Dinuclear 2Fe-2S analogues

Although there are three types of active sites in non-heme iron proteins, namely containing one, two, and four iron atoms per center, in the discussion above only tetrameric Fe-S species have been considered. This topic has been treated with some detail because these compounds meet properly the concept of cluster and are therefore clearly within the scope of this book. That is not the case for the di-iron species which form the 2Fe-2S active sites in the ferredoxins nor for the mononuclear center in the rubredoxins. However, in order to get a better understanding of the chemistry of the iron-sulfur proteins, an overview of the chemistry of the 2Fe-2S analogues is also outlined in this Section. 

The relatively high stability of the species [Fe4S4(SR)4] prevents, in most cases, the direct synthesis of the dimers [Fe2S2(SR)4] . However in some special cases, namely by using ligands as the 1,2-dithiols whose structures are 

In spite of the similarity of the dianionic analogue with the oxidized form of 2Fe-Fd, there are some discrepancies with the reduced form. According to Mossbauer and ENDOR (Electron Nuclear Double Resonance) results, in 2Fe-Fdred the iron atoms are non-equivalent corresponding to Fe(II) and Fe(III) in a situation of trapped valence. That could not be observed in the synthetic analogue. Moreover, the second reduction potential of the analogue i.e. from tri to tetraanion, is only about 0.24-0.28 V more negative than the first one, meanwhile 2Fe-Fdred is apparently unable to accept a second electron. 

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