Transition Metal Ions In Solution Research Articles

Transition Metal Ions in Solution: Complexed Ion Speciation at the Air-Liquid Interface.

We have studied the interfacial properties of Zn2+ vs selected transition metal cations such as Fe2+, Cu2+, and Cr3+ in a water-ethanol mixture using field-induced droplet ionization. This was in particular inspired by the specific surface activity of Zn2+ that has been observed on several occasions and a desire to clarify the root cause for this behavior. We have found that Zn2+, due to its unique electronic configuration and atomic size, is the only ion of those under study that gives rise to specific speciation at the air-liquid interface with three ethanol molecules attached to the central atom for optimal polarity.

Adsorption of copper(II) ions by zeolite modified with organosilicon thiosemicarbazide

The article is devoted to the calculation of the kinetics and thermodynamics of adsorption of copper(II) ions by zeolite activated with hydrochloric acid and modified 1-(3-triethoxysilylpropyl)thiosemicarbazide. For the calculation of the kinetic parameters, we used the Lagergren, Ho, and Mackay equation. The activation energy was found using the Arrhenius equation. For the calculation of thermodynamic parameters, the linearized Van't Hoff equation and the Gibbs-Helmholtz equation were used. A comparison of the determination coefficients of the pseudo-first (from 0.815 to 0.892) and pseudo-second (from 0.995 to 0.999) order models allowed choosing the latter as the leading model for the adsorption of Cu(II) ions. Pseudo-second order rate constants depending on temperature were in the range 0.201-3.915 g⋅mmol-1⋅min.-1. The determined value of the activation energy 42.3 kJ mol-1 was in the range of 40-120 kJ, which is typical for chemisorption of ions Cu(II). Adsorption isotherms were well described by the Langmuir model. Adsorption proceeded with the formation of sorbate monolayer on the outer surface. Adsorption limit values ranged from 0.0016 to 0.0039 mol g-1 depending on temperature from 238 to 309 K. The processes of adsorption of Cu(II) ions on the surface of the modified zeolite occurred spontaneously. A decrease in the Gibbs energy ΔG0 was observed (from -14.20 to -16.29 kJ mol-1) with increasing temperature. Positive enthalpy value Δ𝐻0 (3.6 kJ mol-1) indicated that the adsorption of Cu(II) ions occurred endothermically. Positive entropy value Δ𝑆0 (58.15 J mol-1) indicated an increase in randomness at the interface due to the destruction of the solvation shells surrounding the transition metal ions in solution. Thus, the ratios of kinetic and thermodynamic quantities indicated that a significant contribution to the process of adsorption of Cu(II) ions by a modified zeolite was made by processes associated with the chemical interaction of adsorbate molecules and functional nitrogen and sulphur containing groups of thiosemicarbazide.

Spontaneous Reduction of Transition Metal Ions by One Electron in Water Microdroplets and the Atmospheric Implications

Freshman chemistry teaches that Fe3+ and Cu2+ ions are stable in water solutions, but their reduced forms, Fe2+ and Cu+, cannot exist in water as the major oxidation state due to the fast oxidation by O2 and/or disproportionation. Contrary to these well-known facts, significant fractions of dissolved Fe and Cu species exist in their reduced oxidation states in atmospheric water such as deliquesced aerosols, clouds, and fog droplets. Current knowledge attributes these phenomena to the stabilization of the lower oxidation states by the complexation of ligands and the various photochemical or thermal pathways that can reduce the higher oxidation states. In this study, by spraying the water solutions of transition metal ions into microdroplets, we show the results of the spontaneous reduction of ligated Fe(III) and Cu(II) species into Fe(II) and Cu(I) species, presenting a previously unknown source of reduced transition metal ions in atmospheric water. It is the spontaneously generated electrons in water microdroplets that are responsible for the reduction. Control experiments in the atmosphere and in a glove box filled with precisely controlled gaseous contents reveal that O2, CO2, and NO2 are the major competitors for the electrons, forming O2-, HCO2-, and NO2-, respectively. Taking these findings together, we opine that microdroplet chemistry might play significant but previously underestimated roles in atmospheric redox chemistry.

Affinity of phenolic compounds for transition metal ions immobilized on cation-exchange columns

Immobilized metal ion affinity chromatography (IMAC) is useful in purification of histidine-tagged or histidine-rich proteins and peptides from a variety of hosts. However, phenolic compounds including polyphenols interfere with IMAC due to their high affinities for the transition metals immobilized on the column resins, which hampers the purification of proteins from plant-based host systems. In contrast to extensive knowledge of the mechanism of the interactions between phenolic compounds and transition metal ions in solution, an understanding of the interactions on the columns, where transition metal ions are immobilized on the resins, remains elusive. This study systematically investigated the affinity of phenolic compounds for transition metal ions by varying the number and position of phenolic hydroxyl groups (OH groups) and using different transition metals—Fe(II), Cu(II) and Ni(II)—on various IMACs, in which the columns were fabricated by equilibrating the cation-exchange column with transition metal solutions. It was found that the more OH groups the aromatic compounds have, the higher the affinity for transition metal ions; in particular, methyl gallate and pyrogallol were permanently bound to the IMAC column, which reflected coordinate bond formation with the transition metal ions. Importantly, the phenolic compounds showed no obvious affinity for the Ni(II)-IMAC column, in contrast to the Fe(II)- and Cu(II)-IMAC columns, whereas imidazole and histidine-tagged proteins showed evident binding to the Ni(II)-IMAC column. Ni(II)-IMAC should thus be especially effective in isolating histidine-tagged and histidine-rich species from phenolic compound-containing systems. These results indicate that the affinity between phenolic compounds and transition metal ions on the column is consistent with the results in solution. They also provide a comprehensive view for devising strategies to improve IMAC purification of target proteins and peptides from samples containing phenolic compounds.

Amplified luminescence quenching effect upon binding of nitrogen doped carbon nanodots to transition metal ions.

There is a significant drive to identify a unified emission mechanism hidden behind carbon nanodots (CDs) to attain reliable control over their photoluminescence properties. This issue is addressed here by investigating the fluorescence response of citric acid and urea-based nitrogen doped carbon nanodots (NCDs) towards transition metal ions in solutions of different polarities/viscosities/hydrogen bonding strengths. The photoluminescence from NCDs upon excitation at 400 nm is quenched by metal ions such as chromium(vi), ruthenium(iii) and iron(iii) in two different polar solvents, protic water and aprotic dimethylsulphoxide (DMSO). This amplified luminescence quenching in polar solutions showed significant static quenching contributions. The quenching phenomenon highly depends on the excitation wavelength and solvent environment. The fluorescence quenching sequence reveals that pyridinic nitrogen-bases have a dominant influence on J-like emissive aggregates of NCDs. Similarly, oxygen-containing functional groups play a significant role in constructing H-aggregates of NCDs. The most intense emission is contributed by the J-like assembly of H-aggregates.

X-ray absorption study of the geometry structure of Co2+/Co3+ in ammonia solution

The investigation of the geometrical structure of transition metal ions in solution is mandatory to determine the role of transition metal ions in many chemical reactions. In this contribution, the local structure of Co2+/Co3+ in ammonia solution was investigated by X-ray absorption spectroscopy (XAS). We found that Co2+ ions are coordinated by six NH3 ligands arranged in a Jahn-Teller (JT) distorted octahedral (DT Oc) geometrical configuration with four long Co-Neq bonds (2.17 Å) and two short Co-Nax bonds (2.03 Å). At variance, Co3+ ions in [Co(NH3)6]3+ or [Co(NH3)5Cl]2+ clusters occur depending on the presence of activated carbons during the oxidation reaction process from Co2+ to Co3+. By means of full multiple-scattering theoretical (MXAN) analysis of the Co K-edge XANES (X-ray absorption near edge structure), we show that [Co(NH3)6]3+ is characterized by a regular octahedral (Re Oc) geometrical structure with a Co–N distance 1.91 Å long while [Co(NH3)5Cl]2+ by a distorted octahedral geometrical structure with four Co-Neq bonds (1.94 Å), a shorter Co-Nax bond (1.91 Å) and a Co–Cl bond (2.12 Å).

Mixed ligand complexation of some transition metal ions in solution and solid state: Spectral characterization, antimicrobial, antioxidant, DNA cleavage activities and molecular modeling

Equilibrium studies of Ni(II), Cu(II) and Zn(II) mixed ligand complexes involving a primary ligand 5-fluorouracil (5-FU; A) and imidazoles viz., imidazole (him), benzimidazole (bim), histamine (hist) and L–histidine (his) as co-ligands(B) were carried out pH-metrically in aqueous medium at 310±0.1K with I=0.15M (NaClO4). In solution state, the stoichiometry of MABH, MAB and MAB2 species have been detected. The primary ligand(A) binds the central M(II) ions in a monodentate manner whereas him, bim, hist and his co-ligands(B) bind in mono, mono, bi and tridentate modes respectively. The calculated ΔlogK, logX and logX′ values indicate higher stability of the mixed ligand complexes in comparison to binary species. Stability of the mixed ligand complex equilibria follows the Irving-Williams order of stability. In vitro biological evaluations of the free ligand(A) and their metal complexes by well diffusion technique show moderate activities against common bacterial and fungal strains. Oxidative cleavage interaction of ligand(A) and their copper complexes with CT DNA is also studied by gel electrophoresis method in the presence of oxidant. In vitro antioxidant evaluations of the primary ligand(A), CuA and CuAB complexes by DPPH free radical scavenging model were carried out. In solid, the MAB type of M(II)5-FU(A)his(B) complexes were isolated and characterized by various physico-chemical and spectral techniques. Both the magnetic susceptibility and electronic spectral analysis suggest distorted octahedral geometry. Thermal studies on the synthesized mixed ligand complexes show loss of coordinated water molecule in the first step followed by decomposition of the organic residues subsequently. XRD and SEM analysis suggest that the microcrystalline nature and homogeneous morphology of MAB complexes. Further, the 3D molecular modeling and analysis for the mixed ligand MAB complexes have also been carried out.

Dissecting Local Atomic and Intermolecular Interactions of Transition-Metal Ions in Solution with Selective X-ray Spectroscopy.

Determining covalent and charge-transfer contributions to bonding in solution has remained an experimental challenge. Here, the quenching of fluorescence decay channels as expressed in dips in the L-edge X-ray spectra of solvated 3d transition-metal ions and complexes was reported as a probe. With a full set of experimental and theoretical ab initio L-edge X-ray spectra of aqueous Cr(3+), including resonant inelastic X-ray scattering, we address covalency and charge transfer for this prototypical transition-metal ion in solution. We dissect local atomic effects from intermolecular interactions and quantify X-ray optical effects. We find no evidence for the asserted ultrafast charge transfer to the solvent and show that the dips are readily explained by X-ray optical effects and local atomic state dependence of the fluorescence yield. Instead, we find, besides ionic interactions, a covalent contribution to the bonding in the aqueous complex of ligand-to-metal charge-transfer character.

Electrochemical metal-ion sensors based on a novel manganese phthalocyanine complex

In this paper, we investigated the electrochemical metal-ion sensor activity of a manganese phthalocyanine (MnPc). For this purpose, interactions of MnPc with various transition metal ions were examined in solution and in solid state. MnPc forms aggregates only with Ag+ and Hg2+ ions selectively among various transition metal ions in solution, which indicates possible application of the complex as a metal sensor. For practical application, MnPc was immobilized in a polymeric cation exchange membrane, Nafion, on a glassy carbon electrode and used as an electrochemical receptor element. Sensor properties of MnPc were potentiometrically and amperometrically characterized as metal ion sensor with respect to reproducibility, repeatability, stability, linear concentration range, and sensitivity.

Partitioning of 2,6-Bis(1 H-Benzimidazol-2-yl)pyridine fluorophore into a phospholipid bilayer: Complementary use of fluorescence quenching studies and molecular dynamics simulations

Successful use of fluorescence sensing in elucidating the biophysical properties of lipid membranes requires knowledge of the distribution and location of an emitting molecule in the bilayer. We report here that 2,6-bis(1 H-benzimidazol-2-yl)pyridine (BBP), which is almost non-fluorescent in aqueous solutions, reveals a strong emission enhancement in a hydrophobic environment of a phospholipid bilayer, making it interesting for fluorescence probing of water content in a lipid membrane. Comparing the fluorescence behavior of BBP in a wide variety of solvents with those in phospholipid vesicles, we suggest that the hydrogen bonding interactions between a BBP fluorophore and water molecules play a crucial role in the observed “light switch effect”. Therefore, the loss of water-induced fluorescence quenching inside a membrane are thought to be due to deep penetration of BBP into the hydrophobic, water-free region of a bilayer. Characterized by strong quenching by transition metal ions in solution, BBP also demonstrated significant shielding from the action of the quencher in the presence of phospholipid vesicles. We used the increase in fluorescence intensity, measured upon titration of probe molecules with lipid vesicles, to estimate the partition constant and the Gibbs free energy (ΔG) of transfer of BBP from aqueous buffer into a membrane. Partitioning BBP revealed strongly favorable ΔG, which depends only slightly on the lipid composition of a bilayer, varying in a range from − 6.5 to − 7.0 kcal/mol. To elucidate the binding interactions of the probe with a membrane on the molecular level, a distribution and favorable location of BBP in a POPC bilayer were modeled via atomistic molecular dynamics (MD) simulations using two different approaches: (i) free, diffusion-driven partitioning of the probe molecules into a bilayer and (ii) constrained umbrella sampling of a penetration profile of the dye molecule across a bilayer. Both of these MD approaches agreed with regard to the preferred location of a BBP fluorophore within the interfacial region of a bilayer, located between the hydrocarbon acyl tails and the initial portion of the lipid headgroups. MD simulations also revealed restricted permeability of water molecules into this region of a POPC bilayer, determining the strong fluorescence enhancement observed experimentally for the membrane-partitioned form of BBP.

Optical induction of magnetization and observation of fast spin dynamics in aqueous solutions of copper ions

Optically induced magnetization and its fast dynamics in aqueous solutions of transition-metal ions is studied by the polarization spectroscopy with the pump-probe technique. The fast spin dynamics in solutions at room temperature, which cannot be detected by the conventional magnetic-resonance methods, was observed by an all-optical method. A circularly polarized pump pulse creates the magnetization in the ground state of the copper ions in aqueous solutions of copper sulfate, and the time evolution of the magnetization in the subnanosecond region is monitored through the change of the polarization of the probe pulse. Quantum-beat free-induction decay signals are observed in transverse magnetic fields, whose Fourier transform gives the ESR spectra. The observed concentration dependence of the relaxation rate in no external magnetic field is interpreted by the spin cross relaxation due to the magnetic dipole interaction between the copper ions.

Chemical Control of the DNA Light Switch: Cycling the Switch ON and OFF

The emission of the DNA light-switch complex [Ru(bpy)2(tpphz)]2+ (bpy = 2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' '',3' ''-j]phenazine) can be reversibly turned ON and OFF over several cycles. The tpphz and taptp (taptp = 4,5,9,18-tetraazaphenanthreno[9,10-b] triphenylene) ligands in [Ru(bpy)2(tpphz)]2+ and [Ru(bpy)2(taptp)]2+, respectively, intercalate between the DNA bases, and a 50-fold increase in emission intensity of [Ru(bpy)2(tpphz)]2+ is observed upon DNA intercalation. The [Ru(bpy)2(tpphz)]2+ DNA light switch can be turned OFF statically in the presence of Co2+, Ni2+, and Zn2+, and the emission can be fully restored by the addition of EDTA. Cycling of the DNA light switch OFF and ON can be accomplished through the successive introduction of Co2+ and EDTA, respectively, to solutions of DNA-bound [Ru(bpy)2(tpphz)]2+. Owing to the absence of additional coordination sites, the emission of DNA-intercalated [Ru(bpy)2(taptp)]2+ is not quenched by transition metal ions in solution. To our knowledge, this work presents the first example of a reversible DNA light switch.

Effects of Antioxidants on the Hydrogen Peroxide-Mediated Oxidation of Methionine Residues in Granulocyte Colony-Stimulating Factor and Human Parathyroid Hormone Fragment 13-34

The effects and mechanisms of different antioxidants, methionine, glutathione, acetylcysteine, and ascorbic acid (AscH2), on the oxidation of methionine residues in granulocyte colony-stimulating factor (G-CSF) and human parathyroid hormone fragment 13-34 (hPTH 13-34) by hydrogen peroxide (H2O2) were quantified and analyzed. The rates of oxidation of methionine residues in G-CSF were determined by peptide mapping analyses, and the oxidation of methionine residue in hPTH 13-34 was quantified by reverse-phase HPLC. At pH 4.5, free methionine reduces, glutathione and acetylcysteine have no obvious effect on, and AscH2 promotes the rates of oxidation of methionine residues in G-CSF. The H2O2-induced oxidation rate constants for free methionine, acetylcysteine, and glutathione at pH 4.5 were measured to be 32.07, 1.00, and 1.63 M(-1)h(-1), respectively, while the oxidation rate constant for Met1, the most readily oxidizable methionine residue in G-CSF, is 13.95 M(-1)h(-1). Therefore, the different effects of free methionine, acetylcysteine, and glutathione on the rates of oxidation of methionine residues in G-CSF are consistent with their different reactivity toward oxidation by H2O2. By using hPTH 13-34, the effect of AscH2 on the H2O2-induced oxidation of methionine residue was quantified, and the mechanisms involved were proposed. Because of the presence of trace transition metal ions in solution, at low concentrations, AscH2 is prone to be a prooxidant, increasing the hydroxyl radical (.OH) production rate via Fenton-type reactions. In addition to peroxide oxidation, these radicals lead to the degradation of hPTH 13-34 to smaller peptide fragments. At high concentrations, AscH2 tends to act as an OH scavenger. EDTA inhibits OH production and thus eliminates the degradation of hPTH 13-34 by forming complexes with transition metal ions. However, the rate of oxidation of the methionine residue in hPTH 13-34 increases as the concentration of AscH2 is increased from 0 to 200 mM, and the reason for this is still not clear. Our results demonstrate that free methionine is an effective antioxidant to protect G-CSF against methionine oxidation at pH 4.5. Acetylcysteine and glutathione are not effective antioxidants at pH 4.5. Their oxidation rates at different pH values imply that they would be much more effective antioxidants than free methionine at alkaline conditions. AscH2 is a powerful electron donor. It acts as a prooxidant in the conditions in this study and is unlikely to prevent oxidation by H2O2 in protein formulation, whether or not EDTA is present.

Isotopic fractionation during diffusion of transition metal ions in solution.

Isotope ratios and elemental concentrations were measured in aqueous solutions sampled at varying distances from sources of Fe or Zn ions. The measurements reveal fractionation of isotopes resulting from pure diffusion in solution. Our data demonstrate that diffusion alone can cause changes in (56)Fe/(54)Fe and (66)Zn/(64)Zn isotope ratios in excess of -0.3 per thousand. These findings thus confirm previous suspicions that transport processes contribute to observed variations in isotopic compositions. Diffusion must therefore be considered when attempting to make inferences from isotope measurements on samples originating from aqueous systems where concentration gradients may develop.

Metal Ion Quenching Kinetics of DTDCI in Viscous Solution and Nafion Membranes: Model System for Near Infrared Fluorescence Sensing

Fo¨rster type energy transfer offers opportunities as a sensitive and frequently selective method of monitoring the concentration of analytes in medical sensing applications. However, the fluorescence quenching kinetics observed in microheterogeneous media such as tissue are likely to be less than ideal. In the present article, the quenching kinetics of the carbocyanine dye DTDCI by transition metal ions in solutions and in the microheterogeneous polymer Nafion (a registered trademark of Dupont Corporation) are reported and compared with a view to understanding the complex fluorescence kinetics likely to be encountered in biological media. © 1998 Society of Photo-Optical Instrumentation Engineers.

Field-Dependent Proton NMR Relaxation in Aqueous Solutions of Ni(II) Ions. A New Interpretation

A new model is presented for nuclear-spin relaxation in paramagnetic transition metal complexes in solution, allowing the electron-spin relaxation to be outside the Redfield limit. The novel feature is that the transient zero-field splitting (ZFS), modulated by distortions of the solvation shell, is allowed to be of rhombic rather than cylindrical symmetry. The model, which assumes that the static ZFS is absent, is applicable to aqueous solutions of transition metal ions. The magnetic-field dependence of the proton spin–lattice relaxation rate enhancement in aqueous solution has been investigated, and calculations are presented for anS= 1 system such as Ni2+(aq), using different degrees of rhombicity of the ZFS and different motional conditions (Redfield limit and slow-motion regime). The new model is also applied to fit the previously reported data for the field dependence of proton relaxation in aqueous solution of Ni(ClO4)2at low pH [J. Kowalewski, T. Larsson, and P.-O. Westlund,J. Magn. Reson.74, 56 (1987)]. The inclusion of rhombicity, motivated by recent theoretical work, provides a model which performs as well as the earlierad hocmodel.

Slopes of the energy surface of transition metal ions in solution: A molecular orbital calculation

Application of the semiempirical quantum-chemical INDO/2-MO method in computing the slopes of the energy surface of various ions participating in electron-transfer reactions in solution is presented in this paper. The values of slopes obtained from INDO/2-MO agree with those obtained from the classical improved-average-dipole-orientation (IADO) method. Theoretical results of electronic-transmission coefficients of electron-transfer reactions obtained using values of slopes from INDO/2-MO as well as IADO methods are found in close agreement with those obtained from experimental values of rate constants. The values of transmission coefficient are found to be less than unity and these indicate the non-adiabatic nature of electron-transfer reactions involving the transition metal ions studied in this work.

COORDINATION GEOMETRY OF TRANSITION METAL IONS IN DILUTE SOLUTIONS BY XANES

The coordination geometry of transition metal ions cr3+, ~n~+,~e'+,Fe~+,N i~+ and cu2+ in acqueous solution has been studied by x-ray absorption near edge structure spectroscopy. The spectra have been analyzed in terms of succesive orders of multiple scattering contributions. The effect of the absorbing central atom, oxydation state, interatomic distance of the various metal ions and the particular site structure of cu2+ are discussed. The coordination of transition metal ions in solution has been studied by several methods which probe the pair correlation fun~tion.(~~~)~owever, direct information on the site geometry can only be obtained by the determination of higher orders correlation functions. It has been recently demonstrated that x-ray absorption absorption near edge structure (XANES) spectroscopy is a direct probe of these ~atters(~*~) and the three atom correlation function has been extracted experimentally for the (cro412' and (Mn04)- ions in solution.(8) We report here the metal ion K-edge spectra of cr3+,~n2+,Fe3+, Ni2+ and cu2+ ions in solution with high signal to noise ratio using synchrotron radiation. The data analysis of the XANES spectra for these ions is simplified by the structural disorder of further shells of neighbours which reduces the size of the effective absorbing cluster to the metal ion and the first coordination shell. The multiple scattering data analysis shows that octahedral geometry of water molecules around the central atom is present in all the ions studied. The effect of the ion valence state on the spectra is described in terms of different electronic configuration used to construct the final state potential and by the different metal-ligand interatomic distances. In the case of cu2+ regular octahedral geometry was found and the tetragonal Jahn Teller distortion expected for this ion was not observed. A careful analysis of the EXAFS and XANES spectra supports the conclusion that either the static distortion is less than 0.3 or a dynamical Jahn Teller distortion is present in this ion.

Study of the complexing properties of the dextran carboxymethyl ether with transition metal ions in solutions

The complexing properties of carboxymethyl dextran with ions Cu 2+, Co 2+ and Ni 2+ have been investigated. To do so we carried out potentiometric titration of the solutions varying their concentration and ion strength. Complexing constants were calculated and the stability of the complexes was investigated. It was found that in regard to stability Cu 2+ > Ni 2+ ≈ Co 2+.

Core-electron spectroscopy of alkaline-earth and transition-metal ions in solution

Core-electron spectra of alkaline-earth and transition-metal ions in solution have been recorded. Solvation energies for the core-ionized species are derived using thermodynamic cycles which involve the free-ion to dissolved-ion binding energy shifts. The distinction between “soft” and “hard” ion behaviour is discussed in relation to the solvation energies and multi-line features in the transition-metal core spectra.