Ferric chelates

Lauff JL, DB Steele, LA Coogan, M Breitfeller (1990) Degradation of the ferric chelate of EDTA by a pure culture of w Agrobacterium sp. Appl Environ Microbiol 56 3346-3353. 

A neuropathy caused by clioquinol (iodochlorohydroxyquin, chinoform) and enhanced by the formation of a clioquinol ferric chelate which initiates lipid peroxidation, leads to complete degeneration of retinal neuroblasts within a day. Vitamin E has a potent protective action against the effects of the chelate [75]. Peroxidative damage to DNA in rat brain, induced by methyl ethyl ketone peroxide, a potent initiator of lipid peroxidation, was inhibited by addition of vitamin E to the diet of rats [76]. 

Investigations of the kinetics of the reaction of these new siderophores with iron-saturated transferrin showed a rapid formation of a ternary complex with transferrin, followed by a slow step in which the ferric siderophore was released from the apoprotein. Weitl et al.257 have evaluated the ferric-chelating properties of several of these siderophores and found the following order of effectiveness for removing iron from transferrin enterobactin > MECAMS > MECAM > LICAMS > DFOA > TRIM-CAMS. 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

Oxidation of Ferrous Chelates to Ferric Chelates. Flue gas contains about 5% oxygen. When dissolved, oxygen can oxidize ferrous ions to ferric ions which are inactive for coordination with NO. The oxidation rate of ferrous ions is accelerated in the presence of chelating agents, e.g., EDTA and NTA. This acceleration may be ascribed to the stabilization of the oxidized form by the chelation. 

Reduction of Ferric Chelates by HSO3 and Formation of Dithionate. FeJ+(EDTA) is reduced by HSO3, producing dithionate and a small amount of S0/2 (24). The rate of reduction of Fe +(EDTA) is first order in [HSO3] and [Fe +(EDTA)], and inversely first order in [Fe2+(EDTA)]. 

The improvement over the existing Japanese processes can be made by developing a more efficient ferrous chelate such that it can provide better absorption efficiency for NO, faster reaction rates between NO and S02> and better stability for the ferrous chelate toward oxidation, compared to Fe +(EDTA) or Fe +(NTA) employed in Japanese processes. The development of an efficient and cost-effective method for the reduction of ferric chelate to ferrous chelate without producing dithionate ions could make the process attractive. In addition to these areas, the study of several alternative approaches and novel ideas could develop into a much more efficient and cost-effective scrubber system employing metal chelate additives. 

Ferrioxamines B and E are prime examples of the linear and cyclic species, respectively. Several members of the series have been prepared by total synthesis, thus establishing the sequence of the contained units (For example, four isomers could be constructed from the hydrolytic products of ferrioxamine B). The three hydroxamate functions must be spatially located so as to form a stable, intramolecular hexa-dentate ferric chelate. Acetylation of ferrioxamine B affords ferrioxa-mine Di ferrioxamine G corresponds to component B with succinic replacing acetic acid cyclization of G yields ferrioxamine E components Ai and D2 carry l-amino-4-hydroxyaminobutane in place of a residue of the next higher homologue. 

Pulcherrimin, the ferric chelate of pulcherriminic acid, is believed to consist of a mosaic of dihydroxypyrazinedioxide molecules cemented together by iron atoms as illustrated in Fig. 9 (65). In this form it is highly insoluble. 

Gives an amorphous, 3 1 ferric chelate, C33H570isN Fe. Absorption maximum at 4400 A, bleached by EDTA or acid thus indicating mono-hydroxamic acid structure. Iron retained in strongly alkaline solution. 

Iron-free ferrioxamine B ( Desferal", Ciba Pharmaceutical Company, Sum-mitt, New Jersey) reacts rapidly with ferrous ion, especially at neutral pH, forming the ferric chelate. This transition is blocked by mercaptoacetic acid, hydrosulfite and thiosulfate but not by weaker reducing agents such as ascorbic acid, hydroxylam-ine and sulfite (Nature 205, 281, 1965). 

The French workers devised a medium for production of ferropyri-mine-type pigment in which citrate appears to be an important constituent. In the absence of added iron the synthesis is augmented somewhat, although not to the spectacular degree seen with the ferric chelators (118). 

We conclude that plural mechanisms exist for siderophore iron utilization in the enteric bacteria. The iron may be rapidly removed with ( . coli) or without (S. typhimurium) effective transport of the ligand. The rate of this process is such that labilization of the iron by reduction appears most likely. The intact ferric chelates also pass the cell envelope, but by a generally slower mechanism. An estimate of the number of atoms of iron acquired per bacterium indicates true uptake rather than adsorption to the cell surface has taken place. In both organisms the A-cis chromic coordination isomer of ferrichrome is active, indicating that dissociation and/or isomerization is not obligatory for its transport. 

It is noteworthy that Aust and coworkers have shown that in vitro lipid peroxidation initiated by ferrous-ADP or ferrous-AMP complexes was strongly stimulated by the presence of the analogous ferric complexes, with no effect of either SOD, catalase or OH scavengers [156,157]. This suggests that a ferrous-dioxygen-ferric chelate may serve as a potent free radical initiator of its own. 

In vivo uptake of iron by transferrins usually involves its addition as a ferric-chelate complex, to prevent hydrolytic attack on the ferric ion (211). Complexes such as ferric citrate and ferric nitrilotriacetate are commonly used. Under these conditions, kinetic schemes for the uptake of iron by transferrins have identified five steps in the formation of the specific metal-anion-transferrin ternary complex (120). These may be summarized as follows. 

EC 1.16.1.7 Ferric Chelate reductase 2 erric+nadred=2 ferrous+nadox... 

Recently ascorbic acid has been assigned a significant function in the reactions involving oxygen insertion by model oxygenase and peroxidase systems in which ferric ion or a ferric chelate is considered to be the catalyst. Although reaction mechanisms suggested by earlier workers involved ascorbic acid merely as a reductant to convert Fe(III) to Fe(II), which would then in turn interact with the oxidant, Hamilton... 

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