James M. Harrington

Iron sequestration by small molecules: Thermodynamic and kinetic studies of natural siderophores and synthetic model compounds

Publisher Summary This chapter discusses researches on natural siderophores and synthetic siderophore-like molecules, with an emphasis on results from the laboratories over the past decade in the context of the overall field. These findings are discussed in terms of siderophore structure, the thermodynamics of iron sequestration, the kinetics of iron-exchange reactions in ferri-siderophore systems, and molecular recognition and transport. The chapter also discusses the expanding field of ferri-siderophore chemistry and its application to the design of synthetic iron chelators for the study of siderophore binding and uptake in mimics of biological systems, and in therapeutic applications. The use of iron chelators with high stability and specificity can be linked to the study of iron–siderophore chemistry, as an understanding of the structural and architectural features that microbes use to obtain iron from their environment can be used to design synthetic siderophore-like molecules for chelating iron in biological and engineered systems.

The redox hypothesis in siderophore-mediated iron uptake

The viability of iron(III/II) reduction as the initial step in the in vivo release of iron from its thermodynamically stable siderophore complex is explored.

Unusual Metal Ion Selectivities of the Highly Preorganized Tetradentrate Ligand 1,10-Phenanthroline-2,9-dicarboxamide: A Thermodynamic and Fluorescence Study

Some metal ion complexing properties of the ligand PDAM (1,10-phenanthroline-2,9-dicarboxamide) in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of PDAM as a function of metal ion concentration, log K(1) values in 0.1 M NaClO(4) and at 25 °C are, for Cu(II), 3.56(5); Ni(II), 3.06(5); Zn(II), 3.77(5); Co(II), 3.8(1); Mg(II), 0.1(1); Ca(II), 1.94(4); and Ba(II), 0.7(1). For more strongly bound metal ions, competition reactions between PDAM and EDTA (ethylenedinitrilo-tetraacetic acid) or tetren (1,4,7,10,13-pentaazatridecane), monitored following the UV spectrum of PDAM, gave the following log K(1) values in 0.1 M NaClO(4) and at 25 °C: Cd(II), 7.1(1); Pb(II), 5.82(5); In(III), 9.4(1); and Bi(III), 9.4(1). The very low log K(1)(PDAM) values for small metal ions such as Cu(II) or Zn(II) are unprecedented for a phen-based ligand (phen = 1,10-phenanthroline), which is rationalized in terms of the low basicity of the N donors of the ligand (pK(a) = 0.6) and the fact that PDAM has a best-fit size corresponding to large metal ions of ionic radius ~1.0 Å. Large metal ions with ionic radius ≥1.0 Å show large increases in log K(1) relative to their phen complexes, which in turn produces unparalleled selectivities, such as a 3.5 log units greater log K(1)(PDAM) for Cd(II) than for Cu(II). PDAM shows strong fluorescence in aqueous solution, suggesting that its carboxamide groups do not produce a fluorescence-quenching photon-induced electron transfer (PET) effect. Only Ca(II) produces a weak CHEF (chelation enhanced fluorescence) effect with PDAM, while all other metal ions tested produce a decrease in fluorescence, a CHEQ (chelation enhanced quenching effect). The production of the CHEQ effect is rationalized in terms of the idea that coordination of metal ions to PDAM stabilizes a canonical form of the carboxamide groups that promotes a PET effect.

Characterization of Fe(III) sequestration by an analog of the cytotoxic siderophore brasilibactin A: implications for the iron transport mechanism in mycobacteria.

Mycobacteria such as M. tuberculosis represent a significant health concern throughout much of the developing world. In mycobacteria and other pathogenic bacteria, an important virulence factor is the ability of the bacterium to obtain iron from its host. One means of obtaining iron is through the use of siderophores. Brasilibactin A is a membrane bound siderophore produced by Nocardia brasiliensis with structural similarity to the mycobactin class of siderophore in mycobacteria. A characterization of the protonation constants and Fe(III) affinity of a water soluble Brasilibactin A analog (Bbtan) has been performed. Using protonation constants and competition with EDTA, the stability constant of the 1 : 1 Fe(III)-Bbtan complex was found to be log β(110) = 26.96. The pFe of Bbtan is 22.73, somewhat low for a proposed siderophore molecule. The redox potential of the Fe-Bbtan complex was found to be -300 mV vs. NHE, very high for an iron-siderophore complex. The combination of relatively low complex stability and ease of iron reduction may play a crucial role in the mechanism of mycobactin siderophore-mediated iron uptake in mycobacteria and related organisms.

Amelioration of Metal-Induced Toxicity in Caenorhabditis elegans: Utility of Chelating Agents in the Bioremediation of Metals

The presence of toxic amounts of transition metals in the environment may originate from a range of human activities and natural processes. One method for the removal of toxic levels of metals is through chelation by small molecules. However, chelation is not synonymous with detoxification and may not affect the bioavailability of the metal. To test the bioavailability of chelated metals in vivo, the effects of several metal/chelator combinations were tested in the environmentally relevant organism Caenorhabditis elegans. The effect of metal exposure on nematode growth was used to determine the toxicity of cadmium, copper, nickel, and zinc. The restoration of growth to levels observed in nonexposed nematodes was used to determine the protective effects of the polydentate chelators: acetohydroxamic acid (AHA), cyclam, cysteine, calcium EDTA, desferrioxamine B, 1,2-dimethyl,3-hydroxy,4-pyridinone, and histidine. Cadmium toxicity was removed only by EDTA; copper toxicity was removed by all of the chelators except AHA; nickel toxicity was removed by cyclam, EDTA, and histidine; and zinc toxicity was removed by only EDTA. These results demonstrate the utility of polydentate chelators in the remediation of metal-contaminated systems. They also demonstrate that although the application of a chelator to metal contaminants may be effective, binding alone cannot be used to predict the level of remediation. Remediation depends on a number of factors, including metal complex speciation in the environment.

Fe(III)-complexes of the tripodal trishydroxamate siderophore basidiochrome: potential biological implications.

One method of mobilization of iron by mycorrhizal organisms is through the secretion of small organic chelators called siderophores. Hydroxamate donor chelators are a common type of siderophore that is frequently used by fungal organisms. The primary siderophore that is produced by fungi from the genera Ceratobasidium and Rhizoctonia is the tripodal trishydroxamate siderophore basidiochrome. To gain some insight into the iron uptake mechanisms of these symbiotic fungi, the iron binding characteristics of basidiochrome were determined. It was found that basidiochrome exhibits a log β(110) of 27.8±0.1 and a pFe value of 25.0. These values are similar to those of another fungal trishydroxamate siderophore, ferrichrome. The similarity in iron affinity between the two siderophores suggests that the structure of the backbone has little influence in complex formation due to the length of the pendant arms, although the identity of the terminating groups of the pendant arms is likely related to complex stability. The role of basidiochrome in the biogeochemical cycling of iron is also discussed.

The kinetics of dimethylhydroxypyridinone interactions with iron(III) and the catalysis of iron(III) ligand exchange reactions: implications for bacterial iron transport and combination chelation therapies

Many microbes acquire environmental Fe by secreting organic chelators, siderophores, which possess the characteristics of a high and specific binding affinity for iron(iii) that results in the formation of thermodynamically stable, and kinetically inert iron(iii) complexes. Mechanisms to overcome the kinetic inertness include the labilization of iron(iii) by means of ternary complex formation with small chelators. This study describes a kinetic investigation of the labilization of iron(iii) between two stable binding sites, the prototypical siderophore ferrioxamine B and EDTA, by the bidentate siderophore mimic, 1,2-dimethyl-3-hydroxy-4-pyridinone (L1, H(DMHP)). The proposed mechanism is substantiated by investigating the iron(iii) exchange reaction between ferrioxamine B and H(DMHP) to form Fe(DMHP)3, as well as the iron(iii) exchange from Fe(DMHP)3 to EDTA. It is also shown that H(DMHP) is a more effective catalyst for the iron(iii) exchange reaction than bidentate hydroxamate chelators reported previously, supporting the hypothesis that chelator structure and iron(iii) affinity influence low denticity ligand facilitated catalysis of iron(iii) exchange reactions. The results are also discussed in the context of the design and use of combination chelator therapies in the treatment of Fe overload in humans.