Electron-transfer reactions mixed-valent 3+ center

Two electron transfers from cytochrome c substrate convert the inactive oxidized enzyme with Fe -Cu bimetallic site to an active Fe -Cu state mixed-valent form. The first intermediate in the reaction of O2 is an adduct with cytochrome 3, an Fe-02 moiety with i/p -o = 572cm (from resonance Raman (rR) spectroscopy) closely resembling those observed in oxyhemoglobin. It appears to involve a Fe<23 -02 in an end-on configuration, without any significant interaction with the Cu center, consistent with the crystallographic data. The subsequent reaction was... 

The above theoretical studies based on structural models, and their possible involvement in H2 activation or production, give some insight into the possible redox levels of the active site. However, they focus only on the active center. They do not consider at all the protein environment which is crucial to the reactions that this site promotes, i.e., the availability of energetically favorable pathways for proton and electron transfer in and out of this active site. Nevertheless, these theoretical calculations have reproduced some of the spectroscopic features (i.e., stretching frequencies) of synthetic models, which lends confidence to the reliability of the computations. A reasonable agreement in all these calculations is that H2 binds to an Fe center and not Fe, which is consistent with the experimental fact that the mixed-valent active form of the enzyme has at least one Fe in the +2 oxidation state. This is also in agreement with the formation of stable 7 -H2 complexes of d metals. 

Statement 3 For reactions which involve a formal redox of reaction centers in addition to bond exchanges, the promoted states involve one charge transfer excitation for each pair of centers which undergo formal redox. The rest of the excitations will be of the singlet -> uiplet variety. The odd electrons are coupled across the reactants into bond-pairs to fit the number of bonds which aie formed at the bottom end of the correlation line. These reactions will be termed henceforth mixed-valent reactions , to emphasize the formal requirement for redox. 

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