Oxinobactin, a siderophore analogue to enterobactin involving 8-hydroxyquinoline subunits: synthesis and iron binding ability.

Oxinobactin, a siderophore analogue to enterobactin but possessing 8-hydroxyquinoline instead of catechol complexing subunits, has been synthesized starting from L-serine and 8-hydroxyquinoline. Comparative iron binding studies showed that oxinobactin is as effective as enterobactin for the complexation of Fe(III) at physiological pH but with improved complexing ability at acidic pH.

Vesicles to concentrate iron in low-iron media: an attempt to mimic marine siderophores.

Amphiphilic catechol-type iron chelators were studied with the aim of mimicking the properties of marine bacterial siderophores. The Fe(III) complexation constants and aqueous solution speciation of L(S10), a sulfonated catechol unit that has a C(10) lipophilic carbon chain connected by an amide linkage, were determined by spectrophotometric titration. The calculated value of pFe3+ is 18.1 at pH 7.4. Cryogenic transmission electron microscopy showed that the tris(catecholate) ferric complex formed at physiological pH initially assembles into micelles, in which the catecholate-iron units stay on the exterior of the micelle. The average diameter of these micelles was estimated to be 4.2 nm. The micelles then slowly rearrange into clusters of different sizes, which leads to the formation of unilamellar and bilamellar vesicles. The reorganization processes are comparable to those observed by Butler et al. for the marinobactin siderophores produced by marine bacteria, but in contrast to the marinobactins, vesicles of the Fe3+-L(S10) complex form without an excess of iron relative to ligand concentration. The time-dependent micelle-to-vesicle transition is discussed herein.

Iron-citrate complexes and free radicals generation: is citric acid an innocent additive in foods and drinks?

The generation of free radicals (Fenton chemistry) from various iron citrate complexes has been studied. Spin trapping methods have been used. The results can question concerning the innocence of added citric acid in foods and cold drinks. We concluded that in absence of pathological situation citric acid is probably not dangerous but it may become dangerous in situation of oxidative stress and/or iron overload.

Modulation of cell proliferation and polyamine metabolism in rat liver cell cultures by the iron chelator O-trensox.

The antiproliferative effects of the iron chelator O-trensox and the ornithine-decarboxylase (ODC) inhibitor alpha-difluoromethylornithine (DFMO) were characterized in the rat hepatoma cell line FAO, the rat liver epithelial cell line (RLEC) and the primary rat hepatocyte cultures stimulated by EGF. We observed that O-trensox and DFMO decreased cell viabilty and DNA replication in the three culture models. The cytostatic effect of O-trensox was correlated to a cytotoxicity, higher than for DFMO, and to a cell cycle arrest in G0/G1 or S phases. Moreover, O-trensox and DFMO decreased the intracellular concentration of spermidine in the three models without changing significantly the spermine level. We concluded that iron, but also polyamine depletion, decrease cell growth. However, the drop in cell proliferation obtained with O-trensox was stronger compared to DFMO effect. Altogether, our data provide insights that, in the three rat liver cell culture models, the cytostatic effect of the iron chelator O-trensox may be the addition of two mechanisms: iron and polyamine depletion.

Design of iron chelators: syntheses and iron (III) complexing abilities of tripodal tris-bidentate ligands.

The interest in synthetic siderophore mimics includes therapeutic applications (iron chelation therapy), the design of more effective agents to deliver Fe to plants and the development of new chemical tools in order to study iron metabolism and iron assimilation processes in living systems. The design of ligands needs a rational approach for the understanding of the metal ion complexing abilities. The octahedral arrangement of donor atoms is the most favourable geometry, allowing the maximum possible distance between their formal or partial negative charges. Hexadentate chelators, usually of the tris-bidentate type, can accommodate the metal coordination sphere and are well-suited to obtain high pFe values. The first part of this review is dedicated to selected synthetic routes, taking into account (i) the nature of the chelating subunits, connecting groups and spacers, (ii) the water-solubility and hydrophilic/lipophilic balance, (iii) the chirality and (iv) the possibility of grafting probes or vectors. In the second part, we discuss the role of the molecular design on complexing abilities (thermodynamics and kinetics). The bidentate 8-hydroxyquinoline moiety offers an alternative to the usual coordinating hydroxamic acids, catechols and/or alpha-hydroxycarboxylic acids groups encountered in natural siderophores. The promizing results obtained with the tris-hydroxyquinoline-based ligand O-TRENSOX are summarized. O-TRENSOX exhibits a high and selective affinity for Fe(III) complexation. Its efficiency in delivering Fe to plants, iron mobilization, cell protection, and antiproliferative effects has been evidenced. Other chelators derived from O-TRENSOX (mixed catechol/8-hydroxyquinoline ligands, lipophilic ligands) are also described. Some results question the relevance of partition coefficients to foresee the activity of iron chelators. The development of probes (fluorescent, radioactive, spin labelled) based on the O-TRENSOX backbone is in progress in order to get insights in the complicated iron metabolism processes.

Catecholase activity of a copper(II) complex with a macrocyclic ligand: unraveling catalytic mechanisms.

We report the structure, properties and a mechanism for the catecholase activity of a tetranuclear carbonato-bridged copper(II) cluster with the macrocyclic ligand [22]pr4pz (9,22-dipropyl-1,4,9,14,17,22,27,28,29, 30-decaazapentacyclo[22.2.1.1(4,7).1(11,14). 1(17,20)]triacontane-5,7(28),11(29),12,18, 20(30),24(27),25-octaene). In this complex, two copper ions within a macrocyclic unit are bridged by a carbonate anion, which further connects two macrocyclic units together. Magnetic susceptibility studies have shown the existence of a ferromagnetic interaction between the two copper ions within one macrocyclic ring, and a weak antiferromagnetic interaction between the two neighboring copper ions of two different macrocyclic units. The tetranuclear complex was found to be the major compound present in solution at high concentration levels, but its dissociation into two dinuclear units occurs upon dilution. The dinuclear complex catalyzes the oxidation of 3,5-di-tert-butylcatechol to the respective quinone in methanol by two different pathways, one proceeding via the formation of semiquinone species with the subsequent production of dihydrogen peroxide as a byproduct, and another proceeding via the two-electron reduction of the dicopper(II) center by the substrate, with two molecules of quinone and one molecule of water generated per one catalytic cycle. The occurrence of the first pathway was, however, found to cease shortly after the beginning of the catalytic reaction. The influence of hydrogen peroxide and di-tert-butyl-o-benzoquinone on the catalytic mechanism has been investigated. The crystal structures of the free ligand and the reduced dicopper(I) complex, as well as the electrochemical properties of both the Cu(II) and the Cu(I) complexes are also reported.

Valence tautomerism in octahedral and square-planar phenoxyl-nickel(II) complexes: are imino nitrogen atoms good friends?

The two tetradentate ligands H(2)L and H(2)L(Me) afford the slightly distorted square-planar low-spin Ni(II) complexes 1 and 2, which comprise two coordinated phenolate groups. Complex 1 has been electrochemically oxidized into 1(+), which contains a coordinated phenoxyl radical, with a contribution from the nickel orbital. In the presence of pyridine, 1(+) is converted into 1(Py) (+), an octahedral phenolate nickel(III) complex with two pyridines axially coordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H(2)L(Me), in the presence of pyridine, and the hexadentate ligand H(2)L(Py) in CH(2)Cl(2) afford, respectively, the octahedral high-spin Ni(II) complexes 2(Py) and 3, which involve two equatorial phenolates and two axially coordinated pyridines. At 100 K, the one-electron-oxidized product 2(Py) (+) comprises a phenoxyl radical ferromagnetically coupled to the high-spin Ni(II) ion, with large zero-field splitting parameters, while 3(+) involves a phenoxyl radical antiferromagnetically coupled to the high-spin Ni(II) ion.

The new orally active iron chelator ICL670A exhibits a higher antiproliferative effect in human hepatocyte cultures than O-trensox.

By comparing the antiproliferative effect of the iron chelators ICL670A and O-trensox in the human hepatoma cell line HUH7 and human hepatocyte cultures, we have shown that ICL670A decreased cell viability, inhibited DNA replication and induced DNA fragmentation more efficiently than O-trensox. O-trensox and ICL670A induced a cell cycle blockade in G0-G1 and S phases respectively. In parallel, ICL670A inhibited polyamine biosynthesis by decreasing ornithine decarboxylase and spermidine/spermine N(1)-acetyltransferase activities. O-trensox increased polyamine biosynthesis and particularly putrescine level by stimulating spermidine-spermine N(1)-acetyltransferase activity which could activate the polyamine retro-conversion pathway. Moreover, the two chelators exhibit some cytotoxic effect in the two culture models; ICL670A was more cytotoxic than O-trensox and higher concentrations of the two chelators were necessary to induce a cytotoxicity in primary cultures versus hepatoma cells. These results suggested that ICL670A has the most efficient antitumoral effect, blocks cell proliferation by a pathway different of O-trensox and may constitute a potential drug for anticancer therapy.

Fine tuning of the oxidation locus, and electron transfer, in nickel complexes of pro-radical ligands.

A large number of complexes of the first-row transition metals with non-innocent ligands has been characterized in the last few years. The localization of the oxidation site in such complexes can lead to discrepancies when electrons can be removed either from the metal center (leading to an M((n+1)+) closed-shell ligand) or from the ligand (leading to an M(n+) open-shell ligand). The influence of the ligand field on the oxidation site in square-planar nickel complexes of redox-active ligands is explored herein. The tetradentate ligands employed herein incorporate two di-tert-butylphenolate (pro-phenoxyl) moieties and one orthophenylenediamine spacer. The links between the spacer and both phenolates are either two imines ([Ni(L1)]), two amidates ([Ni(L3)]2-), or one amidate and one imine ([Ni(L2)]-). The structure of each nickel(II) complex is presented. In the noncoordinating solvent CH2Cl2, the one-electron-oxidized forms are ligand-radical species with a contribution from a singly occupied d orbital of the nickel. In the presence of an exogenous ligand, such as pyridine, a Ni(III) closed-shell ligand form is favored: axial ligation, which stabilizes the trivalent nickel in its octahedral geometry, induces an electron transfer from the metal(II) center to the radical ligand. The affinity of pyridine for the phenoxylnickel(II) species is correlated to the N-donor ability of the linkers.

New trends in the chemistry of iron(III) citrate complexes: correlations between X-ray structures and solution species probed by electrospray mass spectrometry and kinetics of iron uptake from citrate by iron chelators.

Despite the crucial role of "iron(III) citrate systems" in the iron metabolism of living organisms (bacteria as well as plants or mammals), the coordination chemistry of ferric citrate remains poorly defined. Variations in the experimental conditions used for the preparation of so-called ferric citrates (iron salt, Fe:cit molar ratio, base, pH, temperature, solvent) lead to several different species, which are in equilibrium in solution. To date, six different anionic complexes have been structurally characterized in the solid state, by ourselves or others. In the work described herein, we have established the experimental conditions leading to each of them. Five were obtained from aqueous solution. With the exception of a nonanuclear species (of which fragments have been detected), all were identified in aqueous solution on the basis of electrospray ionization mass spectrometry. In addition, the spectra revealed a new trinuclear species, which could not be crystallized. Kinetic studies of iron uptake from citrate species by iron chelators confirmed the results indicated by the ESI-MS studies. These studies also allowed the relative molar fraction of mononuclear versus polynuclear complexes to be determined, which depends on the Fe:cit molar ratio.

Substrate binding in catechol oxidase activity: biomimetic approach.

A series of dicopper(II) complexes have been investigated as model systems for the catechol oxidase active site enzyme, regarding the binding of catechol substrate in the first step of the catalytic cycle. The [Cu(2)(L(R))(mu-OH)](ClO(4))(2) and [Cu(2)(L(R))(H(2)O)(2)](ClO(4))(3) complexes are based on the L(R) ligands (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-R-substituted phenol) with -R = -OCH(3), -CH(3), or -F. Binding studies of diphenol substrates were investigated using UV-vis and EPR spectroscopy, electrochemistry, and (19)F NMR (fluorinated derivatives). All the complexes are able to bind two ortho-diphenol substrates (tetrachlorocatechol and 3,5-di-tert-butylcatechol). Two successive fixation steps, respectively fast and slower, were evidenced for the mu-OH complexes (the bis(aqua) complexes are inactive in catalysis) by stopped-flow measurement and (19)F NMR. From the mu-OH species, the 1:1 complex/substrate adduct is the catalytically active form. In relation with the substrate specificity observed in the enzyme, different substrate/inhibitor combinations were also examined. These studies enabled us to propose that ortho-diphenol binds monodentately one copper(II) center with the concomitant cleavage of the OH bridge. This hydroxo ligand appears to be a key factor to achieve the complete deprotonation of the catechol, leading to a bridging catecholate.

Comparative studies on the iron chelators O-TRENSOX and TRENCAMS: selectivity of the complexation towards other biologically relevant metal ions and Al(3+).

Complexation constants have been determined by potentiometric titration and spectrophotometric measurements for several biologically relevant divalent metals (Ca(2+), Cu(2+), Zn(2+)) as well as Al(3+) with the sulfonated tris(8-hydroxyquinolinate) tripodal ligand O-TRENSOX. The values demonstrate great selectivity of O-TRENSOX for Fe(3+) according to the sequence Fe(3+) >>Cu(2+)>Zn(2+)>Ca(2+). This selectivity is compared to that shown by tris(hydroxamate) and tris(catecholate) ligands. (1)H NMR spectroscopy of the diamagnetic complexes have been carried out in (2)H(2)O solutions.

Hydrophilic and lipophilic iron chelators with the same complexing abilities.

A new series of iron chelators with the same coordination sphere as the water-soluble ligand O-trensox, but featuring a variable hydrophilic-lipophilic balance, have been obtained by grafting oxyethylene chains of variable length on a C-pivot tripodal scaffold. The X-ray structure of a ferric complex exhibiting tris(8-hydroxyquinolinate) coordination and solution thermodynamic properties (pK(a) of the ligands, stability constants of the ferric complexes) have been determined. The complexing ability (pFe(III) values) of the ligands are similar to that of O-trensox. Partition coefficients between water and octanol or chloroform have been measured and transport across a membrane has been mimicked ("shuttle process"). The results of biological assays (iron chelation with free ligands or iron nutrition with ferric complexes) could not be correlated with the partition coefficients. These results call into question the role of distribution coefficients (of the ligands and/or complexes) in the biological activities of iron chelators.