A new route to trihydroxamate-containing artificial siderophores and synthesis of a new fluorescent probe

Flow cytometry-based study of model marine microalgal consortia revealed an ecological advantage of siderophore utilization by the dinoflagellate Amphidinium carterae

Investigations of phytoplankton responses to iron stress in seawater are complicated by the fact that iron concentrations do not necessarily reflect bioavailability. Most studies to date have been based on single species or field samples and are problematic to interpret. Here, we report results from an experimental cocultivation model system that enabled us to evaluate interspecific competition as a function of iron content and form, and to study the effect of nutritional conditions on the proteomic profiles of individual species. Our study revealed that the dinoflagellate Amphidinium carterae was able to utilize iron from a hydroxamate siderophore, a strategy that could provide an ecological advantage in environments where siderophores present an important source of iron. Additionally, proteomic analysis allowed us to identify a potential candidate protein involved in iron acquisition from hydroxamate siderophores, a strategy that is largely unknown in eukaryotic phytoplankton.

Photophysical Studies of a Catechol Based Polyfunctional Dipodal Chelator: Application for Optical Probe for Selective Detection of Fe(III)

A novel catechol based dipodal fluorescent chelator N,N'-bis[3-[(E)-(2,3-dihydroxyphenyl)methyleneamino]propyl]propanediamide(MPC), has been developed and its photophysical behaviour was studied by experimental (UV-VIS and fluorescence) and DFT method. The design of the molecule has been inspired from the naturally occurring siderophore enterobactin, a catechol based chelator with amide linkage, that shows an excellent binding efficiency towards Fe(III). The dipodal molecule (MPC) presented here, carries two catechol pendant binding moieties linked to the malonate central unit through propylene spacers by amide linkage. MPC showed good selectivity for Fe(III) at 10^-4 M concentration in aqueous medium amongst the biologically and environmentally important metal ions chosen viz., Na(I), K(I), Al(III), Cr(III), Fe(III), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), by demonstrating a remarkable quenching in the fluorescent emission from 262 a.u. to 55 a.u. at λ_max = 477 nm. Also, the pre-organized assembled ligand favored an efficient Fe (III) encapsulation through coordination by imine nitrogen and catecholate oxygen donors. High formation constant (log β = 31.3) for 1:1 metal-ligand complex evaluated by both potentiometric and spectrophotometric methods, established the strong binding efficiency of the ligand for Fe(III) metal ion. The binding stoichiometry in the complex was also confirmed from Stern -Volmer and Hill Plot analysis. Further investigation on the emission behavior of MPC in a completely DMSO system explored its suitability for extensive applications in the areas such as, metallurgy, material science, iron contamination remedial in the materials etc.. DFT studies suggest that the ligand displays a U-shaped geometry with a parallel π-stacking and the hydrogen bond between two arms. The experimental infrared, electronic, fluorescence, ¹H nmr, ¹³C nmr spectra were correlated with the theoretical results. The nature of electronic transitions were identified from the TDDFT calculation. The ligand forms a hexa-coordinated complex with six Fe-O bonds extending an orthorhombic geometry due distortion from a regular octahedron.

Endocytosis-mediated siderophore uptake as a strategy for Fe acquisition in diatoms

Phytoplankton growth is limited in vast oceanic regions by the low bioavailability of iron. Iron fertilization often results in diatom blooms, yet the physiological underpinnings for how diatoms survive in chronically iron-limited waters and outcompete other phytoplankton when iron becomes available are unresolved. We show that some diatoms can use siderophore-bound iron, and exhibit a species-specific recognition for siderophore types. In Phaeodactylum tricornutum, hydroxamate siderophores are taken up without previous reduction by a high-affinity mechanism that involves binding to the cell surface followed by endocytosis-mediated uptake and delivery to the chloroplast. The affinity recorded is the highest ever described for an iron transport system in any eukaryotic cell. Collectively, our observations suggest that there are likely a variety of iron uptake mechanisms in diatoms besides the well-established reductive mechanism. We show that iron starvation-induced protein 1 (ISIP1) plays an important role in the uptake of siderophores, and through bioinformatics analyses we deduce that this protein is largely diatom-specific. We quantify expression of ISIP1 in the global ocean by querying the Tara Oceans atlas of eukaryotic genes and show a link between the abundance and distribution of diatom-associated ISIP1 with ocean provinces defined by chronic iron starvation.

Siderophores for molecular imaging applications

This review covers publications on siderophores applied for molecular imaging applications, mainly for radionuclide-based imaging. Siderophores are low molecular weight chelators produced by bacteria and fungi to scavenge essential iron. Research on these molecules has a continuing history over the past 50 years. Many biomedical applications have been developed, most prominently the use of the siderophore desferrioxamine (DFO) to tackle iron overload related diseases. Recent research described the upregulation of siderophore production and transport systems during infection. Replacing iron in siderophores by radionuclides, the most prominent Ga-68 for PET, opens approaches for targeted imaging of infection; the proof of principle has been reported for fungal infections using ⁶⁸Ga-triacetylfusarinine C (TAFC). Additionally, fluorescent siderophores and therapeutic conjugates have been described and may be translated to optical imaging and theranostic applications. Siderophores have also been applied as bifunctional chelators, initially DFO as chelator for Ga-67 and more recently for Zr-89 where it has become the standard chelator in Immuno-PET. Improved DFO constructs and bifunctional chelators based on cyclic siderophores have recently been developed for Ga-68 and Zr-89 and show promising properties for radiopharmaceutical development in PET. A huge potential from basic biomedical research on siderophores still awaits to be utilized for clinical and translational imaging.

Xanthurenic acid: a natural ionophore with high selectivity and sensitivity for potassium ions in an aqueous solution

Synopsis: A well-known tryptophan metabolite, xanthurenic acid, a natural non-fluorescent intermediate siderophore, showed a very selective turn-on response to K⁺ over other competing metal ions and the detection limit of this natural ionophore was found to be 53 nM at physiological pH.

A New Schiff Base Chemodosimeter for Fluorescent Imaging of Ferric Ions in Living Cells

A new and efficient chemodosimeter for ferric ions has been developed. The visual and fluorescent behaviors of the compound toward various metal ions were investigated: ferric ions are distinguished from other cations by selective color change and unusual fluorescence enhancement in mixed aqueous solution. Fluorescence microscopy experiments showed that this receptor is effective for detection of Fe(3+) in vitro, developing a good image of the biological organelles. The sensing mechanism is shown to involve a hydrolysis process.

Recognition of iron ions by carbazole-desferrioxamine fluorescent sensor and its application in total iron detection in airbone particulate matter

This work reports on an efficient microwave irradiation synthesis of a new fluorescent chemosensor based on desferrioxamine B (DFO-B) and carbazole moiety. Furthermore, this novel chemosensor was employed for a comparative study of real environmental samples of airbone particulate matter collected from Dunkirk (Northern of France). Among selected relevant metal cations present in its airbone particulate matter, such as Na(+), K(+), Mg(2+), Ca(2+), Al(3+), Cr(3+), Mn(2+) and Zn(2+), this molecular device proved to be outstandingly sensitive toward Fe(3+) with a limit of detection of 1.49 ppb (2.1×10(-8) M) in methanol allowing the estimation of total iron in atmospheric particles.

Development of a novel fluorescence probe capable of assessing the cytoplasmic entry of siderophore-based conjugates

A novel fluorescence probe capable of assessing the cytoplasmic entry of siderophore-based conjugates was synthesized and evaluated by photochemical characterization and cell-based assays.

Adsorption and detection of Escherichia coli using an Au substrate modified with a catecholate-type artificial siderophore-Fe3+ complex

A catecholate-type artificial siderophore with a terminal-NH2 group (1) and its Fe(3+) complex (2) were prepared. Siderophore 1 was characterized by (1)H NMR, FT-IR, and ESI-TOF MS spectroscopy. The corresponding Fe(3+) complex 2 was obtained by reaction of 1 with Fe(acac)3. The absorption band at 500 nm (ε = 4670 M(-1) cm(-1) at pH 7.0) of the electronic absorption spectrum of 2 is assignable as the LMCT (O(catecholate) → Fe(3+)) absorption band. This band indicates the formation of the Fe(3+) complex of 1. The biological activity of 2 with respect to Escherichia coli was clearly confirmed by observing that it permeates into the cell membrane. The self-assembled monolayer of 2 on an Au substrate, 2/Au, was prepared and its preparation was confirmed by FT-IR reflection-absorption spectroscopy (IR-RAS) and cyclic voltammetry (CV). Furthermore, a quartz crystal microbalance (QCM) chip modified with 2 effectively adsorbed E. coli. M. flavescens, an organism which is incapable of synthesizing siderophores and must therefore use exogenous hydroxamate-type siderophores for growth, did not adsorb on 2/Au. In contrast, E. coli did not adsorb on the hydroxamate-type artificial siderophore-Fe(3+) complex (3)-modified Au substrate, 3/Au. These results provide preliminary evidence that microbes recognized Fe(3+) ion-bound siderophores on the surface. The detection limit of 2/Au was ∼10(4) CFU mL(-1).

An efficient and selective flourescent chemical sensor based on 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane as a new fluoroionophore for determination of iron(III) ions. A novel probe for iron speciation

A novel fluorescent chemical sensor for the highly sensitive and selective determination of Fe(3+) ions in aqueous solutions is prepared. The iron sensing system was prepared by incorporating 5-(8-hydroxy-2-quinolinylmethyl)-2,8-dithia-5-aza-2,6-pyridinophane (L) as a neutral Fe(3+)-selective fluoroionophore in the plasticized PVC membrane containing sodium tetraphenylborate as a liphophilic anionic additive. The response of the sensor is based on the strong fluorescence quenching of L by Fe(3+) ions. At pH 5.5, the proposed sensor displays a calibration curve over a wide concentration range from 6.0 × 10(-4) to 1.0 × 10(-7) M, with a relatively fast response time of less than 2 min. In addition to a high stability and reproducibility, the sensor shows a unique selectivity toward Fe(3+) ion with respect to common coexisting cations. The proposed fluorescence optode was applied to the determination of iron(III) content of straw of rice, spinach and different water samples. The fluorescent sensor was also used as a novel probe for Fe(3+)/Fe(2+) speciation in aqueous solution.

Iron(III) selective molecular and supramolecular fluorescent probes

Iron is one of the most important elements in metabolic processes, being indispensable for all living systems and therefore it is extensively distributed in environmental and biological materials. However, both its deficiency and excess from the normal permissible limit can induce serious disorders. Therefore, several analytical techniques have been adopted for the detection of iron. Among the various techniques used for its detection, the method based on fluorescent sensors has received considerable interest in recent years because of its ability to provide online monitoring of very low concentrations without any pre-treatment of the sample together with the advantages of spatial and temporal resolution. In this article, efforts have been made to review the various molecular and supramolecular fluorescent sensors that have been developed for the selective detection of iron(III).

Adsorption behavior of microbes on a QCM chip modified with an artificial siderophore-Fe3+ complex

Three hydroxamate-type artificial siderophores with terminal NH(2) groups, tris[2-{3-(N-acyl-N-hydroxamino)propylamido}propyl]aminomethane (1-3, acyl-R group = Me, Et, and Ph, respectively), and their Fe(3+) complexes, 4-6, were prepared. The stability constant (log β) of 4 was estimated to be about 31 by its EDTA titration. The biological activities of 4-6 for Microbacterium flavescens, which is a hydroxamate-type siderophore, auxotrophic gram-positive microbe, clearly indicated that they permeated the cell membrane depending on their terminal bulky acyl-R groups. These artificial siderophore complexes, 4-6, were modified on Au electrode surfaces with the terminal NH(2) group (4-6/Au). The surface modification of 4-6 was confirmed by several electrochemical measurements. The quartz crystal microbalance (QCM) chips were also modified with 4-6. Microbe adsorption measurements using these modified QCM chips for M. flavescens, Pseudomonas putida, and Eschrichia coli were performed. The QCM chips have the ability to adsorb microbes selectively as a result of the differences in the interactions between the structures of Fe(3+)-artificial siderophore complexes and their receptors or binding proteins within the cell membrane.

Fl-DFO molecules@mesoporous silica materials: highly sensitive and selective nanosensor for dosing with iron ions

Highly sensitive and selective nanosensor for labile iron pool (LIP) determination, has been designed and prepared by immobilization of Fluoresceine-Desferrioxamine (Fl-DFO), a bifunctional fluoro-siderophore probe molecule with great affinity for iron ions (pKf=30.7), into highly ordered mesoporous silica structure. Different immobilization methods of Fl-DFO molecules, such as their encapsulation in surfactant micelles used as templating agents for the synthesis of mesoporous silica, direct impregnation into the mesochannels of as-synthesized mesoporous silica and their surface anchoring by covalent binding with propylamine groups implanted by post-synthesis on the internal surface of mesochannels, have been explored. Each nanohybrid has been fully characterized by small angle XRD, TEM, SEM, solid state (29)Si and (13)C MAS NMR and N(2) adsorption-desorption. The fluorescence properties of nanohybrids obtained have been correlated with the immobilization methods, generating interesting information concerning the localization of Fl-DFO molecules in the channels of mesoporous silica. The leaching of Fl-DFO molecules from mesoporous materials has been investigated. The nanosensor prepared by surface anchoring of Fl-DFO at the internal surface of mesochannels showed high performances with no leaching effect and high sensitivity in regards to its responses to ferric ions. Its fluorescence intensity decreased as soon as first Fe(III) ions are in contact with this nanosensor. A linear relationship between the fluorescence intensity and the ferric ions concentration was observed in low micromolar range. The selectivity of this nanosensor towards other metal ions has also been tested and shown its high affinity to ferric ions. This study can allow the design of a stable, portable, simple, regenerable and cost-effective nanosensor highly sensitive and selective for iron ions with detection limits in the range of cellular LIP in cells, e.g. lower micromolar range.

Synthesis of fluorescent probes based on the pyochelin siderophore scaffold

Pyochelin is a siderophore common to several pathogenic bacterial strains. Two conjugates, 1 and 2, between the NBD (4-nitro-benzo[1,2,5]oxadiazole) fluorophore and an N3''-functionalized pyochelin were synthesized. These fluorescent probes unexpectedly increased their fluorescence in an aqueous medium in the presence of iron(III) and were transported into bacterial cells.

8-Hydroxyquinoline dansylates modified with PAMAM dendrimer as fluorescent Fe+ sensors

A series of fluorescent sensors based on 8-hydroxyquinoline dansylate as core and dendritic PAMAM as shell were synthesized. Their fluorescence characteristic and fluorescent sensing behavior toward metal ions were studied in water/methanol (1: 1, v/v). The sensors exhibit good selectivity and sensitivity for Fe³⁺ ion and to some extent for Cr³⁺ ion. The third generation dendrimer has the better sensitivity than the first and second generations.

Novel near-infrared fluorescent integrin-targeted DFO analogue

Desferrioxamine (DFO), a siderophore initially isolated from Streptomyces pilosus, possesses extraordinary metal binding properties with wide biomedical applications that include chelation therapy, nuclear imaging, and antiproliferation. In this work, we prepared a novel multifunctional agent consisting of (i) a near-infrared (NIR) fluorescent probe-cypate; (ii) an integrin alpha vbeta3 receptor (ABIR)-avid cyclic RGD peptide, and (iii) a DFO moiety, DFO-cypate-cyclo[RGDfK(approximately)] (1, with approximately representing the cypate conjugation site at the side chain of lysine; f is d-phenylalanine). Compound 1 and two control compounds, cypate-cyclo[RGDfK(approximately)] ( 2) and cypate-DFO ( 3), were synthesized by modular assembly of the corresponding protected RGD peptide cyclo[R(Pbf)GD(OBut)fK] and DFO on the dicarboxylic acid-containing cypate scaffold in solution. The three compounds exhibited similar UV-vis and emission spectral properties. Metal binding analysis shows that DFO as well as 1 and 3 exhibited relatively high binding affinity with Fe(III), Al(III), and Ga(III). In contrast to Ga(III), the binding of Fe to 1 and 3 quenched the fluorescence emission of cypate significantly, suggesting an efficient metal-mediated approach to perturb the spectral properties of NIR fluorescent carbocyanine probes. In vitro, 1 showed a high ABIR binding affinity (10 (-7) M) comparable to that of 2 and the reference peptide cyclo(RGDfV), indicating that both DFO and cypate motifs did not interfere significantly with the molecular recognition of the cyclic RGD motif with ABIR. Fluorescence microscopy showed that internalization of 1 and 2 in ABIR-positive A549 cells at 1 h postincubation was higher than 3 and cypate alone, demonstrating that incorporating ABIR-targeting RGD motif could improve cellular internalization of DFO analogues. The ensemble of these findings demonstrate the use of multifunctional NIR fluorescent ABIR-targeting DFO analogues to modulate the spectral properties of the NIR fluorescent probe by the chelating properties of DFO and visualize intracellular delivery of DFO by receptor-specific peptides. These features provide a strategy to explore the potential of 1 in tumor imaging and treatment as well as some molecular recognition processes mediated by metal ions.

Adsorption of microorganisms onto an artificial siderophore-modified Au substrate

The hydroxamate-type artificial siderophore, tris[2-{3-(N-acetyl-N-hydroxamino)propylamido}propyl]aminomethane (TAPPA) and its Fe(III) complex, Fe(III)-TAPPA were prepared and characterized by several spectroscopic methods. Fe(III)-TAPPA exhibits biological activity for the hydroxamate-type siderophore auxotrophic microorganism, Microbacterium flavescens, suggesting that Fe(III)-TAPPA can permeate the cell membrane of the microorganism. The adsorption of the Fe(III)-siderophore complex onto a deposited Au substrate was achieved by a stepwise self-assembling method. The modification of Fe(III)-TAPPA on the surface was confirmed from the cyclic voltammogram of the resultant Au electrode, Fe(III)-TAPPA/Au. The adsorption experiments of M. flavescens with Fe(III)-TAPPA/Au were monitored by optical, scanning electron, and atomic force microscopy and quartz crystal microbalance (QCM) measurements. These results clearly indicate that Fe(III)-TAPPA/Au can immobilize M. flavescens. This adsorption characteristic is due to the interaction between Fe(III)-TAPPA on an Au electrode and a receptor/binding protein within the cell membrane.

Trafficking of siderophore transporters in Saccharomyces cerevisiae and intracellular fate of ferrioxamine B conjugates

We have studied the intracellular trafficking of Sit1 [ferrioxamine B (FOB) transporter] and Enb1 (enterobactin transporter) in Saccharomyces cerevisiae using green fluorescent protein (GFP) fusion proteins. Enb1 was constitutively targeted to the plasma membrane. Sit1 was essentially targeted to the vacuolar degradation pathway when synthesized in the absence of substrate. Massive plasma membrane sorting of Sit1 was induced by various siderophore substrates of Sit1, and by coprogen, which is not a substrate of Sit1. Thus, different siderophore transporters use different regulated trafficking processes. We also studied the fate of Sit1-mediated internalized siderophores. Ferrioxamine B was recovered in isolated vacuolar fractions, where it could be detected spectrophotometrically. Ferrioxamine B coupled to an inhibitor of mitochondrial protoporphyrinogen oxidase (acifluorfen) could not reach its target unless the cells were disrupted, confirming the tight compartmentalization of siderophores within cells. Ferrioxamine B coupled to a fluorescent moiety, FOB-nitrobenz-2-oxa-1,3-diazole, used as a Sit1-dependent iron source, accumulated in the vacuolar lumen even in mutants displaying a steady-state accumulation of Sit1 at the plasma membrane or in endosomal compartments. Thus, the fates of siderophore transporters and siderophores diverge early in the trafficking process.