Siderophore sorption to clays

Siderophores are low molecular weight organic ligands exuded by some aerobic organisms and plants to acquire Fe under Fe-limited conditions. The hydroxamate siderophores may sorb to aluminosilicate clays through a variety of mechanisms depending upon the nature of the clay and of the siderophore along with solution conditions such as pH, ionic strength, and presence of metal cations. They may also affect metal binding to clays. Here, we review previous studies of siderophore sorption to aluminosilicate clays; briefly discuss how the techniques of X-ray diffractometry, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy may be applied to such studies; review effects of siderophores on metal sorption to clays; and highlight some areas for future research.     

X-ray absorption spectroscopy (XAS) of toxic metal mineral transformations by fungi

Fungi can be highly efficient biogeochemical agents and accumulators of soluble and particulate forms of metals. This work aims to understand some of the physico-chemical mechanisms involved in toxic metal transformations focusing on the speciation of metals accumulated by fungi and mycorrhizal associations. The amorphous state or poor crystallinity of metal complexes within biomass and relatively low metal concentrations make the determination of metal speciation in biological systems a challenging problem but this can be overcome by using synchrotron-based element-specific X-ray absorption spectroscopy (XAS) techniques. In this research, we have exposed fungi and ectomycorrhizas to a variety of copper-, zinc- and lead-containing minerals. X-ray absorption spectroscopy studies revealed that oxygen ligands (phosphate, carboxylate) played a major role in toxic metal coordination within the fungal and ectomycorrhizal biomass during the accumulation of mobilized toxic metals. Coordination of toxic metals within biomass depended on the fungal species, initial mineral composition, the nitrogen source, and the physiological state/age of the fungal mycelium.     

Use of surface analysis techniques to determine the mechanisms of metal corrosion in animal buildings

Recognizing the severity of metal corrosion problems in animal buildings has prompted an attempt to finally determine the fundamental causes of corrosion. Based on a two-year field test, this paper investigated the corrosion mechanisms of various metal products exposed in three animal buildings by analyzing the composition of corrosion products, using advanced material surface analysis techniques, including energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) analyses. Data from these analyses showed that the corrosion products were mainly iron oxides (Fe₂O₃, Fe₃O₄, and FeO) on uncoated 1010 carbon steel and zinc oxides, sulfides, and carbonates on galvanized steel and galvalume (ZnO, ZnS, Zn(CO)₃, and Al₂O₃). Thus, it could be concluded that the fundamental mechanisms of metal corrosion in animal buildings are similar to the classic corrosion mechanisms and the high corrosion rates of metal products in animal buildings are due to the presence of high moisture levels.     

Metal imaging in neurodegenerative diseases

Metal ions are known to play an important role in many neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and prion diseases. In these diseases, aberrant metal binding or improper regulation of redox active metal ions can induce oxidative stress by producing cytotoxic reactive oxygen species (ROS). Altered metal homeostasis is also frequently seen in the diseased state. As a result, the imaging of metals in intact biological cells and tissues has been very important for understanding the role of metals in neurodegenerative diseases. A wide range of imaging techniques have been utilized, including X-ray fluorescence microscopy (XFM), particle induced X-ray emission (PIXE), energy dispersive X-ray spectroscopy (EDS), laser ablation inductively coupled mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS), all of which allow for the imaging of metals in biological specimens with high spatial resolution and detection sensitivity. These techniques represent unique tools for advancing the understanding of the disease mechanisms and for identifying possible targets for developing treatments. In this review, we will highlight the advances in neurodegenerative disease research facilitated by metal imaging techniques.     

Synthesis and study of physico-chemical properties of a new chiral Schiff base ligand and its metal complex

A new chiral amino acid Schiff base ligand (Salarg) and its metal complex (Mn-Salarg) have been synthesized using l-Arginine, a naturally occurring chiral diamine with two kinds of asymmetric α-, ε-NH₂ groups. This new Salarg-ligand and Mn-Salarg complex are characterized with the help of ultraviolet, fluorescence and infrared spectroscopy. Their crystal systems are determined by X-ray powder diffraction method. The elemental analysis has been carried out by energy dispersive X-ray analysis (EDAX). The presence and percentage of metal in the complex have been detected and estimated by energy dispersive X-ray fluorescence (EDXRF) spectroscopy. Circular dichroism spectroscopy has revealed the chiral nature of the Salarg-ligand and its metal complex. Furthermore, a comparative study of this new chiral Salarg-ligand and its complex has been made with the well known achiral Salen ligand and its metal complex (Mn-Salen).     

Complexation and detoxification of Zn and Cd in metal accumulating plants

Metal accumulating plants exposed to toxic levels of zinc (Zn) and cadmium (Cd) uptake metals through extracellular and intracellular complexation with inorganic and organic ligand formation. However, little is known about the nature and formation mechanism of these metal–ligand complexes. Though, Zn and Cd have many similar chemical properties, yet their complexation and compartmentalization in plants vary with plant species. In principal, the question arises what factors govern Zn and Cd partitioning in plants? What form of the metal is taken up by the root, and is further distributed and accumulated in both vegetative and reproductive tissues? Therefore, the aim of present study is to address several questions concerning the mechanisms of Zn and Cd coordination and compartmentalization in plants using X-ray absorption spectroscopy (XAS) technique. XAS allows direct determination of elemental oxidation states and coordination environments in different plant tissues. This review article briefly explains some other important techniques of XAS; EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), which are employed for determining Zn and Cd complexation within the plant. Therefore, In present review, the predominant as well as the minor chemical forms of Zn and Cd present in particular plant tissue have been discussed which could give better insight towards metal accumulation and detoxification mechanisms operated in plants. This information could assist in employing suitable hyperaccumulator plants for metal phytoextraction and reclamation of metal contaminated sites.     

Biosurfactant of marine origin exhibiting heavy metal remediation properties

The present study was aimed at elucidating the role of biosurfactant product isolated from a marine bacterium in removing heavy metals from heavy metal containing solutions. In this study, metal removal was biosurfactant-mediated. Efficiency of metal removal depended on the concentration of the metal as well as that of the biosurfactant. At a concentration 5×, the critical micelle concentration (CMC), almost complete removal of 100 ppm of lead and cadmium occurred. Atomic absorption spectroscopy (AAS) studies also showed metal removal at a concentration less than the CMC in contrast to earlier findings that only micelles are involved in metal removal. Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy (EDS) further substantiated these findings.     

Determination of copper binding in <Emphasis Type="Italic">Pseudomonas putida</Emphasis> CZ1 by chemical modifications and X-ray absorption spectroscopy

Previously performed studies have shown that Pseudomonas putida CZ1 biomass can bind an appreciable amount of Cu(II) and Zn(II) ions from aqueous solutions. The mechanisms of Cu- and Zn-binding by P. putida CZ1 were ascertained by chemical modifications of the biomass followed by Fourier transform infrared and X-ray absorption spectroscopic analyses of the living or nonliving cells. A dramatic decrease in Cu(II)- and Zn(II)-binding resulted after acidic methanol esterification of the nonliving cells, indicating that carboxyl functional groups play an important role in the binding of metal to the biomaterial. X-ray absorption spectroscopy was used to determine the speciation of Cu ions bound by living and nonliving cells, as well as to elucidate which functional groups were involved in binding of the Cu ions. The X-ray absorption near-edge structure spectra analysis showed that the majority of the Cu was bound in both samples as Cu(II). The fitting results of Cu K-edge extended X-ray absorption fine structure spectra showed that N/O ligands dominated in living and nonliving cells. Therefore, by combining different techniques, our results indicate that carboxyl functional groups are the major ligands responsible for the metal binding in P. putida CZ1.     

Using synchrotron X-ray fluorescence microprobes in the study of metal homeostasis in plants

Background and Aims

This Botanical Briefing reviews the application of synchrotron X-ray fluorescence (SXRF) microprobes to the plant sciences; how the technique has expanded our knowledge of metal(loid) homeostasis, and how it can be used in the future.

Scope

The use of SXRF microspectroscopy and microtomography in research on metal homeostasis in plants is reviewed. The potential use of SXRF as part of the ionomics toolbox, where it is able to provide fundamental information on the way that plants control metal homeostasis, is recommended.

Conclusions

SXRF is one of the few techniques capable of providing spatially resolved in-vivo metal abundance data on a sub-micrometre scale, without the need for chemical fixation, coating, drying or even sectioning of samples. This gives researchers the ability to uncover mechanisms of plant metal homeostasis that can potentially be obscured by the artefacts of sample preparation. Further, new generation synchrotrons with smaller beam sizes and more sensitive detection systems will allow for the imaging of metal distribution within single living plant cells. Even greater advances in our understanding of metal homeostasis in plants can be gained by overcoming some of the practical boundaries that exist in the use of SXRF analysis.Key words: Metal homeostasis, synchrotron X-ray fluorescence, SXRF, microspectroscopy, microtomography, X-ray absorption spectroscopy, XAS, ionomics, Arabidopsis thaliana, hyperaccumulator     

Active site electronic structure and dynamics during metalloenzyme catalysis

Zinc-dependent enzymes play important roles in many cellular processes. Assignment of their reaction mechanisms is often a subject of debate because the zinc ion is silent in several spectroscopic techniques. We have combined time-resolved X-ray absorption spectroscopy, pre-steady state kinetics and computational quantum chemistry to study the active site zinc ion of bacterial alcohol dehydrogenase during single substrate turnover. We detect a series of alternations in the coordination number and structure of the catalytic zinc ion with concomitant changes in metal-ligand bond distances. These structural changes are reflected in the effective charge of the metal ion. The present work emphasizes the flexibility of catalytic zinc sites during catalysis and provides novel mechanistic insights into alcohol dehydrogenase catalysis.     

Bioreduction of precious and heavy metals by Candida species under oxidative stress conditions

The aim of the present work was to evaluate whether Candida species can reduce both precious and toxic pure metals from the respective molecular ions. From these results, the nanoparticles formed were studied using scanning electron microscopy with energy-dispersive spectroscopy, Raman spectroscopy, X-ray fluorescence spectroscopy and synchrotron radiation. Our results showed that the metal ions were reduced to their corresponding metallic nanoconglomerate or nanoparticles by Candida species. This is the first report on how yeasts of this genus are capable of achieving homeostasis (resilience) in the presence of metal ions of both precious and toxic metals by reducing them to a metallic state.     

Metal complexation of chitosan and its glutaraldehyde cross-linked derivative

The physicochemical characterization of metal complexed with chitosan (CS) and its glutaraldehyde cross-linked derivative (CSGA) was investigated. Seven metal ions from chromium through zinc of the first row of the transition metals were selected for complexation. Structural features pertinent to where and how metals bind into both polymers are our main interest. Studies using solid-state NMR spectroscopy and XRPD (X-ray powder diffraction) supported by ESR spectroscopy, ICP-OES (inductively couple plasma-optical emission spectroscopy) and far-FTIR spectroscopy for metal interaction with nitrogen sites at C-2 of the metal-polymer complexes were performed. Theoretical calculations of the metal-polymer ratio, the approximate charges on nitrogen for both amine and imino-linker, and the proton affinity between an alcohol group from the polymer and an amino/imino group are reported. A helical coiled chitosan model and a 2C1L (two-chitosans with one linker) model are proposed here. The metal uptake mechanism for both polymers is concluded to be absorption within the polymers, rather than adsorption on the polymer surface.     

Structure of the Mn<Subscript>4</Subscript>–Ca cluster as derived from X-ray diffraction

The catalytic centre for light-induced water oxidation in photosystem II (PSII) is a multinuclear metal cluster containing four manganese and one calcium cations. Knowing the structure of this biological catalyst is of utmost importance for unravelling the mechanism of water oxidation in photosynthesis. In this review we describe the current state of the X-ray structure determination at 3.0 A resolution of the water oxidation complex (WOC) of PSII. The arrangement of metal cations in the cluster, their coordination and protein surroundings are discussed with regard to spectroscopic and mutagenesis studies. Limitations of the presently available structural data are pointed out and possible perspectives for the future are outlined, including the combination of X-ray diffraction and X-ray spectroscopy on single crystals.     

Ultrastructural localization of heavy metals in the extraradical mycelium and spores of the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi, obligate symbionts of most plant species, are able to accumulate heavy metals, thereby, protecting plants from metal toxicity. In this study, the ultrastructural localization of Zn, Cu, and Cd in the extraradical mycelium and spores of the arbuscular mycorrhizal fungus Glomus intraradices grown in monoxenic cultures was investigated. Zinc, Cu, or Cd was applied to the extraradical mycelium to final concentrations of 7.5, 5.0, or 0.45 mmol/L, respectively. Samples were collected at time 0, 8 h, and 7 days after metal application and were prepared for rapid freezing and freeze substitution. Metal content in different subcellular locations (wall, cytoplasm, and vacuoles), both in hyphae and spores, was determined by energy-dispersive X-ray spectroscopy. In all treatments and fungal structures analysed, heavy metals accumulated mainly in the fungal cell wall and in the vacuoles, while minor changes in metal concentrations were detected in the cytoplasm. Incorporation of Zn into the fungus occurred during the first 8 h after metal addition with no subsequent accumulation. On the other hand, Cu steadily accumulated in the spore vacuoles over time, whereas Cd steadily accumulated in the hyphal vacuoles. These results suggest that binding of metals to the cell walls and compartmentalization in vacuoles may be essential mechanisms for metal detoxification.     

Comparative studies on metal biosorption by two strains of Cladosporium cladosporioides

Two strains of a fungus, Cladosporium cladosporioides 1 and C. cladosporioides 2 showed different metal biosorption properties. Strain 1 showed preferential sorption of gold and silver, while strain 2 could bind metals such as copper and cadmium in addition to gold and silver. Strain 1 had a cell-wall hexosamine content of 0.1%. X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red spectroscopy (FTIR) analyses indicated that nitrogen was not involved in metal biosorption by the strain. In strain 2 the cell-wall hexosamine content was 150 times that of strain 1. These results indicated that hexosamine was responsible for non-specific metal binding while cell-wall polymers other than hexosamines had a role in conferring selectivity in precious-metal binding.     

Evidence for the absence of photoreduction of the metal centers of cytochrome C oxidase by X-irradiation

Samples of X-irradiated cytochrome c oxidase were examined by electron paramagnetic resonance and optical spectroscopy. Both radiation from the Stanford Synchrotron Radiation Laboratory and a conventional X-ray source (W target) were utilized. The X-ray flux from these sources ranges from 10(9) to 10(13) photons/s. No evidence was found for photoreduction of the metal centers in the enzyme by X-ray photons. These results demonstrate that the integrity of cytochrome c oxidase is maintained using the conditions under which X-ray absorption measurements are presently being made.     

Comparative studies on metal biosorption by two strains of Cladosporium cladosporioides

Two strains of a fungus, Cladosporium cladosporioides 1 and C. cladosporioides 2 showed different metal biosorption properties. Strain 1 showed preferential sorption of gold and silver, while strain 2 could bind metals such as copper and cadmium in addition to gold and silver. Strain 1 had a cell-wall hexosamine content of 0.1%. X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red spectroscopy (FTIR) analyses indicated that nitrogen was not involved in metal biosorption by the strain. In strain 2 the cell-wall hexosamine content was 150 times that of strain 1. These results indicated that hexosamine was responsible for non-specific metal binding while cell-wall polymers other than hexosamines had a role in conferring selectivity in precious-metal binding.