Tetrameric iron-sulfur clusters

Synthesis and Structure. An appropriate route to mercaptide-iron sulfur tet-rameres is the anaerobic reaction of FeCla with 3 equivalents of sodium thiolate in methanol solution followed by treatment of the resulting polymeric solid with alkali sulfide in methanol. From the resulting dark-brown solutions dark-red crystalline products like the oxidized ferredoxins, Fdox, can be isolated by addition of quaternary cations. 

The presence of disulfide among the reaction products reveals that the overall reaction involves iron reduction. Accordingly the product should contain iron in two different oxidation states namely 2Fe(II) + 2Fe(III). Consequently, the same type of products can be obtained from Fe(II) compounds by oxidation with disulfide. 

The structures of several synthetic tetrameres [Fe4S4(SR)4] has been determined by single crystal X-ray diffraction analysis. The structures of the derivatives with R = CH2Ph and Ph are shown in Fig. 5.3. The comparison of these structures with those of the prosthetic groups of P. aerogenes Fdox and 

In both cases the basic symmetry is the same, i.e. a tetrahedron of four Fe atoms interpenetrated by a second, larger tetrahedron of four sulfur atoms. Observed symmetry D2d, which may be considered as resulting from a distortion of a more symmetric arrangement is similar for both synthetic and natural clusters. One of the very important features is that crystallographic data for all studied compounds do not show any difference between the iron atoms constituting the clusters, i.e. there are no localized Fe(II,III) valence states but mixed-valence species. 

5 Synthetic Analogues of the Active Sites of Iron-Sulfur Proteins 

---

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. 

---