-c and the heme-di below it. A single arrow represents intermolecular reduction of heme-c by the external reductant. These reactions are all assumed to be irreversible and to occur with the same, near diffusion controlled, rate constant. The double arrows represent reversible intramolecular electron transfer within a subunit. It is assumed that heme-di is not reduced by in/crmolecular ET, and that there is no intramolecular electron transfer between the two subunits. The asterisk indicates that there ate two fmms of the species. They differ by...

These reactions show that dimers can be formed (and have been observed) if either peptide radicals or side chain radicals on the globin surface react intermolecularly. They also show that if these radicals are close enough to the iron center, electron transfer can occur leading to either reduction or oxidation, depending on the valence state, and to an associated color change. 

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