Iron interconversions between

Even though the iron atoms are separated in haemoglobin by about 25 A, communication between them is still able to occur and this has been postulated to involve a trigger mechanism (Perutz, 1971). The trigger is the movement of the proximal histidine as dioxygen binds to (or is released from) the Fe(n) and results in interconversion between the T and R structures. This movement causes a conformational change which is transmitted through the protein to the other iron sites. X-ray studies indicate that relative shifts of up to 6 A at subunit interfaces occur between the T and R states (Perutz, 1978). 

Table 2.2 summarizes basic crystallographic data for the iron oxides. Iron oxides, hydroxides and oxide hydroxides consist of arrays of Fe ions and 0 or OH ions. As the anions are much larger than the cations (the radius of the 0 ion is 0.14 nm, whereas those of Fe and Fe" are 0.065 and 0.082 nm, respectively), the arrangement of anions governs the crystal structure and the ease of topological interconversion between different iron oxides. Table 2.3 lists the atomic coordinates of the iron oxides. 

A characteristic of the iron oxide system is the variety of possible interconversions between the different phases. Under the appropriate conditions, almost every iron oxide can be converted into at least two others. Under oxic conditions, goethite and hematite are thermodynamically the most stable compounds in this system and are, therefore, the end members of many transformation routes. The transformations which take place between the iron oxides are summarized in Table 14.1. These interconversions have an important role in corrosion processes and in processes occurring in various natural environments including rocks, soils, lakes and biota. In the latter environments, they often modify the availability and environmental impact of adsorbed or occluded elements, for example, heavy metals. Interconversions are also utilized in industry, e.g. in the blast furnace and in pigment production, and in laboratory syntheses. 

Any interconversion in a sample may give rise to temperature-dependent NMR spectra. For example, the 13C NMR spectrum of dimeric cyclopentadienyl iron dicarbonyl is temperature dependent [114]. This was attributed to intermolecular exchanges of carbonyls and interconversion between cis and trans complexes. 

This account will deal in turn with structural features of the one-iron, two-iron, four-iron and three-iron proteins, and also with the most important topic of interconversion between the three-iron and four-iron clusters. Model compounds will be described briefly. Finally, kinetic studies and the role of iron-sulfur proteins in complex enzymes will be discussed. 

Multi-thiolate iron-protein clusters are also found in the ferredoxin family of electron transport proteins. In particular, the interconversion between ferredoxin clusters... 

The fact that the same amino acid polypeptide chain can accomodate these two types of cores raises several questions what is the process that regulates the building up of a three or four iron core Is there interconversion between the two structures (as suggested by reconstitution experiments and participation of Fdll in the phosphoroclastic reaction) Is there a specific biological role for each type of structure ... 

Interestingly when a racemic mixture of the helical complexes M(phen)3 + [M = Fe(II), Co(II) and Zn(II)] was added to the per-C02 - S-CD host, the circular dichroism spectra showed induced Cotton effects which in the case of the iron complex indicates that the per-C02 - S-CD enriches the A-enantiomer of Fe(phen)3 +. This behaviour, known as the Pfeiffer effect, occurs when a chiral complex with labile configuration interacts with optically active species to form a pair of disatereomers with one being more stable and hence interconversion between the A- and the A-enantiomers easily occurs. 

On the other hand, it has also been shown that interconversion between A- and A-enantiomers of [Fe(phen)3] " " occurs easily and the A-isomer is enriched upon complexation of rac-[Fe(phen)3] " " with a right-handed DNA double helix.This behaviour, known as the Pfeiffer effect, was also observed when the same iron complex rac-[Fe(phen)3] " ", which is configurationally labile, was bound to the optically active species such as A-TRISPHAT (4.7) or the heptaanion modified cyclodextrin (4.12) (see Sections 4.1.3 and 4.2). 

An opposite situation is found for the catalytic N O decomposition over Fe/ZSM-5 [46]. Although mononuclear Fe sites are highly active for the dissociation of the first N O molecule, further catalytic reaction is hampered in this case by a very high barrier for the activation of the second N O molecule necessary for the formation and completion of the catalytic cycle. The binuclear [Fe(p-0)2Fe] + and [Fe(p-0)Fe] + complexes are much more catalytically active. Each iron center in such complexes acts as an independent Np dissociation center. This allows the formation of two proximate labile extra-framework O species, which easily recombine to form molecular O. The reaction mechanism in this case is highly complex (Fig. 9). It involves multiple changes in the spin-state of the iron complexes as well as the interconversion between the [Fe(p-0)2Fe] and [Fe(p-0)Fe] active complexes in the course of the catalytic process. The identification of the preferred reaction channel based only on the results of DFT calculations is not possible. 

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