Fe(III) coordination properties of a new saccharide-based exocyclic trihydroxamate analogue of ferrichrome

Antioxidant capacity of two novel bioactive Fe(III)-cyclophane complexes

The cyclophanes 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-1,4,7,10, 24,27,30,33-octaaza-17,40-dioxa[10.1.10.1]paracyclophane (PO) and 2,9,25,32-tetraoxo-4,7,27,30-tetrakis(carboxymethyl)-1,4,7,10,24,27,30,33-octaaza[10.1.10.1]paracyclophane (PC) were coordinated with iron to form cationic binuclear Fe(III) Fe₂PO and Fe₂PC complexes, respectively. Their antioxidant capacity, superoxide dismutase and peroxidase mimetic activity, as well as their toxicity toward peripheral blood mononuclear cells (PBMCs) were evaluated. Both Fe₂PO and Fe₂PC are interesting biomimetics with antioxidant capacity similar to that of ascorbic acid that prevent mortality in cultured PBMCs, with the potential to have bioactive and protective functions in disease animal models.

Fluorescent 3-hydroxy-4-pyridinone hexadentate iron chelators: intracellular distribution and the relevance to antimycobacterial properties

We report the synthesis and characterization of a fluorescent iron chelator (4), shown to be effective in inhibiting the growth of Mycobacterium avium in macrophages, together with the synthesis and characterization of two unsuccessful analogues selected to facilitate identification of the molecular properties responsible for the antimicrobial activity. Partition of the chelators in liposomes was investigated and the compounds were assessed with respect to uptake by macrophages, responsiveness to iron overload/iron deprivation and intracellular distribution by flow cytometry and confocal microscopy. The synthesis of the hexadentate chelators is based on a tetrahedral structure to which three bidentate 3-hydroxy-4-pyridinone chelating units are linked via amide bonds. The structure is synthetically versatile, allowing further addition of functional groups such as fluorophores. Here, we analyse the non-functionalized hexadentate unit (3) and the corresponding rhodamine B (4) and fluorescein (5) labelled chelators. The iron(III) stability constant was determined for 3 and the values log beta = 34.4 and pFe(3+) = 29.8 indicate an affinity for iron of the same order of magnitude as that of mycobacteria siderophores. Fluorescence properties in the presence of liposomes show that 4 strongly interacts with the lipid phase, whereas 5 does not. Such different behaviour may explain their distinct intracellular localization as revealed by confocal microscopy. The flow cytometry and confocal microscopy studies indicate that 4 is readily engulfed by macrophages and targeted to cytosol and vesicles of the endolysosomal continuum, whereas 5 is differentially distributed and only partially colocalizes with 4 after prolonged incubation. Differential distribution of the compounds is likely to account for their different efficacy against mycobacteria.

Iron Chelation Equilibria, Redox, and Siderophore Activity of a Saccharide Platform Ferrichrome Analogue

A complete characterization of the aqueous solution Fe(III) and Fe(II) coordination chemistry of a saccharide-based ferrichrome analogue, 1-O-methyl-2,3,6-tris-O-[4-(N-hydroxy-N-ethylcarbamoyl)-n-butyryl]-alpha-D-glucopyranoside (H3LN236), is reported including relevant thermodynamic parameters and growth promotion activity with respect to both Gram-negative and Gram-positive bacterial strains. The saccharide platform is an attractive backbone for the design and synthesis of ferrichrome analogues because of its improved water solubility and hydrogen-bonding capabilities, which can potentially provide favorable receptor recognition and biological activity. The ligand deprotonation constants (pKa values), iron complex (FeIII(LN236) and FeII(LN236)1-) protonation constants (KFeHxL-236-N), overall Fe(III) and Fe(II) chelation constants (beta110), and aqueous solution speciation were determined by spectrophotometric and potentiometric titrations, EDTA competition equilibria, and cyclic voltammetry. Log betaIII110 = 31.16 and pFe = 26.1 for FeIII(LN236) suggests a high affinity for Fe(III), which is comparable to or greater than ferrichrome and other ferrichrome analogues. The E1/2 for the FeIII(LN236)/FeII(LN236)1- couple was determined to be -454 mV (vs NHE) from quasi-reversible cyclic voltammograms at pH 9. Below pH 6.5, the E1/2 shifts to more positive values and the pH-dependent E1/2 profile was used to determine the FeII(LN236)1- protonation constants and overall stability constant log betaII110 = 11.1. A comparative analysis of similar data for an Fe(III) complex of a structural isomer of this exocyclic saccharide chelator (H3LR234), including strain energy calculations, allows us to analyze the relative effects of the pendant arm position and hydroxamate moiety orientation (normal vs retro) on overall complex stability. A correlation between siderophore activity and iron coordination chemistry of these saccharide-hydroxamate chelators is made.

Superoxide dismutase-like activity of cobalt(ii) complexes based on a sugar platform

A SOD-like activity evaluated by a modified McCord-Fridovich test was evidenced for two Co(II) complexes built from "glycoligands" using a sugar platform derived from d-galactose and D-galactal and functionalized by three 2-picolyl groups.

Synthesis, Solution Behavior, Thermal Stability, and Biological Activity of an Fe(III) Complex of an Artificial Siderophore with Intramolecular Hydrogen Bonding Networks

Previously, an artificial siderophore complex, the iron(III) complex with tris[2-[(N-acetyl-N-hydroxy)glycylamino]ethyl]amine (TAGE), was constructed in order to understand the effect of intramolecular hydrogen bonding interaction on the siderophore function, and its structural characterization in the solid state was reported (Inorg. Chem. 2001, 40, 190). In this paper, the solution behavior of the M(III)-TAGE (M = Fe, Ga) system has been investigated using (1)H NMR, UV-vis, and FAB mass spectroscopies in efforts to characterize the biological implication of hydrogen bonding networks between the amide hydrogens and coordinating aminohydroxy oxygens of the complex. The temperature dependence of (1)H NMR spectra for Ga(III) complex of TAGE indicates that hydrogen bonding networks are maintained in polar solvents such as DMSO-d(6) and D(2)O. The UV-vis spectra of the Fe(III)-TAGE system under various pH conditions have shown that TAGE forms a tris(hydroxamato)iron(III) complex in an aqueous solution in the pH range 4-8. By contrast, tris[2-[(N-acetyl-N-hydroxy)propylamido]ethyl]amine (TAPE; TAGE analogue that is difficult to form intramolecular hydrogen bonding networks), which has been synthesized as a comparison of TAGE, forms both of bis- and tris(hydroxamato)iron(III) complexes in the same pH range. Both the stability constants (log beta(FeTAGE) = 28.6; beta(FeTAGE) = [Fe(III)TAGE]/([Fe(3+)][TAGE(3)(-)])) and pM (-log[Fe(3+)]) value for Fe(III)TAGE (pM 25) are comparable to those of a natural siderophore ferrichrome (log beta = 29.1 and pM 25.2). The kinetic study of the TAGE-Fe(III) system has given the following rate constants: the rate of the ligand exchange reaction between Fe(III)TAGE and EDTA is 6.7 x 10(-4) s(-1), and the removal rates of iron from diferric bovine plasma transferrin by TAGE are 2.8 x 10(-2) and 6.0 x 10(-3) min(-1). These values are also comparable to those of a natural siderophore desferrioxamine B under the same conditions. In a biological activity experiment, TAGE has promoted the growth of the siderophore-auxotroph Gram-positive bacterium Microbacterium flavescens, suggesting that TAGE mimics the activity of ferrichrome. These results indicate that the artificial siderophore with intramolecular hydrogen bonding networks, TAGE, is a good structural and functional model for a natural ferrichrome.

Survival of Aspergillus fumigatus in serum involves removal of iron from transferrin: the role of siderophores

Aspergillus fumigatus is a filamentous fungus which can cause invasive disease in immunocompromised individuals. A. fumigatus can grow in medium containing up to 80% human serum, despite very low concentrations of free iron. The purpose of this study was to determine the mechanism by which A. fumigatus obtains iron from the serum iron-binding protein transferrin. In iron-depleted minimal essential medium (MEM), A. fumigatus growth was supported by the addition of holotransferrin (holoTf) or FeCl(3) but not by the addition of apotransferrin (apoTf). Proteolytic degradation of transferrin by A. fumigatus occurred in MEM-serum; however, transferrin degradation did not occur until late logarithmic phase. Moreover, transferrin was not degraded by A. fumigatus incubated in MEM-holoTf. Urea polyacrylamide gel electrophoresis showed that in MEM-holoTf, holoTf was completely converted to apoTf by A. fumigatus. In human serum, all of the monoferric transferrin was converted to apoTf within 8 h. Siderophores were secreted by A. fumigatus after 8 h of growth in MEM-serum and 12 h in MEM-holoTf. The involvement of small molecules in iron acquisition was confirmed by the fact that transferrin was deferrated by A. fumigatus even when physically separated by a 12-kDa-cutoff membrane. Five siderophores were purified from A. fumigatus culture medium, and the two major siderophores were identified as triacetylfusarinine C and ferricrocin. Both triacetylfusarinine C and ferricrocin removed iron from holoTf with an affinity comparable to that of ferrichrome. These data indicate that A. fumigatus survival in human serum in vitro involves siderophore-mediated removal of iron from transferrin. Proteolytic degradation of transferrin may play a secondary role in iron acquisition.

Iron chelation properties of an extracellular siderophore exochelin MN

The coordination chemistry of an extracellular siderophore produced by Mycobacterium neoaurum, exochelin MN (ExoMN), is reported along with its pK(a) values, Fe(III) and Fe(II) chelation constants, and aqueous solution speciation as determined by spectrophotometric and potentiometric titration techniques. Exochelin MN is of particular interest as it can efficiently transport iron into pathogenic M. leprae, which is responsible for leprosy, in addition to its own parent cells. The Fe(III) coordination properties of ExoMN are important with respect to understanding the Fe(III) acquisition and uptake mechanism in pathogenic M. leprae, as the siderophores from this organism are very difficult to isolate. Exochelin MN has two hydroxamic acid groups and an unusual threo-beta-hydroxy-l-histidine available for Fe(III) chelation. The presence of threo-beta-hydroxy-l-histidine gives rise to a unique mode of Fe(III) coordination. The pK(a) values for the two hydroxamic acid moieties, the histidine imidazole ring and the alkylammonium groups on ExoMN, correspond well with the literature values for these moieties. Proton-dependent Fe(III)- and Fe(II)-ExoMN equilibrium constants were determined using a model involving sequential protonation of the Fe(III)- and Fe(II)-ExoMN complexes. These data were used to develop a model whereby deprotonation reactions on the surface of the complex in the second coordination shell result in first coordination shell isomerization. The overall formation constants were calculated: log beta(110) = 39.12 for Fe(III)-ExoMN and 16.7 for Fe(II)-ExoMN. The calculated pFe value of 31.1 is one of the highest among all siderophores and their synthetic analogues and indicates that ExoMN is thermodynamically capable of removing Fe(III) from transferrin. The E(1/2) for the Fe(III)ExoMN/Fe(II)ExoMN(-) couple was determined to be -595 mV from quasi-reversible cyclic voltammograms at pH = 10.8, and the pH-dependent E(1/2) profile was used to determine the Fe(II)-ExoMN protonation constants.