Iron-catecholate complex

The fruiting bodies of several Cortinarius species have a remarkable deep violet colour. Cortinarius violaceous, which is found in deciduous woodland, can concentrate iron by as much as 100-fold over other typical Basidiomycetes. The violet colour has been ascribed to an iron-catechol complex containing dihydroxyphenyl-p-alanine (7.35). The latter is formed by the rearrangement of l-DOPA. 

Considerable effort has been expended in studies of the interaction of metal ions with catechols with a view to understanding oxygenase activity. In aprotic media, the electrochemical properties of substituted catechols have been examined. Reactions of 3,5-di-tert-butyl-o-quinone with manganese(II) result in stable tris-Mn(IV) or bis-Mn(III) complexes of the corresponding catecholate dianion, Bu C , depending on whether the initial ratio of reactants is 1 3 or 1 2. This flexible redox chemistry may be important for redox catalysis. The O2 oxidation of the iron-catechol complex [Fe(salen)(Bu2CH)] has also been examined in aprotic media. 

The effect of the amino acid spacer on iron(III) affinity was investigated using a series of enterobactin-mimic TRENCAM-based siderophores (82). While TRENCAM (17) has structural similarities to enterobactin, in that it is a tripodal tris-catechol iron-binding molecule, the addition of amino acid spacers to the TRENCAM frame (Fig. 10) increases the stability of the iron(III) complexes of the analogs in the order ofbAla (19)complex stability is attributed to the intramolecular interactions of the additional amino acid side chains that stabilize the iron-siderophore complex slightly. 

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). 

The cleavage of catechols with the incorporation of oxygen is clearly favored in the presence of some of the iron(III) complexes as catalysts. Que and co-workers proposed a substrate activation mechanism for these reactions, wherein the delocalization of the unpaired spin density... 

According to a recent study with iron(III) complexes of tripodal ligands, systematic variation of one ligand arm strongly affects the steric shielding of the iron(III) center and the bonding of catechol substrates (61). It was shown that the dioxygenation reactions of catechols... 

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 values have been obtained for oxidation of benzenediols by [Fe(bipy)(CN)4], including the effect of pH, i.e., of protonation of the iron(III) complex, and the kinetics of [Fe(phen)(CN)4] oxidation of catechol and of 4-butylcatechol reported. Redox potentials of [Fe(bipy)2(CFQ7] and of [Fe(bipy)(CN)4] are available. The self-exchange rate constant for [Fe(phen)2(CN)2] has been estimated from kinetic data for electron transfer reactions involving, inter alios, catechol and hydroquinone as 2.8 2.5 x 10 dm moF s (in dimethyl sulfoxide). 

Alterobactin A is a cyclic mono-catechol-bis-hydroxycarboxylate bacterial siderophore with an extremely high affinity for iron(III) (see Table 12). On hydrolysis n the absence of iron) it gives an acyclic derivative which forms a bis-ligand iron(III) complex. ... 

Several macrocyclic polycatechols, with up to six catechol units incorporated into the ring, have been designed for possible treatment for iron overload and were prepared using high dilution techniques. They form stable iron(III) complexes the complex with the three-catechol ring as ligand has log K= ii.l... 

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. 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

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). 

EXAFS studies and the aforementioned high-resolution crystallography indicate that the catechol snbstrate binds in an asymmetric fashion to the iron(II) center with Fe-Ocatechoiate bond lengths that differ by 0.2 0.4 A. These structural parameters are in excellent agreement with those reported for synthetic iron(II)-monoanionic catecholate complexes, and, on the basis of this comparison, it was proposed that the catechol binds to the iron(n) center as a monoanion. This notion was supported by subsequent UV-resonance Raman andUV-vis studies. " The monoanionic nature of the catechol substrate in extradiol dioxygenases is in sharp contrast with the dianionic catecholate character commonly found in iron(III) complexes. This difference can be rationahzed by the differing Lewis acidities of the metal centers in their divalent and trivalent oxidation states. 

The electron exchange rate constant of the iron(III) complex in DMSO was estimated from the cross reactions with hydroquinone and catechol, which was compared with the rate constant obtained electrochemically. The mechanism of the ascorbic acid oxidation reaction in DMSO is discussed based on the Marcus theory. 

DMSO (Wako Pure Chemicals Inc.) was distilled twice from 4A molecular sieves(Wako) under reduced pressure. Dicyanobis(l,10-phenanthroline)iron(II) [Fe(CN)2(phen)2] was synthesized by mixing 0.03 mol of phen and 0.01 mol of ammonium iron(II) sulfate hexahydrate in 400 cm3 of water, followed by the addition of KCN (0.15 mol). The resulting crude crystals were then dissolved in 30 cm3 of concentrated sulfuric acid followed by the addition of ldm3 of water. Dicyanobis(l,10-phenanthroline)iron(III) nitrate was obtained by the oxidation of corresponding iron(II) complex with concentrated nitric acid. The perchlorate salt was obtained by the addition of sodium perchlorate to the nitrate solution. Analytical grade hydroquinone, catechol, and L-ascorbic acid (Wako) were used without further purification. 

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