Charged Metal Complexes Research Articles

Voltammetric study of the ion-exchange binding of non-electroactive metal cations to DNA-modified surfaces

We describe a simple electrochemical protocol for studying the ion-exchange binding of non-electroactive ions, specifically mono- and divalent metal cations of biological relevance (Mg(2+), Ca(2+), and K(+)), to DNA-modified surfaces. After incubation in a dilute solution of multiply charged transition metal complex (5.0 microM [Ru(NH(3))(6)]Cl(3)), gold electrodes modified with thiolate-DNA monolayers respond to the presence of these non-electroactive metal cations by producing significant changes in the cyclic voltammograms (i.e., decrease of the integrated charge and shift of formal potential) of the surface-bound redox complex ([Ru(NH(3))(6)](3+)). The divalent cations (particularly Mg(2+)) can be detected at very low concentrations (<10 microM), while the on-set value for K(+) is substantially higher (50 mM). The equilibrium binding constants for Mg(2+) and Ca(2+) to DNA-modified surfaces were calculated.

Interaction of Dipicolinatodioxovanadium(V) with Polyatomic Cations and Surfaces in Reverse Micelles

In confined media such as reverse micelles, molecular probes frequently reside at and interact strongly with the interface. If the interface is charged, it is often difficult to separate effects arising from interactions with the charged species from the effect of the interfacial environment. With reverse micelles as a model system, the work reported here explores the interaction of the charged surfactant headgroups at a self-assembled interface with the dipicolinatodioxovanadium(V) coordination complex. The vanadium complex studied in these experiments serves as an excellent probe to investigate how charged metal complexes interact with lipid interfaces. For comparison, measurements were also carried out probing the interaction of the vanadium complex with a model cationic headgroup, tetramethylammonium bromide. The impact of the environment is gauged by changes in the 51V chemical shift, longitudinal relaxation times, and 1H NMR pulsed field gradient measurements. These measurements suggest that while interface component parts, as modeled by the dispersed systems, interact with the vanadium complex, the interfacial environment perturbs the complex substantially more strongly than the sum of the components alone. Coulomb attraction dominates the interaction in all systems probed and surprisingly orients the hydrophobic portion into the bulk water.

An application of electrospray ionization tandem mass spectrometry to probe the interaction of Ca2+/Mg2+/Zn2+ and Cl− with gramicidin A

The peptide, gramicidin A (GrA), has been demonstrated to interact with divalent salts (CaCl2, MgCl2, and ZnCl2) using electrospray ionization mass spectrometry (ESI-MS). The ESI-MS analysis revealed different complexes formed due to the interaction of Val-GrA and Ile-GrA with divalent salts: [Val or Ile-GrA-H+M]+, [Val or Ile-GrA+MCl]+ and [Val or Ile-GrA+M]2+, where M is Ca or Mg or Zn. All these complexes have been subjected to collisionally activated dissociation (CAD). CAD of singly and doubly charged GrA and metal complexes exhibited the losses of water molecules, indicating the ligand preference of GrA. MS/MS and MS3 of [Val or Ile-GrA+MCl]+ resulted in the elimination of chloride ion and water, respectively. The tandem mass spectrometry data of the complex [Val-GrA+MCl]+ suggest that chloride interaction is stronger in the presence of Ca than of Mg and Zn. This study reveals that GrA could interact with Ca, Mg, and Zn in metal ion form as well as in ion pair (MCl) form. The interactions of GrA with Ca support the proposal of a physical basis for the messenger role of Ca (Urry et al., J. Biol. Chem. 1982, 257: 6659-6661).

Influence of pH and ionic strength on the metal profile of impregnation catalysts

In this study the influence of pH and ionic strength on the final metal distribution is investigated. After impregnation, catalysts are usually dried at temperatures between 50°C and 200°C. During this process, a redistribution of the metal occurs, which is a complex function of the drying conditions, the properties of the impregnating solution and the support material. In our model, transport in the gas and liquid phase is described by the dusty gas model, and the Nernst–Plank equation, respectively. The metal adsorption on the porous support is described by the Revised Physical Adsorption model developed by Agashe and Regalbuto (J. Colloid Interface Sci. 185 (1997) 174). In their model, the adsorption constant is a function of the pH and the ionic strength of the liquid solution. The results show that for a positively charged metal complex, the impact of drying is strong when the initial pH of the liquid solution is below the point of zero charge (PZC). In such a case, the metal accumulates at the particle surface when the convective flow is strong (high temperature), and at the particle center when the solute-metal diffusivity is high. A comparison of our model with a drying model that assumes a constant adsorption equilibrium constant shows that the variations of pH and ionic strength cannot be ignored when the initial adsorption constant is low and the pH below the PZC. In such cases, the adsorption constant increases over several orders of magnitude when pH and ionic strength effects are accounted for.

Activity coefficients in mixed electrolyte solutions

An experimental method is illustrated, which opens up an effective line of attack in the fundamental and practically unexplored field of not-necessarily-dilute, not-necessarily-low-charge, multiple electrolytes. The method is based on the idea that ions of a sufficiently enhanced hydrophobic character (typically, charged metal complexes and organic or partially organic ions) can be studied by means of liquid membrane cells without undergoing any interference by highly hydrophilic ions such as SO42−, OH−, H+, Mg2+, La3+, etc. Proofs are given for hexacyanocobaltate(III) and tris(ethylenediamine) cobalt(III) salts. As a first application, the method is used to find otherwise unknown activity coefficients of trace electrolytes in swamping ionic bulks – the typical situation encountered in equilibrium, kinetic constant, and standard potential determinations. Observed salt effects do not prove in any case to be trivial, and in particular, the activity coefficients of 3:3 salts in 2:2 and 3:2 ionic bulks are found to be completely different from expectations.

First-second shell interactions in metal binding sites in proteins: a PDB survey and DFT/CDM calculations.

The role of the second shell in the process of metal binding and selectivity in metalloproteins has been elucidated by combining Protein Data Bank (PDB) surveys of Mg, Mn, Ca, and Zn binding sites with density functional theory/continuum dielectric methods (DFT/CDM). Peptide backbone groups were found to be the most common second-shell ligand in Mg, Mn, Ca, and Zn binding sites, followed (in decreasing order) by Asp/Glu, Lys/Arg, Asn/Gln, and Ser/Thr side chains. Aromatic oxygen- or nitrogen-containing side chains (Tyr, His, and Trp) and sulfur-containing side chains (Cys and Met) are seldom found in the second coordination layer. The backbone and Asn/Gln side chain are ubiquitous in the metal second coordination layer as their carbonyl oxygen and amide hydrogen can act as a hydrogen-bond acceptor and donor, respectively, and can therefore partner practically every first-shell ligand. The second most common outer-shell ligand, Asp/Glu, predominantly hydrogen bonds to a metal-bound water or Zn-bound histidine and polarizes the H-O or H-N bond. In certain cases, a second-shell Asp/Glu could affect the protonation state of the metal ligand. It could also energetically stabilize a positively charged metal complex more than a neutral ligand such as the backbone and Asn/Gln side chain. As for the first shell, the second shell is predicted to contribute to the metal selectivity of the binding site by discriminating between metal cations of different ionic radii and coordination geometries. The first-shell-second-shell interaction energies decay rapidly with increasing solvent exposure of the metal binding site. They are less favorable but are of the same order of magnitude as compared to the respective metal-first-shell interaction energies. Altogether, the results indicate that the structure and properties of the second shell are dictated by those of the first layer. The outer shell is apparently designed to stabilize/protect the inner-shell and complement/enhance its properties.

Selective transport and incorporation of highly charged metal and metal complex ions in self-assembled polyelectrolyte multilayer membranes

The transport of aqueous salts containing mono-, di- and trivalent metal and tetravalent metal complex ions across ultrathin polyvinylammonium/polyvinylsulphate (PVA/PVS) membranes is described. The membranes were prepared by electrostatic layer-by-layer (LBL) assembly of the two polyelectrolytes. Using spectroscopic measurements and permeability studies, it is demonstrated that the transport of copper(II) chloride, lanthanum(III) chloride, barium chloride and potassium hexacyanoferrate(II) is accompanied by the permanent incorporation of the metal and metal complex ions in the membrane. Upon the uptake of copper, lanthanum and hexacyanoferrate ions, the membranes become cross-linked so that the permeation rates of other salts not taken up by the membrane, e.g. sodium chloride, potassium chloride and magnesium chloride, are decreased. The uptake of barium ions leads to a decrease of the cross-linking density of the membrane so that the permeation rate of NaCl is increased. Possible mechanisms for the ion uptake are discussed.

Collision-induced dissociation and photodetachment of singly and doubly charged anionic polynuclear transition metal carbonyl clusters: Ru3Co(CO)13−, Ru6C(CO)162−, and Ru6(CO)182−

Three polynuclear transition metal carbonyl cluster anions, Ru3Co(CO)13−, Ru6C(CO)162−, and Ru6(CO)182− have been studied using energy-dependent electrospray ionization mass spectrometry (EDESI–MS) and photodetachment photoelectron spectroscopy (PES). EDESI–MS maps show a simple collision-induced dissociation (CID) process for Ru3Co(CO)n− by stripping CO down to the metal core. For the doubly charged species, two competing CID channels were observed, viz. loss of neutral CO and loss of CO+e−. It was found that the parent dianions first lose neutral CO down to n=9, producing a series of dianions, Ru6C(CO)n2− (n=9–16) and Ru6(CO)n2− (n=9–18). For n&amp;lt;9, the dianions become electronically unstable against autodetachment, and singly charged anions, Ru6C(CO)n− and Ru6(CO)n− (n=0–9), were observed. The PES spectra of the dianions show the electron binding energies decrease monotonically as n decreases and become ∼0.0 eV for n=9, in exact agreement with the CID patterns that reflect the electronic instability of the doubly charged metal complexes with n&amp;lt;9. All of the PES spectra show congested features, indicating very high density of low-lying electronic states for the transition metal carbonyl clusters. The electron binding energies of Ru3Co(CO)n− are similar for n=4–13, but decrease sharply from n=4 to 0. The intramolecular Coulomb repulsion in the dianions was observed to increase with loss of CO, ranging from ∼1.8 eV for Ru6C(CO)162− to ∼2.4 eV for Ru6C(CO)92−.

Templated Synthesis and Structural Study of Densely Packed Metal Nanostructures in MCM-41 and MCM-48

A template synthetic method to prepare densely packed metal nanostructures in functionalized (MCM)-41 and MCM-48 is described. The intrachannel surface of host silica has been functionalized to carry positive charges for the accommodation of highly concentrated and negatively charged metal complexes. After reduction, Au and Pt nanowire bundles in MCM-41 as well as Pd nanowire networks in MCM-48 are formed. The Pt nanowire bundles in MCM-41 are observed to grow along a preferred direction and stack along Pt {111} planes relative to the pore wall of the host. Furthermore, bimetallic AuPt alloy nanowire bundles in MCM-41 have also been prepared and characterized.

Liquid form of DNA developed

Chemists at the University of North Carolina at Chapel Hill have developed a liquid form of DNA. DNA is normally used as a dilute solution or as a crystalline solid. By combining negatively charged DNA crystals with positively charged metal complexes with a polyether tail, they were able to produce molten DNA at room temperature that is still double-stranded and is able to conduct electricity. Although its immediate practical application is uncertain, liquid DNA might be used in microelectronic circuits in the future that are capable of archiving and transferring electronic information. [Leone, A.M. et al. (2001) J. Am. Chem. Soc. 123, 218–222].

The ultraviolet photofragmentation of doubly charged transition metal complexes in the gas phase: Initial results for [Cu.(pyridine)n]2+ and [Ag.(pyridine)n]2+ ions

Results are presented of a study of the UV photofragmentation patterns of the doubly charged complexes [Cu.(pyridine)n]2+ and [Ag.(pyridine)n]2+ for n=4–7. The two systems behave very differently, with Ag(II) being dominated by the presence of charge transfer products at small values of n. This ease of reduction of Ag(II) can be equated to behavior seen in the condensed phase. At all values of n both types of complex display a remarkable tendency to fragment to the most stable configuration, which can correspond to either a singly or doubly charged ion.

Determination of metal ions by capillary electrophoresis using pre-column complexation with 1,10-phenanthroline

The capillary electrophoretic separation of Fe(II), Cu(II) and Zn(II) ions in an acidic buffer solution (pH 2.5) by complexation with 1,10-phenanthroline is investigated. As 1,10-phenanthroline is a neutral ligand, the positively charged metal complexes formed migrate in the same direction as the EOF, providing a rapid separation of metal ions in acidic buffers. The method was applied to the determination of metal ions in vitamin tablets.

Preparation and Testing of Diffusible Metal Chelate Fungicides

Water soluble chelates were formed by reacting copper, zinc, nickel, cobalt, chromium and rare earth oxides, hydroxides and carbonates with carboxylic acids, natural and synthetic amino acids, hydroxy acids, peptides, acetylacetone and sodium tripolyphosphate. The ligands were selected to give either electrically neutral or negatively charged metal complexes in order to minimise the coordinate bonding between the metal chelate and the timber substrate. Eighteen of the 38 chelates prepared were found to be effective fungicides, which readily diffused through the heartwood of high density eucalypt construction timbers. There was no apparent correlation between the physical properties of the chelates and fungitoxic efficacy, although the majority of the copper and rare earth chelates had good biocidal properties. The amount of metal ion bonded to the timber substrate could be varied by selecting ligands of different stability constants, chelates with low stability constants giving a greater extent of metal ion-wood bonding than chelates with high stability constants.

Gas-phase reactions of doubly charged alkaline earth and transition metal complexes of acetonitrile, pyridine, and methanol generated by electrospray ionization

Formation and low energy collision-induced dissociation (CID) of doubly charged metal(II) complexes ([metal(II)+Ln]2+, metal(II)=Co(II), Mn(II), Ca(II), Sr(II) and L = acetonitrile, pyridine, and methanol) were investigated. Complexes of [metal(II)+Ln]2+ where n≤7 were obtained using electrospray ionization. Experimental parameters controlling the dissociation pathways for [Co(II)+(CH3CN)2]2+ were studied and a strong dependence of these processes on the collision energy was found. However, the dissociation pathways appear to be independent of the cone potential, indicating low internal energy of the precursor ions. In order to probe how these processes are related to intrinsic parameters of the ligand such as ionization potential and metal ion coordination, low energy CID spectra of [metal(II)+Ln]2+ for ligands such as acetonitrile, pyridine, and methanol were compared. For L = pyridine, all metals including the alkaline earth metals Ca and Sr were reduced to the bare [metal(I)]+ species. Hydride transfer was detected upon low energy CID of [metal(II)+Ln]2+ for metal(II)=Co(II) and Mn(II) and L = methanol, and corroborated by signals for [metal(II)+H−]+ and [metal(II)+H−+CH3OH]+, as well as by the complementary ion [CH3O]+.

Gas phase reactions of doubly charged alkaline earth and transition metal(II)-ligand complexes generated by electrospray ionization

Doubly charged metal(II)-complexes of α 1–3, α 1–6 mannotriose and the conserved trimannosyl core pentasaccharide as well as doubly charged complexes of Co(II), Mn(II), Ca(II) and Sr(II) with acetonitrile generated by electrospray ionization were studied by low energy collision induced dissociation (CID). Two main fragmentation pathways were observed for the metal(II)-oligosaccharide complexes. Regardless of the coordinating metal, loss of a neutral dehydrohexose residue (162 Da) from the doubly charged precursor ion is observed, forming a doubly charged product ion. However, if the oligosaccharide is coordinated to Co(II) or Mn(II), loss of a dehydroxyhexose cation is also observed. Investigation of the low mass region of the mass spectra of the metal coordinated oligosaccharides revealed intense signals corresponding to [metal(II) + (CH 3CN) n 2+ (where n = 1–6) species which were being formed by the metal(II) ions and the acetonitrile present in the sample. Analysis of these metal(II)-acetonitrile complexes provided further insight into the processes occurring upon low energy CID of doubly charged metal complexes. The metal(II)-acetonitrile system showed neutral loss and ligand cleavage as observed with the oligosaccharide complexes, as well as a series of six different dissociation mechanisms, most notable among them reduction from [metal(II) + (CH 3CN) n 2+ to the bare [metal(I)] + species by electron transfer. Depending on the metal and collision gas chosen, one observes electron transfer from the ligand to the metal, electron transfer from the collision gas to the metal, proton transfer between ligands, heterolytic cleavage of the ligands, reactive collisions and loss of neutral ligands.

Determination of metal-hydroxide bond energies in doubly charged scandium hydroxide, iron hydroxide, and cobalt hydroxide ions

Bond dissociation energies of the metal hydroxide dications of Sc, Fe, and Co have been obtained through charge stripping of ions produced by fast atom bombardment. The reactions for fragmentation of the doubly charged transition metal complexes FeOH 2+ and CoOH 2+ to produce M + +OH + are exothermic by 0.6±0.5 and 2.2±0.5 eV, respectively. For ScOH 2+ , the corresponding reaction is endothermic by 4.9±0.3 eV

Electrochemical responses of multiply-charged transition metal complexes bound electrostatically to graphite electrode surfaces coated with polyelectrolytes

Several types of cationic and anionic polyelectrolyte molecules that adhere to graphite electrode surfaces have been employed to bind oppositely charged transition metal complexes to the electrode surface. Electrostatic interactions are the basis of the binding. Electrochemical responses from the bound complexes are measured as a function of time, nature of the polyelectrolyte coating and solvent. Examples in which the complex incorporated by the polyelectrolyte experiences significant shifts in formal potential are contrasted with those in which little or no change in formal potential is observed.

A proton magnetic resonance study of ligand exchange on the hexakis(1,1,3,3-tetramethylurea)scandium(III) ion and its NN-dimethylacetamide analogue

Proton n.m.r. spectroscopic studies show that the rate law for tetramethylurea (tmu) exchange on the hexakis-(tetramethylurea)scandium(III) ion is: rate = 6k1[Sc(tmu)63+] where, typically, k1(300 K)= 0.26 ± 0.03 s–1, ΔH‡1= 91.2 ± 2.3 kJ mol–1 and ΔS‡1= 47.8 ± 6.7 J K–1 mol–1 in CD3NO2 diluent, while k1(300 K)= 1.08 ± 0.05 s–1, ΔH‡1= 68.6 ± 1.3 kJ mol–1, and ΔS‡1=–15.7 ± 3.8 J K–1 mol–1 in CD3CN diluent. In contrast, the rate law for ligand exchange on the hexakis (NN-dimethylacetamide)scandium(III) ion is: rate = 6(k1+k2[dma])[Sc(dma)63+] in CD3NO2 and CD3CN diluents. For the nitromethane system, k1(300 K)= 4.6 ± 0.3 s–1, ΔH‡1= 30.3 ± 2.0 kJ mol–1, and ΔS‡1=–132 ± 6 J K–1 mol–1 and k2(300 K)= 112 ± 3 dm3 mol–1 s–1, ΔH‡2= 26.0 ±0.6 kJ mol–1, and ΔS‡2=–119 ± 2 J K–1 mol–1. Similar parameters characterize the acetonitrile solutions. Ligand exchange in solutions of either [Sc(dmf)6][ClO4]3 or [Sc(dmso)6][ClO4]3 lies in the fast-exchange limit of the n.m.r. time scale. The mechanistic implications of these data are discussed and comparisons are made with other tripositively charged metal complex systems.