Iron-Sulfur Redox Chemistry

Pyrite (FeS2 is by far the most abundant sulfide mineral, occurring in most types of geologic formations. Its less common polymorph, marcasite, usually forms in near-surface, low-temperature environments. At 25°C pyrite is more stable than marcasite by about -0.4 kcal/mol. The oxidative breakdown of these minerals as the result of exposure to aerobic conditions due to mining is the chief cause of acid mine waters. 

Pyrite and marcasite do not nucleate and precipitate directly from solution, but result from the successive sulfidation of a series of metastable Fe(II) sulfides. The experimental results of Schoonen 

The subsequent sequence of sulfides leading to pyrite/marcasite is 

Marcasite forms below about pH 5, when the neutral, undissociated polysulfide acids dominate, whereas pyrite is favored at pH values above 6, when polysulfide anions are the major species (Fig. 12.15). Schoonen and Barnes (1991a) offer detailed explanations for such complex behavior. 

Because of the poorly known value for, Fl2(H2S), it has become traditional to write solubility product expressions for the Fe(II) sulfides in terms of bisulfide ion and elemental sulfur instead of S . Assuming for simplicity that the amorphous sulfide and mackinawite are 1 1 solids, their dissolution expressions are  

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Another process option is the LO-CAT process, which employs chelated iron liquid redox chemistry and has been popular for smaller operations. Solution compositions include iron, proprietary chelates, a biocide, and a surfactant that facilitates sulfur sinking to the bottom of the oxidizer, where it is removed as a slurry. Other chelated iron processes include Sulferox and Hiperion (Dalrymple 1989). 

Wu S-P, Wu G, Surerus KK, Cowan JA. 2002c. Iron-sulfur cluster biosynthesis Kinetic analysis of [2Fe-2S] cluster transfer from holo ISU to apo Fd Role of redox chemistry and a conserved aspartate. Biochemistry 41 8876-85. 

This section is organized as follows we first start with a discussion of the electrochemical behavior of the Roussin-type synthetic iron- sulfur clusters for their historic importance and as an interesting introduction to poly iron-sulfur centers redox chemistry. Then we review iron-sulfur centers in proteins and artificial models in the order of increasing iron content. Finally, biological iron-sulfur centers and artificial models directly linked to other inorganic centers, the so-called bridged molecular assemblies, are considered. 

Schumacher, W., C. Holliger, A. J. B. Zehnder, and W. R. Hagen, Redox chemistry of cobalamin and iron-sulfur cofactors in the tetrachloroethene reductase of Dehalobacter restrictus , FEBS Lett., 409, 421-425 (1997). 

The central role of the complexes [Fe(NO)2(SR)2]- in the reaction chemistry of iron-sulfur-nitrosyl complexes and their very ready formation both in in vitro and in vivo (108,123-125,128,129) suggest that the antimicrobial activity of nitrite depends not only upon the disruption of respiration [by destruction of the natural iron-sulfur clusters of redox proteins (70S)] but also specifically upon the formation... 

Iron-sulfur chemistry has developed over recent years to become a very rich area. A feature of many complexes is their redox behaviour, with more than one oxidation state being important. This... 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

Redox catalysis is the catalysis of redox reactions and constitutes a broad area of chemistry embracing biochemistry (cytochromes, iron-sulfur proteins, copper proteins, flavodoxins and quinones), photochemical processes (energy conversion), electrochemistry (modified electrodes, organic synthesis) and chemical processes (Wacker-type reactions). It has been reviewed altogether relatively recently [2]. We will essentially review here the redox catalysis by electron reservoir complexes and give a few examples of the use of ferrocenium derivatives. 

Langen, R., G. Jensen, U. Jacob, P. Stephens and A. Warshel. (1992). Protein Control of Iron-sulfur Cluster Redox Potentials. Journal of Biological Chemistry. 267 25625-25627. 

When the overall oxidation state of a system is desired, unless a water is obviously anaerobic (e.g., it has an H2S odor) one should first attempt to measure dissolved oxygen as an index of system redox state. Eh measurements are unlikely to be stable and thermodynamically meaningful in surface-waters, except in acid waters (where ferrous and ferric species are usually present). Eh measurements may be stable and meaningful in anaerobic sediments or groundwaters, when species of iron, sulfur, and manganese dominate the redox chemistry, but otherwise are of qualitative value only. 

During the thermally driven differentiation of the Earth into core-mantle-crust, numerous reactions would have produced oxidized forms of iron, sulfur and carbon. These would have contributed to the redox chemistry in the early planet development. Volcanic and hydrothermal emission of sulfur dioxide, SO2, delivered oxidants to the oceans and atmosphere. Photodissociation of water vapor in the atmosphere have undoubtedly provided a small but significant source of molecular oxygen. Furthermore, UV-driven ferrous iron oxidation could have been coupled to the reduction of a variety of reactants, for instance, CO2 (Figure 16). 

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