Iron-EDTA complexes

Discussion. Salicylic acid and iron(III) ions form a deep-coloured complex with a maximum absorption at about 525 nm this complex is used as the basis for the photometric titration of iron(III) ion with standard EDTA solution. At a pH of ca 2.4 the EDTA-iron complex is much more stable (higher stability constant) than the iron-salicylic acid complex. In the titration of an iron-salicylic acid solution with EDTA the iron-salicylic acid colour will therefore gradually disappear as the end point is approached. The spectrophotometric end point at 525 nm is very sharp. 

In related work, the reactions of hydrogen peroxide with iron(II) complexes, including Feu(edta), were examined.3 Some experiments were carried out with added 5.5"-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent fa so-called spin trap) for HO. These experiments were done to learn whether HO was truly as free as it is when generated photochemically. The hydroxyl radical adduct was indeed detected. but for some (not all) iron complexes evidence was obtained for an additional oxidizing intermediate, presumably an oxo-iron complex. 

A multiple-path mechanism has been elaborated for dissociation of the mono- and binuclear tris(hydroxamato)-iron(III) complexes with dihydroxamate ligands in aqueous solution. " Iron removal by edta from mono-, bi-, and trinuclear complexes with model desferrioxamine-related siderophores containing one, two, or three tris-hydroxamate units generally follows first-order kinetics though biphasic kinetics were reported for iron removal from one of the binuclear complexes. The kinetic results were interpreted in terms of discrete intrastrand ferrioxamine-type structures for the di-iron and tri-iron complexes of (288). " Reactivities for dissociation, by dissociative activation mechanisms, of a selection of bidentate and hexadentate hydroxamates have been compared with those of oxinates and salicylates. ... 

Thus, antioxidant effects of nitrite in cured meats appear to be due to the formation of NO. Kanner et al. (1991) also demonstrated antioxidant effects of NO in systems where reactive hydroxyl radicals ( OH) are produced by the iron-catalyzed decomposition of hydrogen peroxide (Fenton reaction). Hydroxyl radical formation was measured as the rate of benzoate hydtoxylation to salicylic acid. Benzoate hydtoxylation catalyzed by cysteine-Fe +, ascorbate - EDTA-Fe, or Fe was significantly decreased by flushing of the reaction mixture with NO. They proposed that NO liganded to ferrous complexes reacted with H2O2 to form nitrous acid, hydroxyl ion, and ferric iron complexes, preventing generation of hydroxyl radicals. 

The T relaxation agent increased the sensitivity of the NMR instrument by decreasing several of the mono- and diester phosphate relaxation times by factors of 2-5. In this way the delay time between scans was decreased (43). This change permits an increase in the number of scans observed per unit time. Although the presence of ferric ions creates the potential for precipitation of phosphorus-iron complexes, the addition of a large molar excess of ethylenediaminetetraacetate (EDTA) relative to ferric ions prevented precipitation, even over a large pH region (44). 

Iron should be coordinated by the chelate in such a manner as to prevent direct access of oxygen and hydrogen peroxide. If this is achieved, then hydroxyl-radical production will be reduced to a minimum. Some iron complexes, for instance ethylenediamine tetra-acetic acid (EDTA) (Fig. 3a), generate hydroxyl radicals efficiently while others such as DFO and the hydroxypyridinones (Fig. 3b) do not. By designing chelators that produce extremely stable complexes, the generation of hydroxyl radicals is further minimized. Such stable complex formation would also reduce the tendency for iron redistribution within the body. 

Fig. 3. Comparison of iron complexes of (a) Fe-EDTA and (b) Fe-hydroxypyridin-4-one. EDTA is unable to completely envelope the iron atom due to its limited size. The hydroxypyridinones in contrast completely envelope the iron atom and prevent oxygen and hydrogen peroxide from gaining access...

The widespread occurrence of iron ores, coupled with the relative ease of extraction of the metal, has led to its extensive use as a constructional material with the result that the analysis of steels by both classic wet and instrumental methods has been pursued with vigour over many years.3 Iron complexes are themselves widely used as the basis of convenient analytical methods for the detection and estimation of iron down to parts per million. Familiar tests for iron(III) in aqueous solution include the formation of Prussian blue as a result of reaction with [Fe(CN)6]4, and the formation of the intensely red-coloured [Fe(H20)5SCN]2+ on reaction with thiocyanate ion.4 Iron(II) forms particularly stable red tris chelates with a,a -diimines such as 1,10-phenanthroline or 2,2 -bipyridine that have been used extensively in spectrophotometric determinations of iron and in the estimation of various anions.5 In gravimetric estimations, iron(III) can be precipitated as the insoluble 8-hydroxyquinoline or a-nitroso-jS-naphthol complex which is then ignited to Fe203.6 In many situations the levels of free [Fe(H20)6]3+ may be controlled through complex formation by addition of edta. 

Some trace-metal transport systems are even more complex than the one described in Figure 5 and involve the release of metallophores into the medium. The archetypes of these—and the only ones characterized so far—are the side-rophores produced by various species of marine bacteria to acquire iron. In the model organisms in which they have been characterized, the mechanisms of uptake are quite varied and complex, often involving intermediate siderophores in the peri-plasmic space and several transport proteins (Neilands, 1981). The effect of such siderophores on iron bioavailability is clearly not the same as that of EDTA. While complexation by a siderophore makes iron directly available to the bacteria which take up the complex (and whose rate of iron uptake is proportional to FeY), it drastically reduces the bioavailability of iron to most other organisms (whose rate of iron uptake is proportional to Fe ). For organisms which are able to promote the release of iron from the siderophore, e.g., by reduction of Fe(III), the effect of complexation is a less drastic decrease in iron... 

EDTA is a common food preservative. Foods contain ions of iron, zinc, magnesium, and other metals. These are natural components of food substances, but they hasten the chemical reactions which cause flavor and color to deteriorate. EDTA added to foods forms strong, stable bonds to the metal ions, blocking their chemical activity. EDTA is also used to treat lead poisoning in human beings. The EDTA-lead complex is safely excreted in body waste. 

Dexrazoxane is the 5-( + )-isomer of razoxane. It is a potent intracellular chelating agent that is used for cardiopro-tection in patients receiving doxorubicin. - Dexrazoxane has two imide groups that open intracellularly to form a compound related to EDTA. This compound complexes with iron and interferes with free radical generation as.sociated with doxorubicin-iron complexes. 

The diphenylcarbazide method is almost specific for chromium(Vl). Interferences result only from Fe, V, Mo, Cu, and Hg(II) present at much higher concentrations than the chromium. Iron(lll) can be masked by phosphoric acid or EDTA. Iron(III) can also be separated as Fe(OH>3, after chromium has been oxidized to Cr(VI), or by extraction. Vanadium can be separated from Cr(VI) by extraction as its oxinate at pH -4. Molybdenum is masked with oxalic acid, and Hg(II) is converted into the chloride complex. 

FIGURE 3.4. Schematic structure of the AIF3-, binuclear AV-PO4. and EDTA-iron(ll) complex... 

There are few peroxide adducts of synthetic non-heme iron complexes that are well characterized (Table VI). Perhaps the best known adduct is that derived from Fe (EDTA) under basic conditions. This purple complex has an absorption maximum near 520 nm (e 528 M cm ) (163). These are absorptions characteristics associated with the peroxide-to-Fe" charge-transfer band in oxyHr however, the coordination mode of peroxide in the complex appears to be different from that in oxyFIr. After some debate in the literature, it has been concluded that the peroxide is T --bound on the basis of isotope effects observed in the Raman spectrum of the complex (158, 164). The v(O-O) of the H2 Oi complex is found at 815 cm". When is used, the v(O-O) shifts to 794 cm and appears as a peak of comparable line width. Were the peroxide only ri -bound, two peaks due to the Fe-O -O " and the Fe-" 0- 0 isotopomers would have been expected, as in oxyHr. An ri -peroxo structure is also proposed for [Fe(TPP)02] and has been determined for the corresponding [Mn(TPP)Oi] complex (165). 

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