New insight into the oxidation of Fe(II) by desferrioxamine B (DFB): spectrophotometric and capillary electrophoresis (CE) study

Cr(VI) Reduction by Siderophore Alone and in Combination with Reduced Clay Minerals

Siderophores and iron-containing clays are known to influence the transformation of chromium in the environment. The role of clays in hexavalent chromium [Cr(VI)] reduction has been reported extensively. However, the mechanisms of Cr(VI) reduction by siderophores and their combination with iron-bearing clays are poorly known. Herein, we report the kinetics and products of Cr(VI) reduction by a siderophore alone or in combination with reduced clays. Results showed that Cr(VI) reduction by a tri-hydroxamate siderophore─desferrioxamine B (DFOB)─at a pH of 6 was achieved by one-electron transfer via the formation of Cr(V) intermediate. The formed Cr(V) was further reduced to organically complexed Cr(III). The Cr(VI) reduction rate and extent in the presence of both DFOB and reduced clays unexpectedly decreased relative to that with reduced clays alone, despite both serving as Cr(VI) reductants. The interaction between DFOB and clays (e.g., adsorption/intercalation, dissolution, and/or oxidation) was primarily responsible for Cr(VI) reduction inhibition. The extent of inhibition increased at higher DFOB concentrations in the presence of iron-rich nontronite but decreased in the presence of iron-poor montmorillonite, which may be related to their different Cr(VI) reduction mechanisms. This study highlights the importance of siderophores in chromium transformation and its impact on the reactivity of iron-bearing clays toward heavy metal reduction in the environment.

Oxidation of Fe(II) by Flavins under Anoxic Conditions

Flavin-mediated electron transfer is an important pathway for Fe(III) reduction by dissimilatory iron-reducing bacteria. Although the mechanisms and kinetics of Fe(III) reduction by reduced flavins have been widely studied, the reaction between Fe(II) and oxidized flavins is rarely investigated. Results of this study show that under anoxic conditions, Fe(II) can be oxidized by the oxidized forms of riboflavin (RBF) and flavin mononucleotide (FMN) at pH 7-9. For instance, at pH 9, 73% of 17.8 μM Fe(II) was oxidized by 10 μM RBF within 20 min. Both the rate and extent of oxidation increased with increasing concentrations of oxidized flavins and increasing solution pH. Thermodynamic calculations and kinetic analyses implied that the oxidation of Fe(II) proceeded predominantly via the autodecomposition of Fe²⁺-RBF^- and Fe²⁺-FMN^- complexes, along with minor contributions from direct oxidation of Fe(II) by flavins and flavin radicals. Our findings suggest that the reoxidation of Fe(II) by oxidized flavins may be a rate-controlling factor in microbial Fe(III) reduction via flavin-mediated electron transfer.

Mobilization and partitioning of rare earth elements in the presence of humic acids and siderophores

Developing rare earth elements (plus yttrium, REY) as a group of environmental tracer requires comprehensive understandings in their geochemical behaviors associated with natural organic matter. Recent work highlighted the promotions on REY mobilization and cerium oxidation by siderophores during silicate dissolution, but the mechanism remained ambiguous. Here, we performed batch fluid-rock interaction experiments to explore the functions of siderophore desferrioxamine B (DFOB) and humic acids (HA) towards REY mobility and partitioning during REY-bearing ferrihydrite dissolution. To acquire in-depth knowledge of organic controls on REY, we used multiple strategies, including elemental, multispectral, and electrochemical analyses, to investigate the organic regulation on REY geochemistry. This study sheds light on the function of ligand-specific selectivity and solid-fluid organic molecular fractionation, primarily dependent on hydrochemical settings (pH, organic compounds, ionic strength, and oxicity). Our results confirm the catalytic oxidation ability of ligand, which forms DFOB-Ce(IV) (K = 10⁴², electrochemistry), producing positive Ce anomalies in solutions by ligand-driven redox shifting. Both HA and DFOB showed high affinities to HREY, and facilitated LREY/HREY partitioning. The mobilization of REY and the development of Ce anomalies were limited by HA coatings that modified surface properties and disturbed the approach of DFOB. Excess siderophores attack inert HA coatings, facilitating REY liberation and Ce redox activities. The release of REY and catalytic oxidation of Ce can be inhibited at high ionic strength or under oxygen deficiency. Our study reveals that natural organic matter significantly influences the fate of REY in iron oxides, and crucial for the biogeochemical cycles of REY in nature.

Electrochemical determination of Ga(III) through formation of Ga(III)-deferrioxamine B nanostructures on the glassy carbon electrode surface

Selective and sensitive determination of Ga(III) in the presence of Fe(III), as the main interfering ion is studied by using glassy carbon electrode modified with deferrioxamine B (GC-DFO). Characterization and analytical application are performed by different methods including cyclic and differential pulse voltammetry (CV and DPV), electrochemical impedance spectroscopy (EIS), and Field Emission Scanning Electron Microscopy (FESEM). The DPV measurements showed two reduction peaks around -0.630 and -0.830V. While the current of both peaks varied linearly with Ga(III) concentration of the accumulation solution, the latter was more sensitive and used for construction of the calibration curve. The experimental parameters are studied and optimized. A dynamic calibration curve (6.0×10(-11) to 1.4×10(-9)molL(-1)), including a linear part, from 6.0×10(-11) to 1.0×10(-9)molL(-1) with mean RSDs of 5.3% for n=3 at 4.0×10(-10)molL(-1) Ga(III), and a detection limit of 2.0×10(-11) mol L(-1) Ga(III) is observed at the optimized conditions. The validity of the method and applicability of the sensor are successfully tested by determining of Ga(III) in natural (river) waters, rice and coal samples. The experimental data are presented and discussed from which the new sensor is characterized.

Reactivity of inorganic Mn and Mn desferrioxamine B with O2, O2(-), and H2O2 in seawater

Manganese (Mn) is a required element for oceanic phytoplankton as it plays a critical role in photosynthesis, through its unique redox chemistry, as the active site in photosystem II, and in enzymes that act as defenses against reactive oxygen species (ROS), most notably for protection against superoxide (O2(-)), through the action of superoxide dismutase (SOD), and against hydrogen peroxide (H2O2) via peroxidases and catalases. The distribution and redox speciation of Mn in the ocean is also apparently controlled by reactions with ROS. Here we examine the connections between ROS and dissolved Mn species in the upper ocean using field and laboratory experimental data. Our results suggest it is unlikely that significant concentrations of Mn(III) are produced in the euphotic zone, as in the absence of evidence for the existence of strong Mn(III) ligands, Mn(II) reacts with O2(-) to form the short-lived transient manganous superoxide, MnO2(+), which may react rapidly with other redox species in a manner similar to O2(-). Experiments with the strong Mn(III) chelator, desferrioxamine B (DFB), in seawater indicated that the Mn(III) species are unlikely to form, as formation of the precursor Mn(II) complex is hindered due to the stability of the Ca complex with DFB.

Quantitative structure-activity relationships for aqueous metal-siderophore complexes

Siderophores, biogenic chelating agents that facilitate the solubilization and uptake of ferric iron, form stable complexes with a wide range of nutrient and contaminant metals and thus may profoundly affect their fate, transport, and biogeochemical cycling. To understand more comprehensively the factors that control the stability and reactivity, as well as the potential for microbial uptake, of metal-siderophore complexes, we probed the structures of complexes formed between the trihydroxamate siderophore desferrioxamine B (DFOB) and Cu(II), Ga(III), Mn(II), Ni(II), and Zn(II) in solution by using extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that all metals studied are dominantly in octahedral coordination, with significant Jahn-Teller distortion of the Cu(II)HDFOB(0) complex. Additionally, log-transformed complex stability constants correlate not only with the charge-normalized interatomic distances within the complex, affirming and expanding existing predictive relationships, but also with the Debye-Waller parameter of the first coordination shell. The derived structure-activity relationships not only quantitatively relate the measured physical architecture of aqueous complexes to their observed stability but also allow for the prediction of siderophore-metal stability constants.

Determining subnanomolar iron concentrations in oceanic seawater using a siderophore-modified film analyzed by infrared spectroscopy

Iron is a bioactive trace element in seawater that regulates photosynthetic carbon dioxide drawdown and export from surface waters by phytoplankton in upward of 40% of the world's oceans. While autonomous sensor arrays are beginning to provide high-resolution data on temporal and spatial scales for some key oceanographic parameters, current analytical methods for iron are not amenable to autonomous platforms because of the need for user involvement and wet chemistry-based approaches. As a result, very large gaps remain in our understanding of iron distribution and chemistry in seawater. Here we present a straightforward nanostructure-based method to measure dissolved iron in natural seawater. The device comprises an iron-specific chelating biomolecule, desferrioxamine B (DFB), covalently immobilized on a mesoporous silica film. Changes in infrared spectral signatures of the immobilized DFB upon Fe(III) complexation provide an accurate and precise measure of iron on the surface of a chip exposed to seawater. The current system has a detection limit of approximately 50 pM for a 1-L sample at pH 1.7 and was used to measure dissolved iron in subarctic Pacific waters without interference from other elements in seawater. This technology provides a major step toward obtaining accurate iron measurements on autonomous research platforms.

N-phthaloyl-glycine-hydroxamic acid as serum iron chelator in rats

The aim of this study was to investigate the activity of N-phthaloyl-glycine-hydroxamic acid (Phth-Gly-HA) as a new iron chelator in vivo to be used in iron overload diseases. After intraperitoneal application of Phth-Gly-HA to male rats (1 mg kg(-1) body mass) once a day for seven days, iron serum level decreased by 21%, whereas the iron value dropped by 32% in female rats (1.5 mg kg(-1) body mass). The results indicate that the tested substance has the ability to bind serum iron by complexation. Besides transferrin iron release, mobilization of ferritin iron is also possible.

Complexation of desferricoprogen with trivalent Fe, Al, Ga, In and divalent Fe, Ni, Cu, Zn metal ions: effects of the linking chain structure on the metal binding ability of hydroxamate based siderophores

Complexes of the natural siderophore, desferricoprogen (DFC), with several trivalent and divalent metal ions in aqueous solution were studied by pH-potentiometry, UV-Vis spectrophotometry and cyclic voltammetry. DFC was found to be an effective metal binding ligand, which, in addition to Fe(III), forms complexes of high stability with Ga(III), Al(III), In(III), Cu(II), Ni(II) and Zn(II). Fe(II), however, is oxidized by DFC under anaerobic conditions and Fe(III) complexes are formed. By comparing the results with those of desferrioxamine B (DFB), it can be concluded that the conjugated beta-double bond slightly increases the stability of the hydroxamate chelates, consequently increases the stability of mono-chelated complexes of DFC. Any steric effect by the connecting chains arises only in the bis- and tris-chelated complexes. With metal ions possessing a relatively big ionic radius (Cu(II), Ni(II), Zn(II), In(III)) DFC, containing a bit longer chains than DFB, forms slightly more stable complexes. With smaller metal ions the trend is the opposite. Also a notable difference is that stable trinuclear complex, [Cu(3)L(2)], is formed with DFC but not with DFB. Possible bio-relevance of the Fe(II)/Fe(III) results is also discussed in the paper.

Capillary electrophoresis of inorganic ions: An update

This review as a sequel of three earlier similar reports gives a summary of the progress and significant methodological developments, starting from 2002, in the use of capillary electrophoresis (CE) for inorganic ion analysis. As substantiated by the illustrative number of relevant references, improvements in sensitivity achieved both in and outside a CE system, advances in manipulating the separation selectivity, novel hardware configurations, and system performance innovations are continually being reported over the review period. Specifically viewed are the recent advancements in elemental (bio)speciation analysis, which remains one of the most fertile areas of CE research, as well as in three recently booming research topics: contactless conductivity detection, separations on microchips, and transient isotachophoretic preconcentration. A state-of-the-art picture of technique's potentialities within the field of interest presented here demonstrates that CE has become recognized and is growing in acceptance as a reliable alternative to traditional analytical methods such as high-performance liquid chromatography (HPLC).