Compound iron-sulfur

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The mimicking of the iron-sulfur clusters by synthetic chemistry has been quite successful over the years. One of the last synthetic clusters to be obtained was the [3Fe-4S] cluster (109, 110). This new synthetic compound was useful for the demonstration of interconversion pathways, as well as for the formation of different heterometal clusters beyond those produced in proteins (111). The [3Fe-4S] core... 

A preliminaiy characterization of a new iron—sulfur protein isolated from Desulfovibrio vulgaris Hildenborough was reported in 1989 124). The protein contained approximately 6 iron and 6 inorganic sulfur atoms per molecule. The FPR spectrum of the dithionite reduced protein exhibited an S = signal similar to what was found for synthetic compounds with a [6Fe-6S] core (prismane core). No other FPR signals were reported at this time, and based on the observed similarity it was suggested that this peculiar iron-sulfur protein contained a [6Fe-6S] cluster. Because it had no known function, the pro-... 

Although heme is absent in Clostridia, it was early recognized that anaerobic bacteria may contain substantial levels of iron (44). To date the best characterized iron compounds from this source are the iron-sulfur proteins termed ferredoxins and rubredoxins. Molecular structures of representatives of both types of protein have been worked out by Jensen and his colleagues by X-ray diffraction analysis (see below). 

See pyrophoric iron-sulfur compounds See other corrosion incidents... 

See Other METAL SULFIDES, PYROPHORIC IRON-SULFUR COMPOUNDS... 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

For updating the information presented in this chapter, a literature search on the keyword nitrogenase modified with structure, X ray, Mossbauer, iron sulfur cluster, or model compound will generate citations referring to the newest research results. A search of the Protein Data Bank (PDB) at the website address http //www.rcsb.org/pdb/ will yield the latest updates on X ray, NMR, and other submitted structural data. 

The interactions of NO with the iron-sulfur cluster moieties of several enzymes generate iron-sulfur-nitrosyl cluster compounds [156]. However, synthetic nitrosyl clusters such as Roussins black salt (RBS), Roussins red salt (RRS), Roussins red ester (RRE) and [FeNOS]4 (Fig. 5.3) are well known [129, 157] and can be synthesized easily [158-164]. For example, the RBS can be prepared by mixing FeS04 with NaN03 and (NH4)2S in aqueous solution [158]. RRE salts are generally insoluble in water, but recently the water soluble sulfonated derivative, Na2[Fe2(SCH2CH2S03)2(N0)4], has been prepared [165]. 

Up to now, two types of iron compounds have been studied with ENDOR, namely heme compounds (hemoproteins and some heme model compounds) and iron-sulfur proteins. For comprehensive summaries of the corresponding EPR work, the reader is referred to the literature234-2371. 

Fe-S proteins contain four basic core structures, which have been characterized crystal-lographically both in model compounds (Rao and Holm, 2004) and in iron-sulfur proteins. These are (Figure 3.6), respectively, (A) rubredoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom liganded to four Cys residues—the iron atom... 

It is necessary to consider a number of equilibrium reactions in an analysis of a hydrometallurgical process. These include complexing reactions that occur in solution as well as solubility reactions that define equilibria for the dissolution and precipitation of solid phases. As an example, in analyzing the precipitation of iron compounds from sulfuric acid leach solutions, McAndrew, et al. (11) consider up to 32 hydroxyl and sulfate complexing reactions and 13 precipitation reactions. Within a restricted pH range only a few of these equilibria are relevant and need to be considered. Nevertheless, equilibrium constants for the relevant reactions must be known. Furthermore, since most processes operate at elevated temperatures, it is essential that these parameters be known over a range of temperatures. The availability of this information is discussed below. 

McAndrew, Wang and Brown (11) Precipitation of iron compounds from sulfuric and leach solutions temp, up to 140°C. 

ALKYLALUMINIUM DERIVATIVES, ALKYLBORANES, ALKYLHALOBORANES ALKYLHALOPHOSPHINES, ALKYLHALOSILANES, ALKYLMETALS ALKYLNON-METAL HYDRIDES, ALKYLPHOSPHINES, ALKYLSILANES ARYLMETALS, BORANES, CARBONYLMETALS, COMPLEX ACETYLIDES COMPLEX HYDRIDES, HALOACETYLENE DERIVATIVES HEXAMETHYLNITRATODIALUMINATE SALTS, METAL HYDRIDES NON-METAL HYDRIDES, ORGANOMETALLICS, PYROPHORIC ALLOYS PYROPHORIC CATALYSTS, PYROPHORIC IRON-SULFUR COMPOUNDS PYROPHORIC METALS... 

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