CLUSTERS oxidation states

ISP contain four basic core structures which have been characterized crystallographically both in model compounds and in ISP (Rao and Holm, 2004). These are (Figure 13.15), respectively, (A) rubedoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom bound to four Cys residues—the iron atom can be in the +2 or +3 valence (B) rhombic two-iron-two-sulfide [Fe2-S2] clusters—typical stable cluster oxidation states are +1 and + 2 (the charges of the coordinating cysteinate residues are not considered) ... 

The additional electron in Fdred is associated with only one of the iron sites, resulting in a so-called trapped-valence structure. The [Fe2S2(SR)4] cluster oxidation state, containing two ferrous ions, can be produced in vitro when strong reductants are used. 

Evolution of the metal-metal intracluster bond depending on the cluster oxidation state... 

Figure 1 The three most common types of biological iron-sulfur clusters, with the common oxidation states indicated for each. For biological iron-sulfur clusters, cluster oxidation states are typically stated as the sum of the charges on the iron ions and the sulfide ions, ignoring the charges on the exogenous ligands (L in this figure). The nominal oxidation states of the iron ions in each cluster are indicated, although it should be emphasized that delocalized mixed-valent states are common in these clusters.

When the rate of interconversion between two altanate cluster oxidation states is comparable to 7", saturation of a peak for one oxidation state (designated F)... 

Standard cycling between the expected alternate cluster oxidation states,... 

Whereas the change in solely the cluster oxidation state can be effected within milliseconds,redox reactivity of the disulfide was found extraordinarily sluggish at 30°, some 70° below the physiological temperature for Pf. A disulfide bond that is redox active at approximately the same potential as the cluster has been reported for the 3Fe Fd from the mesophile Dg, but the disulfide reactivity is rapid such that only very small amounts of the intermediate states are populated at equilibrium, and hence no structural information could be obtained on the two intermediate oxidation states. For Pf Fd, the sluggish disulfide reactivity, likely because of its remarkable hyperthermostability, allows the preparation of the two metastable intermediate oxidation states, Fd and Fd , in relatively pure conditions and over times sufficient to carry out structural characterization of the four distinct redox states. ... 

Reactions With Changes in Cluster Oxidation State... 

In the following description of cluster reactions involving changes in cluster oxidation states both electrochemical and chemical processes will be considered. In order to systematize the chemical redox processes, they have been roughly cataloged as reduction, oxidation, or additive oxidation reactions. 

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