Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions

Inactivation of siderophore iron-chelating moieties by the fungal wheat root symbiont Pyrenophora biseptata

We investigated the ability of four plant and soil-associated fungi to modify or degrade siderophore structures leading to reduced siderophore iron-affinity in iron-limited and iron-replete cultures. Pyrenophora biseptata, a melanized fungus from wheat roots, was effective in inactivating siderophore iron-chelating moieties. In the supernatant solution, the tris-hydroxamate siderophore desferrioxamine B (DFOB) underwent a stepwise reduction of the three hydroxamate groups in DFOB to amides leading to a progressive loss in iron affinity. A mechanism is suggested based on the formation of transient ferrous iron followed by reduction of the siderophore hydroxamate groups during fungal high-affinity reductive iron uptake. P. biseptata also produced its own tris-hydroxamate siderophores (neocoprogen I and II, coprogen and dimerum acid) in iron-limited media and we observed loss of hydroxamate chelating groups during incubation in a manner analogous to DFOB. A redox-based reaction was also involved with the tris-catecholate siderophore protochelin in which oxidation of the catechol groups to quinones was observed. The new siderophore inactivating activity of the wheat symbiont P. biseptata is potentially widespread among fungi with implications for the availability of iron to plants and the surrounding microbiome in siderophore-rich environments.

A comparative study on the metal complexes of an anticancer estradiol-hydroxamate conjugate and salicylhydroxamic acid

Hydroxamic acids bearing an (O,O) donor set are well-known metal-chelating compounds with diverse biological activities including anticancer activity. Since steroid conjugation with a pharmacophoric moiety may have the potential to improve this effect, a salicylhydroxamic acid-estradiol hybrid molecule (E2HA) was synthesized. Only minimal effect of the conjugation on the proton dissociation constants was observed in comparison to salicylhydroxamic acid (SHA). The complexation with essential metal ions (iron, copper) was characterized, since E2HA may exert its cytotoxicity through the binding of these ions in cells. UV-visible spectrophotometric and pH-potentiometric titrations revealed the formation of high-stability complexes, while the Fe(III) preference over Fe(II) was proved by cyclic voltammetry and spectroelectrochemical measurements. Complex formation with half-sandwich Rh(III)(η⁵-Cp*) and Ru(II)(η⁶-p-cymene) organometallic cations was also studied as it may improve the anticancer effect and the pharmacokinetic profile of the ligand. At equimolar concentration the speciation is complicated because of the presence of mononuclear and binuclear complexes. The complexes readily react with small molecules e.g. glutathione, 1-methylimidazole and nucleosides, having major effect on solution speciation, namely mixed-ligand complex formation and ligand displacement occur. These processes serve as models for the interactions with biomolecules in the body. E2HA exerted moderate anticancer activity (IC₅₀ = 25-59 μM) in the tested three human cancer cell lines (Colo205, Colo320 and MCF-7), while being non-toxic on non-cancerous MRC-5 cells. Meanwhile, SHA was inactive in the same cells. Complexation with half-sandwich Rh(III) and Ru(II) cations had only a minor improvement on the cytotoxic effect of E2HA.

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.

Reversible Metal Ion/Complex Binding to Chitin Controlled by Ligand, Redox, and Photochemical Reactions and Active Movement of Chitin on Aquatic Arthropods

There is strong adsorption of metal ions and their complexes to chitin, which depends on both the oxidation and complexation states of many of the said elements (whereas others display chemical reactions detectable via electrochemical methods while being retained by chitin); thus, ad- and desorption at ambient water concentrations (often in the nMol/L range) are controlled by the presence and photochemical properties (concerning Eu and probably U and Ag) of mainly biogenic organic matter (both DOC and POC, and DON). With chitin forming the outer hull of mobile organisms (animals), this biopolymer is expected to take part in metal distribution in aquatic (limnetic and riverine) ecosystems. Having studied the attachment of many different elements to both crayfish and grafted (marine shrimp) chitin, with the highest accumulations observed in Bi, V, Ni, and LREEs, one should consider secondary biochemical transformations which take place at different water and sediment levels. After chitin had been embedded into sediment, methanogenesis (which requires Ni), Bi, and Sb biomethylations and photodesorption in the illuminated water column will occur if there are appropriate organics, causing the vertical separation of Eu from other REEs, at least during the daytime. Eutrophication will enhance both the production and especially the photooxidation rates of organics in water because phosphorylated sugars and lipids are formed quantitatively within min P, which enter water and undergo Eu-mediated photooxidation much more readily. Another biopolymer, gelatin, acts as an inert matrix-enhancing organic photooxidation product via Eu, producing chemical waves, indicating autocatalysis upon light impact. From the redox-related photodesorption of metal analytes from chitin, both sensors and devices for (light-assisted) electrochemical energy conversion are being developed by our workgroup. The electrochemical determination of adsorption thermodynamics on chitin is thus directly linked to its applications in environmental monitoring and technology.

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.

Trends and Exceptions in the Interaction of Hydroxamic Acid Derivatives of Common Di- and Tripeptides with Some 3d and 4d Metal Ions in Aqueous Solution

By using various techniques (pH-potentiometry, UV-Visible spectrophotometry, ¹H and ¹⁷O-NMR, EPR, ESI-MS), first time in the literature, solution equilibrium study has been performed on complexes of dipeptide and tripeptide hydroxamic acids—AlaAlaNHOH, AlaAlaN(Me)OH, AlaGlyGlyNHOH, and AlaGlyGlyN(Me)OH—with 4d metals: the essential Mo(VI) and two half-sandwich type cations, [(η⁶-p-cym)Ru(H₂O)₃]²⁺ as well as [(η⁵-Cp*)Rh(H₂O)₃]²⁺, the latter two having potential importance in cancer therapy. The tripeptide derivatives have also been studied with some biologically important 3d metals, such as Fe(III), Ni(II), Cu(II), and Zn(II), in order to compare these new results with the corresponding previously obtained ones on dipeptide hydroxamic acids. Based on the outcomes, the effects of the type of metal ions, the coordination number, the number and types of donor atoms, and their relative positions to each other on the complexation have been evaluated in the present work. We hope that these collected results might be used when a new peptide-based hydroxamic acid molecule is planned with some purpose, e.g., to develop a potential metalloenzyme inhibitor.

Unravelling the modus-operandi of chromenylium-cyanine fluorescent probes: a case study

Small-molecule fluorescent probes having optimized optical properties, such as high photostability and brightness, local microenvironment sensitivity and specific subcellular localizations, are increasingly available. Although the basis for designing efficient fluorophores for bioimaging applications is well established, implementing an improvement in a given photophysical characteristic always tends to compromise another optical property. This problem has enormous consequences for in vivo imaging, where ensuring a specific localization and precise control of the probe response is challenging. Herein we discuss a fluorescent probe, CC334, as a case study of the chromenylium-cyanine family that commonly exhibits highly complex photophysical schemes and highly interfered bioanalytical responses. By an exhaustive and concise analysis of the CC334 optical responses including detailed spectroscopic calibrations, steady-state microenvironment effects, ultrafast photophysics analysis and computational studies, we elucidate a new strategy to apply the probe in the singlet oxygen reactive oxygen species (1O2-ROS) monitoring using in vitro and in vivo models. The probe provides a new avenue for designing fluorescent probes to understand the dynamic behavior of subcellular environments.

The kinetics of siderophore-mediated olivine dissolution

Silicate minerals represent an important reservoir of nutrients at Earth's surface and a source of alkalinity that modulates long-term geochemical cycles. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient immobilization by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of micro-organisms to actively scavenge these nutrients via redox alteration and/or organic ligand production. In this study, we use targeted laboratory experiments with olivine and the siderophore deferoxamine B to explore how microbial ligands affect nutrient (Fe) release and the overall rate of mineral dissolution. Our results show that olivine dissolution rates are accelerated in the presence of micromolar concentrations of deferoxamine B. Based on the non-linear decrease in rates with time and formation of a Fe³⁺ -ligand complex, we attribute this acceleration in dissolution rates to the removal of an oxidized surface coating that forms during the dissolution of olivine at circum-neutral pH in the presence of O₂ and the absence of organic ligands. While increases in dissolution rates are observed with micromolar concentrations of siderophores, it remains unclear whether such conditions could be realized in natural environments due to the strong physiological control on microbial siderophore production. So, to contextualize our experimental results, we also developed a feedback model, which considers how microbial physiology and ligand-promoted mineral dissolution kinetics interact to control the extent of biotic enhancement of dissolution rates expected for different environments. The model predicts that physiological feedbacks severely limit the extent to which dissolution rates may be enhanced by microbial activity, though the rate of physical transport modulates this limitation.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were clinically developed for a variety of diseases, including tuberculosis, viral infections, malaria, and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity.

CRITICAL ISSUES

In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim of bridging the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO), which are used for the treatment of iron overload.

FUTURE DIRECTIONS

We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways in a complex and multi-faceted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g., complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

Photochemistry of Acetohydroxamic Acid in Solid Argon. FTIR and Theoretical Studies

The products formed during exposure of the CH₃CONHOH/Ar (AHA/Ar) matrices to the full output of the Xe lamp and to 225 nm OPO radiation are studied. The irradiation promotes the isomerization, 1Z → 1E, and AHA photodissociation reactions. Four pairs of coproducts are experimentally found to appear in the photolysis, they form the complexes: CH₃OH···HNCO (1), H₂O···CH₃NCO (2), H₂O···CH₃CNO (3) and CO···CH₃NHOH (4). The structures of the complexes were optimized at the MP2 computational level with the 6-311++G(2d,2p) and aug-cc-pVTZ basis sets. Three local minima were predicted for the complex (1), two for the complexes (2) and (3) and four local minima were found for the complex (4). The comparison of the theoretical spectra with the experimental ones allowed us to determine the structures of the complexes formed in the matrix. The mechanisms of the reaction channels leading to formation of the four coproducts are proposed. It is concluded that the first step in formation of the (1), (2) and (3) complexes is the scission of the N-O bond whereas the creation of the complex (4) is due to the cleavage of the C-N bond.

Ferrous iron content of intravenous iron formulations

The observed biological differences in safety and efficacy of intravenous (IV) iron formulations are attributable to physicochemical differences. In addition to differences in carbohydrate shell, polarographic signatures due to ferric iron [Fe(III)] and ferrous iron [Fe(II)] differ among IV iron formulations. Intravenous iron contains Fe(II) and releases labile iron in the circulation. Fe(II) generates toxic free radicals and reactive oxygen species and binds to bacterial siderophores and other in vivo sequestering agents. To evaluate whether differences in Fe(II) content may account for some observed biological differences between IV iron formulations, samples from multiple lots of various IV iron formulations were dissolved in 12 M concentrated HCl to dissociate and release all iron and then diluted with water to achieve 0.1 M HCl concentration. Fe(II) was then directly measured using ferrozine reagent and ultraviolet spectroscopy at 562 nm. Total iron content was measured by adding an excess of ascorbic acid to reduce Fe(III) to Fe(II), and Fe(II) was then measured by ferrozine assay. The Fe(II) concentration as a proportion of total iron content [Fe(III) + Fe(II)] in different lots of IV iron formulations was as follows: iron gluconate, 1.4 and 1.8 %; ferumoxytol, 0.26 %; ferric carboxymaltose, 1.4 %; iron dextran, 0.8 %; and iron sucrose, 10.2, 15.5, and 11.0 % (average, 12.2 %). The average Fe(II) content in iron sucrose was, therefore, ≥7.5-fold higher than in the other IV iron formulations. Further studies are needed to investigate the relationship between Fe(II) content and increased risk of oxidative stress and infections with iron sucrose.

Synergistic effect of reductive and ligand-promoted dissolution of goethite

Ligand-promoted dissolution and reductive dissolution of iron (hydr)oxide minerals control the bioavailability of iron in many environmental systems and have been recognized as biological iron acquisition strategies. This study investigated the potential synergism between ligands (desferrioxamine B (DFOB) or N,N'-Di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED)) and a reductant (ascorbate) in goethite dissolution. Batch experiments were performed at pH 6 with ligand or reductant alone and in combination, and under both oxic and anoxic conditions. Goethite dissolution in the presence of reductant or ligand alone followed classic surface-controlled dissolution kinetics. Ascorbate alone does not promote goethite dissolution under oxic conditions due to rapid reoxidation of Fe(II). The rate coefficients for goethite dissolution by ligands are closely correlated with the stability constants of the aqueous Fe(III)-ligand complexes. A synergistic effect of DFOB and ascorbate on the rate of goethite dissolution was observed (total rates greater than the sum of the individual rates), and this effect was most pronounced under oxic conditions. For HBED, macroscopically the synergistic effect was hidden due to the inhibitory effect of ascorbate on HBED adsorption. After accounting for the concentrations of adsorbed ascorbate and HBED, a synergistic effect could still be identified. The potential synergism between ligand and reductant for iron (hydr)oxide dissolution may have important implications for iron bioavailability in soil environments.

From α-nucleophiles to functionalized aggregates: exploring the reactivity of hydroxamate ion towards esterolytic reactions in micelles

Hydroxamate ions as α-nucleophiles for esterolytic reactions in water and micelles.

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.

Gold-deferrioxamine nanometric interface for selective recognition of Fe(III) using square wave voltammetry and electrochemical impedance spectroscopy methods

Deferrioxamine, a bacterial hydroxamic siderophore having high binding affinity for Fe(III), is used in its immobilized form, as self-assembled monolayer on Au, for accumulation and recognition of Fe(III) from the solution phase. The accumulated Fe(III) is detected via both active mode based on faradaic reduction current of Fe(III), and inactive mode based on impedimetric effect of accumulated Fe(III) against redox reaction of a suitable probe. Appropriate electrochemical techniques, square wave voltammetry and electrochemical impedance spectroscopy, are used for the transduction of analytical signals obtained by this sensor. Then, the parameters influencing the sensor response are optimized. In the best conditions, a linear response, from 1.0×10(-10) to 1.0×10(-7)M Fe(III) in logarithmic scale with a detection limit of 2.0×10(-11)M, and mean relative standard deviation of 1.7% for n=4 is observed. The results show that the sensor can be used for determination of Fe(III) in the presence of various inorganic ions and biological species. Validity of the method and applicability of the sensor are successfully tested by determination of Fe(III) in various real samples including plant tissue (corn leaves), industrial alloy (Ferrotitanium), and pharmaceutical samples (Venofer(®) ampoule, Ironorm(®) capsule, and V.M. Protein(®) powder).

Discovery of 4-(4-(2-((5-Hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)(propyl)amino)ethyl)piperazin-1-yl)quinolin-8-ol and its analogues as highly potent dopamine D2/D3 agonists and as iron chelator: in vivo activity indicates potential application in symptomatic and neuroprotective therapy for Parkinson's disease

The role of iron in the pathogenesis of Parkinson's disease (PD) has been implicated strongly because of generation of oxidative stress leading to dopamine cell death. In our overall goal to develop bifunctional/multifunctional drugs, we designed dopamine D2/D3 agonist molecules with a capacity to bind to iron. Binding assays were carried out with HEK-293 cells expressing either D2 or D3 receptor with tritiated spiperone to evaluate inhibition constants (K(i)). Functional activity of selected compounds was carried out with GTPgammaS binding assay. SAR results identified compounds (+)-19a and (-)-19b as two potent agonists for both D2 and D3 receptors (EC(50) (GTPgammaS); D2 = 4.51 and 1.69 nM and D3 = 1.58 and 0.74 nM for (-)-19b and (+)-19a, respectively). In vitro complexation studies with 19b demonstrated efficient chelation with iron. Furthermore, the deoxyribose assay with 19b demonstrated potent antioxidant activity. In PD animal model study, (-)-19b exhibited potent in vivo activity in reversing locomotor activity in reserpinized rats and also in producing potent rotational activity in 6-OHDA lesioned rats. This reports initial development of unique lead molecules that might find potential use in symptomatic and neuroprotective treatment of PD.

Preparation and Characterization of Polyamidoxime Chelating Resin from Rubberwood Fibre-G-Polyacrylonitrile

Grafted rubberwood fibre was converted to polyamidoxime ion-exchange resin in order to remove heavy metal ions from aqueous solution. The cation-exchange resin existed predominantly in the syn-hydroxyamino form. The water uptake by the resin was ca. 31 g/g dry resin while its hydrogen ion capacity was 3.6 mmol/g. The adsorption capacity of the resin towards different metal ions from wastewater was determined at different pH values within the range 1–6. The prepared chelating ion-exchanger exhibited the highest adsorption capacity towards Cu2+ ions (3.83 mmol/g), followed by Cd2+, Fe3+, Pb2+, Ni2+ and Co3+ ions, respectively. The results showed that the adsorption capacity depended on the solution pH. Polyamidoxime ion-exchange resin was also used to separate Co3+ and Ni2+ ions from Cu2+ ions using a column technique. On passing Cu2+/Ni2+ and Cu2+/Co3+ ion mixtures through the resin at pH 3, Cu2+ ions were adsorbed by the resin but no sorption of Ni2+ or Co3+ ions was detected. Approximately 98% of the Cu2+ ions could be desorbed from the resin. FT-IR spectroscopy was used to confirm the conversion of polyacrylonitrile-g-rubberwood fibre to polyamidoxime.

Coupled biogeochemical cycling of iron and manganese as mediated by microbial siderophores

Siderophores, biogenic chelating agents that facilitate Fe(III) uptake through the formation of strong complexes, also form strong complexes with Mn(III) and exhibit high reactivity with Mn (hydr)oxides, suggesting a pathway by which Mn may disrupt Fe uptake. In this review, we evaluate the major biogeochemical mechanisms by which Fe and Mn may interact through reactions with microbial siderophores: competition for a limited pool of siderophores, sorption of siderophores and metal-siderophore complexes to mineral surfaces, and competitive metal-siderophore complex formation through parallel mineral dissolution pathways. This rich interweaving of chemical processes gives rise to an intricate tapestry of interactions, particularly in respect to the biogeochemical cycling of Fe and Mn in marine ecosystems.

Peptides Selected for the Protein Nanocage Pores Change the Rate of Iron Recovery from the Ferritin Mineral

Pores regulate access between ferric-oxy biomineral inside and reductants/chelators outside the ferritin protein nanocage to control iron demineralization rates. The pore helix/loop/helix motifs that are contributed by three subunits unfold independently of the protein cage, as observed by crystallography, Fe removal rates, and CD spectroscopy. Pore unfolding is induced in wild type ferritin by increased temperature or urea (1-10 mM), a physiological urea range, 0.1 mM guanidine, or mutation of conserved pore amino acids. A peptide selected for ferritin pore binding from a combinatorial, heptapeptide library increased the rate of Fe demineralization 3-fold (p<0.001), similarly to a mutation that unfolded the pores. Conjugating the peptide to Desferal (desferrioxamine B mesylate), a chelator in therapeutic use, increased the rates to 8-fold (p<0.001). A second pore binding peptide had the opposite effect and decreased the rate of Fe demineralization 60% (p<0.001). The peptides could have pharmacological uses and may model regulators of ferritin demineralization rates in vivo or peptide regulators of gated pores in membranes. The results emphasize that small peptides can exploit the structural plasticity of protein pores to modulate function.

Self-assembly and cytotoxicity study of waterwheel-like dinuclear metal complexes: The first metal complexes appended with multiple free hydroxamic acid groups

Two waterwheel-like dinuclear complexes [M(2)(PHA)(4)(H(2)O)(2)] (M = Cu(II) (1), Zn(II) (2); HPHA = phthal-hydroxamic acid) appended with four free hydroxamic acid groups, namely, free hydroxamic acid metal complexes (FHAMCs) have been synthesized and characterized. The crystal structure of complex 1 was determined by single crystal X-ray diffraction, which adopts the paddlewheel motif with four bidentate carboxylate ligands joining two Cu(II) ions. The relative cytotoxicities of compounds 1 and 2 against SMMC-7721 and HO-8910 cell lines are similar and more predominant than HPHA (IC(50): Cu(II)>Zn(II)>>HPHA). The synergic effect of the bound water molecules, multiple free hydroxamic acid groups and dimetal active sites with bridging carboxylate may have significant impacts on their pharmacological activity. As the prototype for a new class of hydroxamic acid derivatives, the self-assembly of FHAMCs presents a promising new strategy in designing multiple hydroxamic acids with remarkable bioactivities.

Investigation of Solid Surfaces Modified by Langmuir−Blodgett Monolayers Using Sum-Frequency Vibrational Spectroscopy and X-ray Photoelectron Spectroscopy

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.

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.

Structure of carbohydrate-bound polynuclear iron oxyhydroxide nanoparticles in parenteral formulations

Intravenous iron therapy is used to treat anemia associated with chronic kidney disease. The chemical structures of parenteral iron agents have not been characterized in detail, and correlations between structure, efficiency of iron delivery, and toxicity via catalysis of oxygen-derived free radical creation remain to be established. In this study, two formulations of parenteral iron have been characterized by absorption spectroscopy, X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), and elemental analysis. The samples studied were Venofer (Iron Sucrose Injection, USP) and Ferrlecit (Sodium Ferric Gluconate in Sucrose Injection). The 250-800-nm absorption spectra and the XRD patterns showed that both formulations contain a mineral core composed of iron oxyhydroxide in the beta-FeOOH mineral polymorph known as akaganeite. This was further confirmed for each formulation by imaging using TEM and AFM. The average core size for the nanoparticles, after dialysis to remove unbound or loosely bound carbohydrate, was approximately 3+/-2 nm for the iron-sucrose, and approximately 2+/-1 nm for the iron-gluconate. Each of the nanoparticles consists of a mineral core, surrounded by a layer of bound carbohydrate. The overall diameter of the average bead in the dialyzed preparations was approximately 7+/-4 nm for the iron-sucrose, and 3+/-1 nm for the iron-gluconate. Undialyzed preparations have particles with larger average sizes, depending on the extent of dilution of unbound and loosely bound carbohydrate. At a dilution corresponding to a final Fe concentration of 5 mg/mL, the average particle diameter in the iron-sucrose formulation was approximately 22+/-9 nm, whereas that of the iron-gluconate formulation was approximately 12+/-5 nm.

Kinetics and Equilibria of the Interactions of Hydroxamic Acids with Gallium(III) and Indium(III)

The thermodynamics and kinetics of the binding of Ga(III) and In(III) to two hydroxamic acids, C6H5-C(O)N(OH)H (BHA) and C6H5-C(O)N(OH)C6H5 (PBHA), have been investigated in acidic media. Spectrophotometric titrations in the UV region reveal that, with excess metal, only the chelate ML forms, whereas the concentration of the protonated species, MHL, is negligible. The thermodynamic parameters indicate that the driving force for formation of ML from MOH2+ and HL is mainly enthalpic, with entropic contributions favoring InL2+ and disfavoring GaL2+ formation. The kinetic (stopped-flow) experiments are interpreted on the basis of two parallel reaction paths both involving reaction of the undissociated ligand (HL): (a) M + HL <==> MHL <==> ML + H where MHL is in a steady state and (b) MOH + HL <==> ML + H2O. Whereas gallium binding to BHA and PBHA proceeds mainly through path b, indium binding to PBHA proceeds through both a and b paths. The rates of both the a and b steps are ligand dependent. Two alternative mechanisms are proposed. The first is based on the electronic characteristics of the ligands and is of the Ia type. The second, of the Id type, assumes that a considerable fraction of the ligand is unreactive owing to intramolecular hydrogen bonding (possibly including a water molecule) which blocks the reaction site. The reasons for preferring the former mechanism are discussed.

NMR studies of phenylbenzohydroxamic acid and kinetics of complex formation with nickel(II)

The behavior of N-phenylbenzohydroxamic acid (PBHA) in organic solvents has been investigated by (1)H and (13)C NMR spectroscopy. Measurements in acetone at different temperatures and concentrations enable one to individualized two signals, in a 20/80 area ratio, which were ascribed to partition of PBHA between two isomers, HZ (cis) and HE (trans). The dependence of the low-intensity peak on concentration and temperature strongly suggests dimer formation. Since only the HZ form can give dimers, it may be concluded that in acetone PBHA is present mainly in the HE form. (13)C NMR measurements in methanol yielded a 50/50 [HZ]/[HE] ratio. The equilibria and kinetics of complex formation in aqueous solutions between Ni(II) and PBHA were investigated by spectrophotometric and stopped-flow techniques at 25 degrees C and 0.2 M ionic strength. Two reaction paths, involving the binding of Ni(2+) to the neutral PBHA and to its anion, were observed. The rate constants of the forward and reverse steps are k(1) = (7.1 +/- 0.3) x 10(2) M(-)(1) s(-)(1) and k(-1) = (4.9 +/- 0.6) s(-)(1) for the step involving the undissociated PBHA and k(2) = (5.5 +/- 0.7) x 10(4) M(-)(1) s(-)(1) and k(-2) = (1.2 +/- 0.1) s(-)(1) for the step involving the anion. The k(2) value indicates that the PBHA anion reacts with Ni(2+) according to Eigen's mechanism and that in water the cis form prevails. The k(1) value is lower by a factor of 13 compared to the value estimated on the basis of Eigen's mechanism, suggesting that at least 90% of the neutral ligand is present in a nonreactive conformation.