Iron hydroxamate complexes

Fe(III) displacement of Al(III), Ga(III), or In(III) from their respective complexes with these tripodal ligands, have been determined. The M(III)-by-Fe(III) displacement processes are controlled by the ease of dissociation of Al(III), Ga(III), or In(III) Fe(III) may in turn be displaced from these complexes by edta (removal from the two non-equivalent sites gives rise to an appropriate kinetic pattern) (343). Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore-hydroxamate iron(III) complexes chloride and, to lesser extents, bromide and nitrate, catalyze ligand dissociation through transient coordination of the added anion to the iron (344). A catechol derivative of desferrioxamine has been found to remove iron from transferrin about 100 times faster than desferrioxamine itself it forms a significantly more stable product with Fe3+ (345). 

Table XVI shows a selection of stability constants and redox potentials for iron(II) and iron(III) complexes. This Table covers a wide range of the latter, showing how the relative stabilities of the iron(II) and iron(III) complexes are refiected in. B (Fe /Fe ) values. A more detailed illustration is provided by the complexes of a series of linear hexadentate hydroxypyridinonate and catecholate ligands, where again high stabilities for the respective iron(III) complexes are refiected in markedly negative redox potentials (213). The combination of the high stabilities of iron(III) complexes of hydrox5rpyridinones, as of hydroxamates, catecholates, and siderophores, and the low stabilities of their iron(II) analogues is also apparent in Fig. 8. Here redox potentials for hydroxypyranonate and hydroxypyridinonate complexes of iron are placed in the overall context of redox potentials for iron(III)/iron(II) couples. The -(Fe /Fe ) range for e.g., water, cyanide, edta, 2,2 -bipyridyl, and (substituted) 1,10-phenanthrolines is...

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. ... 

In contrast to the tris-catecholate siderophores, which form charged iron(III) complexes, the hydroxamate-based ferri-siderophore complexes are electrically neutral, which may influence their transport through biological membranes. 

The degeneracy of the non-chiral complexes can be removed by incorporating chiral centers, usually as resolved amino acids, into the arms at close vicinity to the hydroxamate iron binding sites. Thus, only one of the energetically non-equivalent diastereomers predominates, leading to pure enantiomeric iron(III) complexes with defined hehcity that allows assessing stereospecific recognition by the ferrichrome receptor. 

In accordance with Emery s retro-hydroxamate ferrichrome, mentioned above, two retro analogs of the linear ferrioxamine G and cyclic desferrioxamine E (129 and 130, respectively) were prepared. The iron-chelating properties were compared to DFO, showing that the linear retro-desferrioxamine G (131) binds iron faster and the cyclic retro desferrioxamine E (132) has improved affinity to iron, compared to the linear DFO. Based on these resnlts, many retro-hydroxamate ferrioxamines were prepared. In a later paper, Akiyama and coworkers reported the attachment of -cyclodextrin, a cyclic oligosaccharide, composed of seven a-D-glucopyranoside units, linked from position 1 to position 4, to linear retro-hydroxamate ferrioxamines (133 and 134), which formed 1 1 iron(III) complexes. Influenced by the chiral -cyclodextrin gronp, 133 and 134 formed A-selective coordination around the metal ion. In addition, Akiyama proposed that the... 

The presence of any of these functional types may be established chemically by applying the hydroxamic acid test. These compounds react with hydroxyl-amine in the presence of sodium hydroxide to form the sodium salt of the corresponding hydroxamic acid. On acidification and addition of iron(m) chloride solution the magenta coloured iron(m) complex of the hydroxamic acid is formed. 

Anderegg, G., F. L Eplattenier, and G. Schwarzenbach Hydroxamate Complex, III. Iron (III) exchange between sideramins and complexones. Helv. Chim. Acta 46, 1409 (1963). 

Schwarzenbach, G. and K. Schwarzenbach Hydroxamate complexes, I. The stability of the iron (III) complex of simple hydroxamic acids and ferrioxamine B. Helv. Chim. Acta 46, 1390 (1963). 

The structure of the simple hydroxamate complex tris(benzohy-droxamato) iron (III) (R = , R = H in Figure 1) has shown the most stable crystalline form of the solid to be the racemic cis isomer (11). (The convention for symbols of absolute configurations A and A are those of the IUPAC Proposal (12). The cis isomer is defined as the isomer... 

The markedly negative redox potentials of tris-catecholate and tris-hydroxamate iron complexes (Figure 4) may be ascribed to the high stabilities of the iron(III) complexes and the rather low stabilities of their iron(II) analogues. Table 9 details the relevant data (interconnected by a thermochemical cycle earlier applied to amino acid pentacyanoferrate complexes ), and documents the remarkably higher stabilities of tris-catecholate than of tris-hydroxamate complexes of iron(III). 

Figure 6 The geometry and dimensions of the iron-coordination octahedron in a natural tris(hydroxamate) complex, triacetylfiisari-nine. (Reprinted with permission from Ref. 71. 1980 American Chemical Society)...

A comparison of the stability constants of the naturally occurring siderophores uncovers a difference of f7 orders of magnitude between enterobactin (K most stable hydroxamate complex, ferrioxamine E. Using the more comparable pM values, enterobactin remains stiU eight orders of magnitude more effective than ferrioxamine E. Enterobactin has the highest affinity for Fe ion of any biological iron chelator tested so far. 

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