Determination Of Stability Constants Research Articles

Experimental and Theoretical Approaches Toward Anion‐Responsive Tripod–Lanthanide Complexes: Mixed‐Donor Ligand Effects on Lanthanide Complexation and Luminescence Sensing Profiles

A new series of tripods were designed to form anion-responsive, luminescent lanthanide complexes. These tripods contain pyridine, thiazole, pyrazine, or quinoline chromophores combined with amide carbonyl oxygen and tertiary nitrogen atoms. Crystallographic and EXAFS studies of the 10-coordinated tripod-La(NO(3))(3) complexes revealed that each La(3+) cation was cooperatively coordinated by one tetradentate tripod and three bidentate NO(3)(-) anions in the crystal and in CH(3)CN. Quantum chemical calculations indicated that the aromatic nitrogen plays a significant role in lanthanide complexation. The experimentally determined stability constants of complexes of the tripod with La(NO(3))(3), Eu(NO(3))(3), and Tb(NO(3))(3) were in good agreement with the theoretically calculated interaction energies. Complexation of each tripod with lanthanide triflate gave a mixture of several lanthanide complex species. Interestingly, the addition of a coordinative NO(3)(-) or Cl(-) anion to the mixture significantly influenced the lanthanide complexation profiles. The particular combination of tripod and a luminescent Eu(3+) center gave anion-selective luminescence enhancements. Pyridine-containing tripods exhibited the highest NO(3)(-) anion-selective luminescence and thus permit naked-eye detection of the NO(3)(-) anion.

Potentiometric and Spectrophotometric Study of 3-Hydroxyflavone−La(III) Complexes

Flavonoids are a large family of natural compounds that includes flavones, flavonols, and chalcones, among others. These compounds are investigated due to their varied biological properties and their capacity to complex metal ions. In this work, the complexation of four 3-hydroxyflavones with the lanthanum(III) ion was studied. A series of novel complexes of La(III) with the selected 3-hydroxyflavones, 3,7-dihydroxyflavone (L1), 3,7,3′-trihydroxyflavone (L2), 3,5,7,3′,4′-pentahydroxyflavone (quercetin) (L3), and 3,5,7,2′,4′-pentahydroxyflavone (morin) (L4), were studied by means of potentiometric and spectrophotometric methods at 26.3 °C and 0.1 M ionic strength in a water−dioxan (1:1, v/v) medium. Their formation constants were evaluated by the Calvin−Bjerrum and Irving−Rossotti methods. Complex 1:1 was found for all four ligands, whereas the 2:1 ligand−metal stoichiometry was found for only three of the four ligands. The obtained protonation constants of the ligands were: L1, log KH1 = 9.128 ± 0.664; L2, log KH1 = 11.091 ± 0.134, log KH2 = 8.822 ± 5·10−4; L3, log KH1 = 9.310 ± 0.119, log KH2 = 8.382 ± 0.109; L4, log KH1 = 10.021 ± 0.355, log KH2 = 7.173 ± 0.576. The determined stability constants were: L1−La(III), log K1 = 7.269 ± 0.387; L2−La(III), log K1 = 9.164 ± 0.020, log K2 = 5.200 ± 0.185; L3−La(III), log K1 = 8.143 ± 0.284, log K2 = 5.632 ± 0.585; L4−La(III), log K1 = 7.407 ± 0.596, log K2 = 4.830 ± 0.165.

The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

Geochemical calculations were employed for the solubility and speciation of neodymium in a model soil solution as a function of pH. The calculations were based on the recently determined stability constants for Nd and solubility product for the Nd end-member of mineral monazite (NdPO(4)). Simulations were carried out at near neutral pH (pH 6.0 to pH 7.5) and 25 degrees C at the atmospheric CO(2) partial pressure. Additional calculations were also performed to assess Nd mobility at the extreme temperature conditions (300 degrees C) at neutral pH. Our results suggest that relatively dilute (Ionic Strength, I = 0.1), low-temperature waters may transport very small quantities of rare earth elements and actinides to the surficial environment at near neutral pH conditions. Evidently, higher temperature or extreme fluid composition may have a greater potential for mobilization of these elements. The results are pertinent to researchers interested in engineering applications for the precipitation of Nd and surrogate actinides from aqueous nuclear wastes.

Interaction between nitroheterocyclic compounds with β-cyclodextrins: Phase solubility and HPLC studies

Chagas disease is a serious health problem for Latin America. Nitrofurazone (NF) and Hidroxymethylnitrofurazone (NFOH) are active against Trypanosoma cruzi. The effect of β-cyclodextrin (β-CD) and dimethyl-β-cyclodextrin (DM-β-CD) complexation on the UV absorption and retention time of nitrofurazone (NF) and its hydroxymethylated analog (NFOH) were studied in solution. The retention behavior was analyzed on a reversed phase C 18 column and the mobile phase used was acetonitrile-water (20/80 v/v), in which cyclodextrins (β-CD or DM-β-CD) were incorporated as a mobile phase additive. The decrease in the retention times of NF (or NFOH) with increasing concentration of HP-β-CD enables the determination of the complex stability constants by HPLC. A phase-solubility study was performed, according to the method reported by Higuchi and Connors, to evaluate the changes of NF/NFOH in the complexation state, and the diagrams obtained suggested that it forms complexes with a stoichiometry of 1:1. This is an important study for the characterization of potential formulations to be used as therapeutic options for the treatment of Chagas disease.

Theoretical study of the spectral behavior of the complex of a recently synthesized aminoxanthone derivative with copper(II) ion in water and methanol

The complexation of a recently synthesized aminoxanthone derivative, 1-({2-[(2-aminoethyl)amino]ethyl}amino)-6-methyl-9H-xanthen-9-one with Cu(II) in methanol and water solutions, was investigated by spectrofluorimetric and spectrophotometric methods. The possible molecular geometries, bindings, and spectroscopic properties for the formed complex were theoretically studied in detail by the Hyperchem program. The stoichiometry of the complex was determined from spectrophotometric molar ratio methods at 20°C. The spectral data were further treated by KINFIT program to calculate the formation constant of the 1:3 complex and its molar absorptivity. Program DATAN was used to calculate the spectral behavior of the complex in each solution for different mole ratios. A simple equation for the determination of complex stability constant was also evaluated from titration data. Under the optimal conditions, the ligand can be used as a fluorometric indicator for measuring Cu2+ ions in aqueous solution.

Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II)

We describe an ultraviolet (UV) absorbance titration method that can be used to determine complexing capacities ( C L) and conditional stability constants (log K) of humic substances (HSs) with metal ions such as Cu(II) and Hg(II). Two fulvic acids (FA) and one humic acid (HA) were used for this study. UV absorbance of HSs gradually increased with the addition of Cu(II) or Hg(II) after blank correction, and these increases followed the theoretical 1:1 (ligand:metal ion) binding model. The results from the absorbance titration calculation for HSs with Cu(II) and Hg(II) compared well with those from fluorescence quenching titration. The titration of the model compound l-tyrosine with Cu(II) proved the validity of this method, and the K and C L were within 2.3% and 7.4% of the fluorescence quenching titration. The results suggest that the UV absorbance titration can be used to study the binding capacities of HSs and/or dissolved organic matter (DOM) with trace metals. The advantages and disadvantages of the absorbance titration method were also discussed.

On the explicit determination of stability constants for linearized geodetic boundary value problems

The theory of GBVPs provide the basis to the approximate methods used to compute global gravity models. A standard approximation procedure is least squares, which implicitly assumes that data, e.g. gravity disturbance and gravity anomaly, are given functions in L2(S). We know that solutions in these cases exist, but uniqueness (and coerciveness which implies stability of the numerical solutions) is the real problem. Conditions of uniqueness for the linearized fixed boundary and Molodensky problems are studied in detail. They depend on the geometry of the boundary; however, the case of linearized fixed boundary BVP puts practically no constraint on the surface S, while the linearized Molodensky BVP requires the previous knowledge of very low harmonics, for instance up to degree 25, if we want the telluroid to be free to have inclinations up to 60°.

Ultraviolet absorbance titration for the determination of conditional stability constants of Hg(II) and dissolved organic matter

Strong interaction between natural dissolved organic matter (DOM) and Hg(II) may influence the transport, conversion, toxicity and bio-validity of mercury in the environment. In this paper ultraviolet (UV) absorbance titration was employed for the first time for the determination of the conditional stability constants of Hg(II) and (DOM). With increasing Hg(II) concentrations, the UV absorbance of fulvic acid, humic acid, and DOM in river increases progressively. By linear and non-linear model fitting, the conditional stability constants (lgK) of Hg(II) and DOM were worked out to be 3.54–4.93 and 3.64–4.85, respectively. The results are consistent with those acquired by the typical fluorescence quenching titration method, with the maximum relative error of lgK being 2.6% and the average relative error being 0.2%. The UV absorbance titration method has the advantages of rapid determination, simple performance, and it will probably become a new approach to studying interactions between DOM and trace metallic ions.

Gallium(III) Ion Hydrolysis under Physiological Conditions

The hydrolysis of gallium(III) has been studied using potentiometric techniques under physiological conditions of temperature 37 C and ionic strength 0.15 moldm-3 NaCl and at different metal ion concentrations. Changes in pH were monitored with a glass electrode calibrated daily in hydrogen ions concentrations. The titration data within the pH range of 2.5-9.99 were analyzed to determine stability constants of hydroxide species using the SUPERQUAD program. Several different species were considered during the calculation procedure and the following hydroxides have been characterized: Ga(OH)3, Ga(OH)4- Ga3(OH)112-, Ga4(OH)11+ and Ga6(OH)153+. Speciation calculations based on the determined constants were then used to simulate the species distribution.

Interaction of water-soluble calix[4]arene with l-tryptophan studied by fluorescence spectroscopy

The complexing ability of water-soluble calix[4]arene for l-tryptophan ( l-try) was investigated by a variety of techniques. The spectrofluorometry titrations were performed at different temperatures to determine stability constants, as well as to evaluate the thermodynamic parameters of the obtained complex. The effect of pH on the complexation process was quantitatively assessed. Moreover, to obtain information about the binding mechanism of the interaction, 1H NMR studies were carried out. Molecular modeling showed that water-soluble calix[4]arene accommodated part of l-amino acid in its cavity meanwhile the aliphatic chain of l-tryptophan stuck out of the cavity. Based on the experiment data, the association process of complexes was established. The water-soluble calix[4]arene was found to be able to adjust its conformation to fit the size of aromatic l-tryptophan, and the benzene ring of amino acid penetrated into the hydrophobic cavity of calix[4]arene.

Stability of Lead(II) Complexes of Alginate Oligomers

The current work reports on the Pb(ll) complexes formed with oligomeric uronic acids (carboxylated saccharide residues) found polymerized in the cell walls and envelopes of algae and bacteria alike. The application of partial acid hydrolysis, size-exclusion chromatography (SEC), 1H NMR, and scanned deposition stripping chronopotentiometry (SSCP) has permitted the determination of stability constants for Pb(II) with both mannuronic (M) and guluronic (G) acid oligomers ranging from the dimer to the pentamer. The determined logarithm of the stability constants range between 4.11 +/- 0.05 and 5.00 +/- 0.04 mol(-1) x dm3 for the eight oligomers studied (pH 6; I = 0.1 mol x dm(-3)). Additional experiments under the same experimental conditions employing galacturonic and glucuronic acid oligomers yielded slightly lower values (2.19 +/- 0.10 to 4.02 +/- 0.07 mol(-1) x dm3) that were expected based on their structure, whereby the monomers which were not included in the alginate oligomer series (unavailable by SEC), yielded the lowest stability constants. This work demonstrates the applicability of the SSCP technique for the determination of stability constants for metal-ligand complexes in which the ligands display relatively low molecular mass. Previous studies on heavy metal interaction with the matrix polysaccharide alginate have largely been restricted to the whole polymer that forms a gel upon binding to network bridging ions such as calcium. The results will be discussed in this context with the emphasis being placed on the relevance of these findings to processes occurring at the biointerface and results from the relevant literature.

Determination of stability constants of valinomycin complexes with ammonium and alkali metal ions by capillary affinity electrophoresis

Capillary affinity electrophoresis (CAE) has been employed to investigate quantitatively the interactions of valinomycin, macrocyclic depsipeptide antibiotic ionophore, with univalent cations, ammonium and alkali metal ions, K(+), Cs(+), Na(+), and Li(+), in methanol. The study involved measuring the change in effective electrophoretic mobility of valinomycin while the cation concentrations in the BGE were increased. The corresponding apparent stability (binding) constants of the valinomycin-univalent cation complexes were obtained from the dependence of valinomycin effective mobility on the cation concentration in BGE using a nonlinear regression analysis. The calculated apparent stability constants of the above-mentioned complexes show the substantially higher selectivity of valinomycin for K(+) and Cs(+) ions over Li(+), Na(+), and NH(4)(+) ions. CAE proved to be a suitable method for the investigation of both weak and strong interactions of valinomycin with small ions.

Study of the interaction between hydroxymethylnitrofurazone and 2-hydroxypropyl-β-cyclodextrin

Chagas disease is a serious health problem in Latin America. Hidroxymethylnitrofurazone (NFOH) is a nitrofurazone prodrug more active than nitrofurazone against Trypanosoma cruzi. However, NFOH presents low aqueous solubility, high photodecomposition and high toxicity. The present work is focused on the characterization of an inclusion complex of NFOH in 2-hydroxypropyl-β-cyclodextrin (HP-β-CD). The complexation with HP-β-CD was investigated using reversed-phase liquid chromatography, solubility isotherms and nuclear magnetic resonance. The retention behavior was analyzed on a reversed-phase C 18 column, using acetonitrile–water (20/80, v/v) as the mobile phase, in which HP-β-CD was incorporated as a mobile phase additive. The decrease in the retention times with increasing concentrations of HP-β-CD enables the determination of the apparent stability constant of the complex ( K = 6.2 ± 0.3 M −1) by HPLC. The solubility isotherm was studied and the value for the apparent stability constant ( K = 7.9 ± 0.2 M −1) was calculated. The application of continuous variation method indicated the presence of a complex with 1:1 NFOH:HP-β-CD stoichiometry. The photostability study showed that the formation of an inclusion complex had a destabilizing effect on the photodecomposition of NFOH when compared to that of the “free” molecule in solution. The mobility investigation (by NMR longitudinal relaxation time) gives information about the complexation of NFOH with HP-β-CD. In preliminary toxicity studies, cell viability tests revealed that inclusion complexes were able to decrease the toxic effect ( p < 0.01) caused by NFOH.

Speciation studies of aluminium(III)–acetate complexes under physiological conditions

The aluminium complexes of acetic acid (ACT) have been studied using Potentiometric titrations under physiological conditions of temperature (37°C) and ionic strength (0.15 dm−3 dm−3 NaCI) and at different ligand to metal ratios. The variations of pH were measured with the help of a glass electrode calibrated daily in hydrogen ion concentrations. Results obtained within the pH range of 2.6–4.2 were analysed to determine stability constants using the SUPERQUAD program. Different complex combinations were considered during the calculation procedure, and evidence was found for ML2 mononuclear species beside binuclear hydroxo-complexes M2L(OH)2 and M2L(OH)3 and metal ion hydroxides. Speciation calculations based on the corresponding constants were then used to simulate species distributions.

Complex Formation of Acetic Acid with Ca(II) and Mg(II) Under Physiological Conditions

Potentiometric titrations of aqueous acetic acid alone and in the presence of Ca(II) or Mg(II) ions have been carried out under physiological conditions at the temperature 37 °C and ionic strength 0.15 mol⋅dm−3 (NaCl) at different ligand-to-metal ratios. Changes in pH were monitored with a glass electrode calibrated daily in terms of the hydrogen ion concentrations. Titration data within the pH range 2.5 to 6.6 were analyzed to determine stability constants using the SUPERQUAD program. Different combinations of complexes were considered during the calculation procedure for both systems, but evidence was found only for mononuclear ML and ML2 species. Speciation calculations based on the corresponding constants were then used to simulate the species’ distributions.

Cyclodextrin Complexes: Chiral Recognition and Complexation Behaviour

Cyclodextrins are well known in supramolecular chemistry as host molecules capable of engulfing molecules in their hydrophobic cavity via noncovalent interactions. Although cyclodextrins are frequently used for chiral separation of racemates, the mechanism of chiral recognition has not yet been fully characterised. The current investigation was aimed at examining chiral recognition mechanism in order to construct an in silico method for prediction of chiral recognition. Amino acids were selected as model guest, whereas alphaCD was used as model host. The results of molecular docking and molecular dynamic calculations were compared to results of stability constant determination and capillary electrophoresis measurements of enantioseparations. Positive correlation between binding strength and chiral separation ability was found. However, the small energy differences between interaction energy of each enantiomer with alphaCD fell into the range of standard error of molecular docking calculations limiting its applicability for in silico prediction. Examining the stability of complex geometry during molecular dynamics simulation revealed that stable complex geometry is likely to be a prerequisite for chiral recognition. This hypothesis was tested on methylderivatized tryptophan. Indeed, chiral separation of beta-methyl-tryptophans by alphaCD could be successfully predicted by examining the complex geometries during molecular dynamic simulation.

Stability constants of complexes formed by new Schiff-base lariat ethers derived from 4,13-diaza-18-crown-6 with Ag+, Pb2+, Cu2+ cations determined by competitive potentiometry

The stability of complexes formed by a series of Schiff-base lariat ethers, derived from 4,13-diaza-18-crown-6, 1 with Ag+, Pb2+, Cu2+ cations, has been comparatively determined, in methanol: dichloromethane solution. We present here the synthesis and an interesting competitive potentiometry method useful for the stability constant determination for a new family of Schiff-base bibracchial lariat ethers. The stability constants and the selectivity in competitive complexation of Ag+, Pb2+ and Cu2+ cations by macrocyclic receptors 1–7 (L), can be accurately evaluated and species distribution diagrams can be calculated for individual system. In all cases further functionalization of bibracchial lariat ethers 2–7 is accompanied by an increasing of the selectivity, relative to the complexes of the initial 4,13-diaza-18-crown-6 macrocycle 1.

Physicochemical Studies on Some Metal Chelates of Cyclodiphosphazane and Thiourea Monophosphazene Derivatives

The interaction of Cr(III), Mn(II), and Fe(III) ions with cyclodiphosphazane and thiourea monophosphazene derivatives was followed potentiometrically with the aim of determination of stability constants of (metal : ligand) 1:1 and 1:2 binary, and 1:1:1 ternary chelates at 25°C and μ = 0.1 mole dm− 3 NaNO3 in 10% (v/v) H 2 O-EtOH mixture. The stability of the complexes of both ligands was found to be Fe (III) > Mn(II) < Cr(III). The concentration distribution of the species formed in solution was evaluated. Binary and ternary solid complexes were prepared with the purpose of structural characterization using elemental analyses, molar conductance, IR, 1HNMR and electronic spectra and thermogravimetric analysis. The data show that the ligands behave as neutral bidentate ligands, forming neutral chelates of monomeric nature.

Competitive Ion Complexation to Polyelectrolytes: Determination of the Stepwise Stability Constants. The Ca2+/H+/Polyacrylate System

This work presents a new methodology aimed at obtaining the stepwise stability constants corresponding to the binding of ions (or other small molecules) to macromolecular ligands having a large number of sites. For complexing agents with a large number of sites, very simple expressions for the stepwise stability constants arise. Such expressions are model-independent; that is, they allow the determination of the stepwise stability constants without making any previous assumption of the detailed complexation mechanism. The formalism is first presented for a single complexing ion and further extended to competitive systems where the competing ions can display, in general, different stoichiometric relationships. These ideas are applied to the analysis of experimental titrations corresponding to competitive binding of calcium ions to poly(acrylic acid) for different pH values and ionic strengths. Intrinsic stability constants were estimated from the stepwise stability constants (by removing the corresponding statistical factor), and split into specific and electrostatic contributions (by means of the Poisson-Boltzmann equation). After this treatment, the specific proton binding energies showed almost no dependence on the coverage and ionic strength. Likewise, for the range of concentrations studied, the specific component of the intrinsic stability constants of the calcium ions, calculated assuming bidentate binding of Ca to neighboring groups of a linear chain, is almost independent of the calcium and proton coverage and ionic strength.

Synthesis, Stability Constant Determination, and Structural Study of Some Complexes of a Zinc Triad Containing Pyridyl‐amine‐quinoline and Pyridyl‐thio‐quinoline

AbstractThe potentially fluorescent and terdentate ligands containing nitrogen or sulfur as coordinating atom were synthesized and used in the formation of perchlorate or chloride complexes of the metals of the Zn triad. The reaction of the ligands with metal perchlorate salts yields the corresponding bis‐chelate derivatives, whereas the same ligands react with metal chloride salts to give monochelate complexes. All of these complexes undergo NMR‐scale fast fluxional rearrangement in solution. Some structural X‐ray diffractometric studies were also performed and the ensuing data confirm the surmised structures and the solution rearrangement in the case of monochelate substrates and of one Hg derivative, respectively. The equilibrium constants of formation in the case of monochelate derivatives were also determined in water by means of spectrophotometric titration of the studied ligand with the metal ions. The values of the equilibrium constants were confirmed by supplementary determination taking into account the exchange between two different metals and/or dissociation equilibria. The fluorescence activity of ligands and complexes was eventually studied and notably onefluorescent silent ligand gives rise to fluorescent zinc derivatives. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)