Bioinorganic chemistry of Parkinson's disease: structural determinants for the copper-mediated amyloid formation of alpha-synuclein

The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD). A central, unresolved question in the pathophysiology of PD relates to the role of AS-metal interactions in amyloid fibril formation and neurodegeneration. Our previous works established a hierarchy in alpha-synuclein-metal ion interactions, where Cu(II) binds specifically to the protein and triggers its aggregation under conditions that might be relevant for the development of PD. Two independent, non-interacting copper-binding sites were identified at the N-terminal region of AS, with significant difference in their affinities for the metal ion. In this work we have solved unknown details related to the structural binding specificity and aggregation enhancement mediated by Cu(II). The high-resolution structural characterization of the highest affinity N-terminus AS-Cu(II) complex is reported here. Through the measurement of AS aggregation kinetics we proved conclusively that the copper-enhanced AS amyloid formation is a direct consequence of the formation of the AS-Cu(II) complex at the highest affinity binding site. The kinetic behavior was not influenced by the His residue at position 50, arguing against an active role for this residue in the structural and biological events involved in the mechanism of copper-mediated AS aggregation. These new findings are central to elucidate the mechanism through which the metal ion participates in the fibrillization of AS and represent relevant progress in the understanding of the bioinorganic chemistry of PD.     

Site-specific interactions of Cu(II) with alpha and beta-synuclein: bridging the molecular gap between metal binding and aggregation

The aggregation of alpha-synuclein (AS) is a critical step in the etiology of Parkinson's disease (PD) and other neurodegenerative synucleinopathies. Protein-metal interactions play a critical role in AS aggregation and might represent the link between the pathological processes of protein aggregation and oxidative damage. Our previous studies established a hierarchy in AS-metal ion interactions, where Cu(II) binds specifically to the protein and triggers its aggregation under conditions that might be relevant for the development of PD. In this work, we have addressed unresolved structural details related to the binding specificity of Cu(II) through the design of site-directed and domain-truncated mutants of AS and by the characterization of the metal-binding features of its natural homologue beta-synuclein (BS). The structural properties of the Cu(II) complexes were determined by the combined application of nuclear magnetic resonance, electron paramagnetic resonance, UV-vis, circular dichroism spectroscopy, and matrix-assisted laser desorption ionization mass spectrometry (MALDI MS). Two independent, noninteracting copper-binding sites with significantly different affinities for the metal ion were detected in the N-terminal regions of AS and BS. MALDI MS provided unique evidence for the direct involvement of Met1 as the primary anchoring residue for Cu(II) in both proteins. Comparative spectroscopic analysis of the two proteins allowed us to deconvolute the Cu(II) binding modes and unequivocally assign the higher-affinity site to the N-terminal amino group of Met1 and the lower-affinity site to the imidazol ring of the sole His residue. Through the use of competitive chelators, the affinity of the first equivalent of bound Cu(II) was accurately determined to be in the submicromolar range for both AS and BS. Our results prove that Cu(II) binding in the C-terminal region of synucleins represents a nonspecific, very low affinity process. These new insights into the bioinorganic chemistry of PD are central to an understanding of the role of Cu(II) in the fibrillization process of AS and have implications for the molecular mechanism by which BS might inhibit AS amyloid assembly.     

EFFECT OF THE DEGREE OF DVB CROSSLINKING ON THE METAL ION SPECIFICITY OF POLYACRYLAMIDE-SUPPORTED GLYCINES

Metal ion specificity studies of divinylbenzene (DVB)-crosslinked polyacrylamide-supported glycines in different structural environments were investigated. The effect of the degree of crosslinking on the specific rebinding of the desorbed metal ion was investigated towards Co(II), Ni(II), Cu(II), and Zn(II) ions. The metal ion-desorbed resins showed specificity for the desorbed metal ion and the specificity characteristics increases with an increasing degree of the crosslinking agent. The polymeric ligands and metal complexes were characterized by IR, UV-visible and EPR spectra, and by SEM analysis. The swelling and solvation characteristics of the crosslinked polymers, polymeric ligands and metal complexes, the effect of the pH dependence on metal ion binding and rebinding and the kinetics of metal ion binding and rebinding were also followed. The complexation resulted in the downfield shift of the carboxylate peak in the IR spectra. The EPR parameters are in agreement with a distorted tetragonal geometry. The Cu(II) ion-desorbed resins selectively rebinds Cu(II) ions from a mixture of Cu(II) and Co(II) and Cu(II) and Ni(II) ions. The resin could be regenerated several times without loss of capacity and effective for the specific and selective rebinding of Cu(II) ions.     

Structural characterization of a high affinity mononuclear site in the copper(II)-α-synuclein complex

Human α-Synuclein (aS), a 140 amino acid protein, is the main constituent of Lewy bodies, the cytoplasmatic deposits found in the brains of Parkinson's disease patients, where it is present in an aggregated, fibrillar form. Recent studies have shown that aS is a metal binding protein. Moreover, heavy metal ions, in particular divalent copper, accelerate the aggregation process of the protein. In this work, we investigated the high affinity binding mode of truncated aS (1-99) (aS99) with Cu(II), in a stoichiometric ratio, to elucidate the residues involved in the binding site and the role of copper ions in the protein oligomerization. We used Electron Paramagnetic Resonance spectroscopy on the Cu(II)-aS99 complex at pH 6.5, performing both multifrequency continuous wave experiments and pulsed experiments at X-band. The comparison of 9.5 and 95 GHz data showed that at this pH only one binding mode is present. To identify the nature of the ligands, we performed Electron Spin Echo Envelope Modulation, Hyperfine Sublevel Correlation Spectroscopy, and pulsed Davies Electron-Nuclear Double Resonance (Davies-ENDOR) experiments. We determined that the EPR parameters are typical of a type-II copper complex, in a slightly distorted square planar geometry. Combining the results from the different pulsed techniques, we obtained that the equatorial coordination is {N(Im), N(-), H(2)O, O}, where N(im) is the imino nitrogen of His50, N(-) a deprotonated amido backbone nitrogen that we attribute to His50, H(2)O an exchangeable water molecule, and O an unidentified oxygen ligand. Moreover, we propose that the free amino terminus (Met1) participates in the complex as an axial ligand. The MXAN analysis of the XAS k-edge absorption data allowed us to independently validate the structural features proposed on the basis of the magnetic parameters of the Cu(II)-aS99 complex and then to further refine the quality of the proposed structural model.     

Symposium on Biomedical Polymers

ABSTRACT

Glycine functions were incorporated into 2–20 mol% N,N′-methylene-bis-acrylamide (NNMBA)-crosslinked polyacrylamides by transamidation with excess sodium salt of glycine. Complexation characteristics of glycine functions in different structural environments were investigated towards Co(II), Ni(II), Cu(II) and Zn(II) ions. The observed trend in complexation was found to be: Cu(II) > Ni(II) > Co(II) > Zn(II). The polymeric ligands and metal complexes were characterized by various spectral techniques. The polymeric ligands could be recycled several times and the metal ion desorbed resins showed specificity to the desorbed metal ion than other metal ions. This originates from the development of certain ‘pockets’ left by the desorbed metal ion or the ‘memory’ of the polymeric ligands for the desorbed metal ion. This lightly crosslinked systems showed much faster rebinding kinetics. The conditions of metal ion binding and rebinding were optimized to exploit the specific and selective separation of metal ions using metal ion desorbed systems. The specificity and selectivity characteristics depend on the degree of NNMBA crosslinking.     

The Chaperone‐like Activity and Structure of Mutant H119G of Rat Lens αB‐crystallin: A Study of Divalent Metal Ion Binding Site

The molecular chaperone αB‐crystallin, the major player in maintaining the transparency of the eye lens, preventing the aggregation of stress‐damaged and aging lens proteins from aggregation. In nonlenticular cells, it is involved in various neurological diseases, diabetes, and cancer. The role of some metal ions in the αB‐crystallin biology has been reported. Theoretical calculations have proposed that the coordination sites involving His101, His119, Lys121, His18 and Glu99 of human αB‐crystallin were the binding sites for divalent metal ions. Our previous mutagenesis study suggested that His18 rat lens αB‐crystallin is a crucial binding site for Cu(II) and Zn(II) in terms of chaperone‐like activity and structure. In this study mutant H119G of rat lens αB‐crystalin was cloned and expressed to investigate whether His119 is the coordination binding site. Copper and zinc at 1 mM concentration significantly increase the chaperone‐like activity in wild type αB‐crystalin, whereas zinc, copper and magnesium at 1 mM reduced the activity of H119G significantly. The results from chaperone‐like activity, ANS fluorescence measurement and Far‐and Near‐UV CD studies suggest that the replacement of His119 with Glycine resulted in a conformational and minor environmental changes that decrease chaperone‐like activity in the presence of divalent ions suggested that His119 was a crucial binding site for Cu(II) and Zn(II), which was similar to our previous study results of His18. Both results together suggest that His18 and His119 coordinates each other for the binding site of Cu(II) and Zn(II) in terms of improving the chaperone‐like activity and stability of crystallin/metal ion complex.     

Synthetic metal-binding protein surface domains for metal ion-dependent interaction chromatography. II. Immobilization of synthetic metal-binding peptides from metal ion transport proteins as model bioactive protein surface domains.

This preliminary investigation tests the premise that biologically relevant (1) peptide-metal ion interactions, and (2) metal ion-dependent macromolecular recognition events (e.g., peptide-peptide interactions) may be modeled by biomimetic affinity chromatography. Divinylsulfone-activated agarose (6%) was used to immobilize three different synthetic peptides representing metal-binding protein surface domains from the human plasma metal transport protein histidine-rich glycoprotein (HRG). The synthetic peptides represented 1-3 multiple repeat units of the 5-residue sequence (Gly-His-His-Pro-His) found in the C-terminal of HRG. By frontal analyses, immobilized HRG peptides of the type (GHHPH)nG, where n = 1-3, were each found to have a similar binding capacity for both Cu(II) ions and Zn(II) ions (31-38 mumol/ml gel). The metal ion-dependent interaction of a variety of model peptides with each of the immobilized HRG peptide affinity columns demonstrated differences in selectivity despite the similar internal sequence homology and metal ion binding capacity. The immobilized 11-residue HRG peptide was loaded with Cu(II) ions and used to demonstrate selective adsorption and isolation of proteins from human plasma. These results suggest that immobilized metal-binding peptides selected from known solvent-exposed protein surface metal-binding domains may be useful model systems to evaluate the specificity of biologically relevant metal ion-dependent interaction and transfer events in vitro.     

Kinetic analysis of a photosensitive chelator and its complex with Zn(II)

The reversible sequestration and release of metal ions is an important objective in biological and environmental research. Unfortunately, although there have been dramatic examples of metal ion activity control, there are very few quantitative investigations of stoichiometry, equilibria and kinetics. A significant contributor to this lack of quantitative work is the complexity of many photochromic systems. Therefore, we have attempted to create a simple, reversible photochromic metal-ion chelator that can be analyzed quantitatively. The chelator should have certain other attributes as well, namely, that it binds to divalent metal ions (because of their extreme biological importance) and that it binds metal ions in the dark so that light is used to release metal ions rather than sequester them. The photochromic chelator (1) binds to divalent metal ions [Zn(II), Cu(II), Pb(II), Hg(II), Fe(II), Co(II) and Cd(II); other metal ions have not yet been tested] in the dark with a significant binding strength. In both methanol (by spectrophotometry) and methanol-water (by voltammetry), the stoichiometry of the 1-Zn(II) complex is 2:1. The binding constant (K1K2) is on the order of 10(12)-10(14) M(-2) in methanol and 5.0 x 10(8) M(-2) in 50% aqueous methanol. The chelator 1 is photolabile, yielding 2 with a quantum efficiency of 0.91. In a solution containing excess Zn(II), so that over 99% of the ligand exists as the monodentate complex, photolysis produces 2 with a quantum efficiency of 0.15. A kinetic analysis leads to the conclusion that the complex itself is photolabile.     

Adsorption of Cu(II) and Ni(II) on solid humic acid from the Azraq area, Jordan

Isotherms of adsorption of Cu(II) and Ni(II) onto solid Azraq humic acid (AZHA) were studied at different pH (2.0-3.7) values and 0.1 M NaClO4 ionic strength. The Langmuir monolayer adsorption capacity was found to range from 0.1 to 1.0 mmol metal ion/g AZHA, where Cu(II) has higher adsorptivity than Ni(II). The previously reported NICA-Donnan parameters for sorption of Cu(II) on HA fit the amount of Cu(bound) determined in the present study at pH 3.7 but underestimates those at pH values of 3.0, 2.4, and 2.0. The contribution of low affinity sites to binding of metal ions increases with decreasing pH and increasing metal ion loading. The aggregation of HA, which is facilitated by decreasing pH and increasing metal loading, may increase the ability of low-affinity sites to encapsulate metal ions. The binding of Ni(II) to HA exhibits less heterogeneity and less multidentism than that of Cu(II). AZHA loaded with Cu(II) and Ni(II) was found to be insoluble in water with no measurable amount of desorbed metal ions.     

Polyols as scaffolds in the development of ion-selective polymer-supported reagents: the effect of auxiliary groups on the mechanism of metal ion complexation

In developing ion-selective polymer-supported reagents, the inherent affinity of a given ligand for a targeted metal ion is found to be affected by auxiliary groups on a scaffold. A series of polyols (ethylene glycol, glycerol, tris(hydroxymethyl)ethane, pentaerythritol, and pentaerythritol triethoxylate) are immobilized onto cross-linked poly(vinylbenzyl chloride), then monophosphorylated. The pentaerythritol, glycerol, and pentaerythritol triethoxylate polymers have the highest affinities for both trivalent and divalent ions. The distribution coefficients of divalent ions (Pb(II), Cd(II), Cu(II), Ni(II), and Zn(II)) correlate with the Misono softness parameter, reflecting a single-site interaction between the metal ion and the phosphoryl oxygen. The distribution coefficients for trivalent ions are in the order Fe(III) < Al(III) < Y(III) less, approximately < La(III) approximately Eu(III) approximately Lu(III). For example, the phosphorylated pentaerythritol polymer has distribution coefficients (also reported as percent complexed) for Fe of 68.4 (75.3%); for Al of 182 (88.5%); and for the rare earth ions Y, Lu, Eu, and La of 374 (94.4%), 1390 (98.4%), 1690 (98.4%), and 708 (96.9%), respectively, from solutions at pH 2.0. The opposite trend (i.e., Fe(III) > Al(III) > (rare earths)) correlates with their hardness, acidity, electron affinity, electronegativity, and formation constants with soluble complexants, including tributyl phosphate. A binding mechanism is proposed wherein the polymer initially has the auxiliary -OH groups hydrogen-bonded to the phosphate ligand; then, binding to the polarizable phosphoryl oxygen with the divalent ions dominates, while the trivalent ions are drawn closer to the phosphoryl oxygen because of their greater charge and, once closer, bind in a multisite interaction with both the phosphate and -OH groups.     

Soil-modified carbon paste electrode: a useful tool in environmental assessment of heavy metal ion binding interactions

Carbon paste electrodes (CPEs) modified with different soils in their native form were prepared to create a soil-like solid phase suitable for application in studies of heavy metal ion uptake and binding interactions. The preparation of CPEs modified with five different soils was examined and their heavy metal ion uptake behavior investigated using a model Cu(II) aqueous solution. Metal ions were accumulated under open circuit conditions and were determined after a medium exchange using differential pulse anodic stripping voltammetry, applying pre-electrolysis at –0.7 V. The soil-modified CPE accumulation behavior, including the linearity of the current response versus Cu(II) concentration, the influence of the pH on the solution, and the uptake kinetics, was thoroughly investigated. The correlation between the soil-modified CPE uptake capability and the standard soil parameters, such as ion exchange capacity, soil pH, organic matter and clay content, were evaluated for all five examined soils. The influence of selected endogenous cations (K(I), Ca(II), Fe(III)) on the transfer of Cu(II) ions from a solution to the simulated soil solid phase was examined and is discussed. Preliminary examinations of the soil-modified CPE uptake behavior with some exogenous heavy metal ions of strong environmental interest (Pb(II), Hg(II), Cd(II) and Ag(I)) are also presented. This work demonstrates some attractive possibilities for the application of a soil-modified CPE in studying soil-heavy metal ion binding interactions, with a further potential use as a new environmental sensor appropriate for fast on-site testing of polluted soils.     

Soil-modified carbon paste electrode: a useful tool in environmental assessment of heavy metal ion binding interactions

Carbon paste electrodes (CPEs) modified with different soils in their native form were prepared to create a soil-like solid phase suitable for application in studies of heavy metal ion uptake and binding interactions. The preparation of CPEs modified with five different soils was examined and their heavy metal ion uptake behavior investigated using a model Cu(II) aqueous solution. Metal ions were accumulated under open circuit conditions and were determined after a medium exchange using differential pulse anodic stripping voltammetry, applying preelectrolysis at -0.7 V. The soil-modified CPE accumulation behavior, including the linearity of the current response versus Cu(II) concentration, the influence of the pH on the solution, and the uptake kinetics, was thoroughly investigated. The correlation between the soil-modified CPE uptake capability and the standard soil parameters, such as ion exchange capacity, soil pH, organic matter and clay content, were evaluated for all five examined soils. The influence of selected endogenous cations (K(I), Ca(II), Fe(III)) on the transfer of Cu(II) ions from a solution to the simulated soil solid phase was examined and is discussed. Preliminary examinations of the soil-modified CPE uptake behavior with some exogenous heavy metal ions of strong environmental interest (Pb(II), Hg(II), Cd(II) and Ag(I)) are also presented. This work demonstrates some attractive possibilities for the application of a soil-modified CPE in studying soil-heavy metal ion binding interactions, with a further potential use as a new environmental sensor appropriate for fist on-site testing of polluted soils.     

Insight into the binding of divalent cations to Sepia eumelanin from IR absorption spectroscopy

IR absorption spectroscopy is used to examine the binding of the divalent cations Mg(II), Ca(II), Zn(II) and Cu(II) to melanin granules isolated from the ink sacs of Sepia officinalis. The functional groups of the melanin granules interacting with the bound metal ions are deduced by examining the effect of metal concentration on transition frequencies associated with the COOH, NH and OH moieties of the pigment. The coordinating groups vary with metal ion and with concentration. For the experimental conditions used (initial solution pH of 4, ionic strength of 100 mM and a melanin concentration of 1 mg mL(-1)) Mg(II), Ca(II) and Zn(II) bind to carboxylate groups and Cu(II) binds predominantly to phenolic (catechol) groups However, at a concentration of 10 mM Cu(II) also shows evidence of binding to carboxylate and amine groups, reflecting a secondary binding site that becomes populated as the catechol sites are depleted.     

Metal ligand-induced alterations in the surface structures of lactoferrin and transferrin probed by interaction with immobilized copper(II) ions

We have evaluated immobilized Cu(II) ions as a potential site-directed molecular probe to monitor ligand-induced alterations in protein surface structures. Metal ion-induced alterations in the surface structures of different lactoferrins (human and porcine), transferrins (human and rabbit), and ovotransferrin (chicken) were examined. Although these 78,000-dalton glycoproteins are related gene products with similar overall structure and function, they differ greatly in the number and distribution of surface-exposed electron-donor groups thought to interact with Cu(II) ions. Each of these proteins interacted with immobilized Cu(II) ions through sites which are distinct from the two specific high affinity metal binding sites identified for iron. In both the presence and absence of bound iron, transferrins interacted more strongly with the immobilized Cu(II) ions than did lactoferrins; ovotransferrin interacted only weakly. Although iron binding increased the affinities of lactoferrins for immobilized Cu(II), iron binding decreased the affinities of transferrins and ovotransferrin for immobilized Cu(II) ions. Iron-saturated and iron-free lactoferrins were resolved by pH gradient elution, but only in the presence of 3 M urea; they were not resolved by imidazole affinity elution. Conversely, the iron-saturated and iron-free forms of transferrin were only separated by imidazole affinity elution. Urea did not influence the resolution of apo and holo ovotransferrins by imidazole. The differential effects of urea and imidazole suggest the participation of different types of surface electron-donor groups. The progressive site-specific modification of surface-exposed histidyl residues by carboxyethylation revealed several lactoferrin forms of intermediate affinity for immobilized iminodiacetate-Cu(II) ions. In summary, independent of species, the affinity for immobilized Cu(II) ions increased as follows: iron-saturated ovotransferrin less than metal-free ovotransferrin less than apolactoferrin less than hololactoferrin much less than diferric or holotransferrin less than monoferric transferrin less than apotransferrin. We have demonstrated the use of immobilized Cu(II) ions to distinguish and to monitor ligand-induced alterations in protein surface structure. The results are discussed in relation to protein surface-exposed areas of electron-donor groups.     

Theoretical investigation on sulfur‐containing chelating resin‐divalent metal complexes

The complex structures and interactions of sulfur‐containing chelating resin poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfide (PVBS), poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfoxide (PVBSO), and poly[4‐vinylbenzyl‐(2‐hydroxyethyl)]sulfone (PVBSO₂) with divalent metal chlorides (Cu(II), Ni(II), Zn(II), Cd(II), and Pd(II)) were investigated theoretically. Results indicate that PVBS tends to coordinate with metal ions by sulfur and oxygen atoms forming five‐membered ring chelating complexes; while PVBSO and PVBSO₂ prefer to interact with metal ions by the oxygen atom of the sulfoxide or sulfone and hydroxyl group to form six‐membered ring chelating compounds. Theoretical calculations reveal that sulfur atoms of PVBS are the main contributor when coordinate with metal ions, while oxygen atoms also take part in the coordination with Cu(II), Zn(II), and Cd(II). As for PVBSO, the oxygen atoms of sulfoxide group play a key role in the coordination, but sulfur and hydroxyl oxygen also participate in the coordination. Similarly, sulfone group oxygen atoms of PVBSO₂ dominate the coordination of Ni(II), Cu(II), and Pd(II), while the affinities of Zn(II) and Cd(II) are mainly attributed to the hydroxyl oxygen atoms. The computational results are in good agreement with the XPS analysis. Combined the theoretical and experimental results, further understanding of the structural information on the complexes was achieved and the adsorption mechanism was confirmed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Characterisation by immobilised metal ion affinity chromatographic procedures of the binding behaviour of several synthetic peptides designed to have high affinity for Cu(II) ions.

In this investigation, several peptides containing an increasing number of histidine residues have been designed and synthesised. The peptides involved repeat units of either the pentameric EAEHA or the tetrameric HLLH sequence motifs. Adsorption isotherms for these synthetic peptides and hexahistidine (hexa-His) as a control substance were measured under batch equilibrium binding conditions with an immobilised Cu(II)-iminodiacetic acid (IDA) sorbent. The experimental data were analysed in terms of Langmuirean binding behaviour. In common with previous studies with synthetic peptides, these investigations have demonstrate that the sequential organisation of the histidine side chains in these peptides can affect the selectivity of the coordination interactions with borderline metal ions in immobilised metal ion affinity chromatographic systems. The results also confirm that peptides selected on the basis of their potential to form amphipathic secondary structures with their histidine residues presented on one face of the molecule can exhibit equivalent or higher affinity constants towards copper ions than hexa-His, although they contain fewer histidine residues. These findings are thus relevant to the selection of peptides produced inter alia by combinatorial synthetic procedures to have enhanced binding properties for Cu(II) or Ni(II) ions, or intended for use as peptide tags in the fusion handle approach for the affinity chromatographic purification of recombinant proteins.     

2D sheet-like architectures constructed from main-group metal ions, 4,4'-bpno and 1,2-alternate p-sulfonatothiacalix[4]arene

Two novel 2D sheet-like complexes 1 and 2 have been prepared by reaction of p-sulfonatothiacalix[4]arene (TC4AS) with main-group metal ions (lead(II) or barium(II)) and 4,4'-bipyridine-N,N'-dioxide (4,4'-bpno). In both complexes 1 and 2, TC4AS molecules prefer 1,2-alternate conformation, forming 2D sheet-like layers in the presence of divalent ions and 4,4'-bpno ligand. The layers extend to 3D architectures via strong hydrogen bonding interactions. In addition, complexes 1 and 2 are thermally stable up to ca. 335 and 305 °C, respectively. It seems that the strong hydrogen bonds between the sheet-like layers impart the extraordinarily high degrees of stability to the structures. Further analysis indicates that complexes 1 and 2 are significantly different: (a) the divalent ions have different coordination environments due to the nature of the different ions; (b) TC4AS within complexes 1 and 2 connects to four Pb(II) or six Ba(II) ions, respectively; (c) 4,4'-bpno shows different connection modes; (d) in the solid state, complex 1 exhibits intense orange luminescence with triexponential decays, while complex 2 does not exhibit any obvious luminescence at cryogenic temperatures.     

Solid phase extraction and determination of metal ions in aqueous samples using Quercetin modified Amberlite XAD-16 chelating polymer as metal extractant

A new chelating polymer has been developed using Amberlite XAD-16 anchored with Quercetin. The modified polymer was characterised by Fourier Transform Infra Red (FTIR) spectroscopy, thermogravimetric analysis, surface area analysis and elemental analysis. The Quercetin anchored polymer showed superior binding affinity for Cr(III), Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) with greater than 95% adsorption under optimum conditions. The optimum pH conditions for the quantitative sorption of metal ions were studied. The developed method showed superior extraction qualities with high metal loading capacities of 387, 313, 195, 473, 210 and 320 µmol g^?1 for Cu(II), Co(II), Cr(III), Fe(III), Mn(II) and Ni(II), respectively. The rate of metal ion uptake i.e. kinetics studies performed under optimum levels, showed t _1/2 for Co(II), Cu(II), Cr(III), Fe(III), Mn(II) and Ni(II) is 20, 15, 25, 10, 30 and 15 min, respectively. Desorption of metal ions was effective with 10 mL of 2 M HCl prior to analysis using flame atomic absorption spectrophotometer. The chelating polymer was highly ion selective in nature even in the presence of interferent ions, with a high preconcentrating ability for the metal ions of interest. The developed chelating polymer was tested on its utility with synthetic and real samples like river, tap water samples and also with multivitamin tablets. It showed relative standard deviation (R.S.D.) values of/less than 3.0% reflecting on the accuracy and reproducibility of data using the newly developed chelating polymer.     

Adsorptive accumulation of Cd(II), Co(II), Cu(II), Pb(II), and Ni(II) from water on montmorillonite: influence of acid activation

The present work investigates the influence of acid activation of montmorillonite on adsorption of Cd(II), Co(II), Cu(II), Ni(II), and Pb(II) from aqueous medium and comparison of the adsorption capacities with those on parent montmorillonite. The clay-metal interactions were studied under different conditions of pH, concentration of metal ions, amount of clay, interaction time, and temperature. The interactions were dependent on pH and the uptake was controlled by the amount of clay and the initial concentration of the metal ions. The adsorption capacity of acid-activated montmorillonite increases for all the metal ions. The interactions were adsorptive in nature and relatively fast and the rate processes more akin to the second-order kinetics. The adsorption data fitted both Langmuir and Freundlich isotherms, indicating that strong forces were responsible for the interactions at energetically nonuniform sites. The Langmuir monolayer capacity of the acid-activated montmorillonite is more than that of the parent montmorillonite (Cd(II): 32.7 and 33.2 mg/g; Co(II): 28.6 and 29.7 mg/g; Cu(II): 31.8 and 32.3 mg/g; Pb(II): 33.0 and 34.0 mg/g; and Ni(II): 28.4 and 29.5 mg/g for montmorillonite and acid-activated montmorillonite, respectively). The thermodynamics of the rate processes showed the adsorption of Co(II), Pb(II), and Ni(II) to be exothermic, accompanied by decreases in entropy and Gibbs free energy, while the adsorption of Cd(II) and Cu(II) was endothermic, with an increase in entropy and an appreciable decrease in Gibbs free energy. The results have established the potential use for montmorillonite and its acid-activated form as adsorbents for Cd(II), Co(II), Cu(II), Ni(II), and Pb(II) ions from aqueous media.     

Post-synthetic modification of isomorphic coordination layers: exchange dynamics of metal ions in a single crystal to single crystal fashion

A diverse set of guest molecules template the consistent formation of fifteen iso-structural layered structures containing a pyridine based tripodal ligand, divalent metal ions and anions. These materials have shown a remarkable ability to interconvert from Zn(II) to Cu(II), Cd(II) to Cu(II), Cu(II) to Cd(II) complexes in a SCSC (single crystal-to-single crystal) fashion.