Determination Of Stability Constants Research Articles

Incorporation of Carboxylate Pendant Arms into 18-Membered Macrocycles: Effects on [nat/203Pb]Pb(II) Complexation.

We present a detailed investigation on the coordination chemistry of [nat/203Pb]Pb(II) with chelators H4PYTA and H4CHX-PYTA. These chelators belong to the family of ligands derived from the 18-membered macrocyclic backbone PYAN and present varying degrees of rigidity due to the presence of either ethyl or cyclohexyl spacers. A complete study of the stable Pb(II) complexes is carried out via NMR, X-Ray crystallography, stability constant determination and computational studies. While these studies indicated that Pb(II) complexation is achieved, and the thermodynamic stability of the resulting complexes is very high, a certain degree of fluxionality does exist in both cases. Nevertheless, radiolabeling studies were carried out using SPECT (single photon emission computed tomography) compatible isotope lead-203 (203Pb, t1/2=51.9 h), and while both chelators complex the radioisotope, the incorporation of carboxylate pendant arms appears to be detrimental towards the stability of the complexes when compared to the previously described amide analogues. Additionally, incorporation of a cyclohexyl spacer does not improve the kinetic inertness of the system.

Role of Imidazole and Chelate Ring Size in Copper Oxidation Catalysts: An Experimental and Theoretical Study.

In this work, the structural, solution, electrochemical, and catalytic properties of the complexes with ligands derived from imidazole and pyridines were studied. A comparative study of five bioinspired copper catalysts with or without coordinated imidazole and with different chelate ring sizes is presented. Catalytic efficiency on the oxidation of 3,5-di-tert-butylcatechol (DTBC) and ortho-aminophenol (OAP) in a MeOH/H2O medium was assessed by means of the Michaelis-Menten model. Catalysts comprising imidazole-containing ligands and/or a six-membered chelate ring proved to be more efficient in both oxidation reactions. Determination of stability constants and electrochemical parameters of the copper complexes supported the explanation of the catalytic behavior. A catalytic cycle similar for both reactions has been proposed. The results of density functional theory (DFT) free energy calculations for all five complexes and both catalytic reactions agree with the experimental results.

First principle determination of stability constants of metal complexes by cellmetry method

AbstractThe stability constants of 81 metal complexes were calculated using the newly‐developed method which was previously used for determining pK values of acid–base dissociation constants by plane wave functions. Fifteen metal ions and 36 ligands were chosen, and a wide range of the complexes was studied to calculate the logK and logß stability constants. The large variety of geometries and coordination modes allowed us to see the limitation of the method. In some cases, good agreements could be found with the experimental data with an accuracy of .5–1 unit. Results howed that for smaller structural changes, the stability constants can be calculated easily by using the determined parameters. For larger structural differences, rough estimations by 2–4 logK units can be conducted. Correlations could be found among the data which can help further studies as well as aid in the prediction of some unknown complexes. The pK of a hexaaqua metal complex was determined as well.

Speciation of the Ternary Cobalt(II) Picolinic and Dipicolinic Acid Complexes with Small Bioligands

This study reports on the determination of stability constants for ternary cobalt(II) complexes using primary ligands, pyridine carboxylic acids (picolinic (HPic) and dipicolinic (H2Dipic) acids), along with secondary ligands: lactic acid (HLac, HL), oxalic acid (H2Ox, H2L), citric acid (H3Cit, H3L), and phosphoric acid (H3PO4, H3L). Potentiometric data for binary cobalt(II) bioligand complexes and ternary cobalt(II) pyridine carboxylic acid and bioligand complexes were analyzed using the computational program LETAGROP. The experiments were conducted in an ionic medium of 1.0 mol dm–3 NaNO3 at 25 °C. Species distribution diagrams were generated, and relative stability parameters Δlog10 K, log10 X, and % R.S. were calculated. These findings may have important implications for the design of new cobalt(II) complexes for various applications.

Tristhiolato Pseudopeptides Bind Arsenic(III) in an AsS3 Coordination Environment Imitating Metalloid Binding Sites in Proteins.

The AsIII binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand L1) or d-penicillamine residues (ligand L2) as potential coordinating groups for soft semimetals or metal ions, was studied by experimental (UV, CD, NMR, and ESI-MS) and theoretical (DFT) methods. All of the experimental data, obtained with the variation of the AsIII:ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS3-type coordination mode. The UV-monitored titration of the ligands with arsenous acid at pH = 7.0 provided an absorbance data set that allowed for the determination of apparent stability constants of the forming species. The obtained stabilities (logK' = 5.26 (AsL1) and logK' = 3.04 (AsL2)) reflect high affinities, especially for the sterically less restricted cysteine derivative. DFT calculated structures correlate well with the spectroscopic results and, in line with the 1H NMR data, indicate a preference for the all-endo conformers resembling the AsIII environment at the semimetal binding sites in various metalloproteins.

Combining batch experiments and spectroscopy for realistic surface complexation modelling of the sorption of americium, curium, and europium onto muscovite

For a safe enclosure of contaminants, for instance in deep geological repositories of radioactive waste, any processes retarding metal migration are of paramount importance. This study focusses on the sorption of trivalent actinides (Am, Cm) and lanthanides (Eu) to the surface of muscovite, a mica and main component of most crystalline rocks (granites, granodiorites). Batch sorption experiments quantified the retention regarding parameters like pH (varied between 3 and 9), metal concentration (from 0.5 µM Cm to 10 µM Eu), or solid-to-liquid ratio (0.13 and 5.25 g·L−1). In addition, time-resolved laser fluorescence spectroscopy (TRLFS) using the actinide Cm(III) identified two distinct inner-sphere surface species. Combining both approaches allowed the development of a robust surface complexation model and the determination of stability constants of the spectroscopically identified species of (S-OH)2M3+ (logKo −8.89), (S-O)2M+ (logKo −4.11), and (S-O)2MOH (logKo −10.6), with all values extrapolated to infinite dilution. The inclusion of these stability constants into thermodynamic databases will improve the prognostic accuracy of lanthanide and actinide transport through groundwater channels in soils and crystalline rock systems.

Selective union of ATP vs ADP to an amino/amide macrocycle and its Cu2+ complexes: potentiometric, spectrophotometric and theoretical approximations

The protonation equilibria of a new amine/amide type macrocycle and its complexation with Cu(II) in solution have been studied by potentiometry and UV-Visible Spectrophotometry. The ligand shows four protonation constants and one unusual deprotonation constant. Six mononuclear and four binuclear copper II complexes were found and reported its stability constant values. Some species predominate in specific ranges of pH, which might be important for biomimetic studies. It was found that the studied ligand forms stable mononuclear and binuclear complexes with Cu (II) and these species have similar geometries, squared pyramid, above pH 7. An evaluation through its potentiometrically determined stability constants of the selective recognition of ATP over ADP by this macrocycle and its Cu (II) complexes is presented. With or without the presence of Cu (II) ions, the ligand forms more stable complexes with ATP over ADP, the stability of these species increase with pH. Both, the stability quotients and the species distribution diagrams for the ATP-ADP mixed systems shows the inclination of the studied systems to selectively bind ATP over ADP, so, in principle, they can be used for the selective recognition of ATP or as catalysts in the hydrolysis of this nucleotide.

The novel U(VI) and Np(VI) mixed-ligand complexes based on quinaldic acid zwitterion

Interaction of U(VI) and Np(VI) with quinaldic acid in the presence of various inorganic (nitrate, chloride, bromide, perrhenate, selenate, sulfate) or organic (dipicolinic acid) counter-ions allowed obtaining a series of complexes where quinaldic acid presents in the zwitterionic form. In all compounds, except one complex, quinaldic acid acts as a monodentate ligand and coordinates metal centers via one of the carboxylic oxygen atoms. In all these cases, isolated molecular complexes are formed. In contrast, uranyl sulfate exhibits 2D layered structure where both sulfate-ions and molecules of quinaldic acid function as linkers. Determination of stability constants for 1:1 complexes has shown that the complexation of U(VI) and Np(VI) with quinaldic acid in solution likely occurs via coordination of the carboxylic group. Prevalence of the zwitterionic form and the coordination via carboxylic group both in solution and solid-state is unexpected taking into account similarity in structures of quinaldic and picolinic acids.

Potentiometric Determination of Stability Constants of Binary and Ternary Complexes of L-Tryptophan and Anti-Inflammatory Drugs with Zn(II)

Potentiometric Determination of Stability Constants of Binary and Ternary Complexes of L-Tryptophan and Anti-Inflammatory Drugs with Zn(II)

SupraFit – An Open Source Qt Based Fitting Application to Determine Stability Constants from Titration Experiments**

AbstractA novel application to determine stability constants from supramolecular titration experiments is presented. The focus lies on NMR titration and ITC experiments for pure 1 : 1 systems, as well as mixed 2 : 1/1 : 1, 1 : 1/1 : 2 and 2 : 1/1 : 1/1 : 2 systems.SupraFitprovides global and local fitting and a global search tool. Statistical methods are implemented and can be applied to analyse the results of nonlinear regression. Monte Carlo simulations, combined with the percentile methods and F‐Test approaches to calculate confidence intervals are supported. The implemented statistical approaches are illustrated and discussed on model functions. All methods are accessible through an intuitive user interface, providing charts for all (kind of) data produced.SupraFitis written in C++, using the Qt Toolkit for the Graphical User Interface (GUI) and the Eigen library for nonlinear regression and is released under the GNU Public License (GPL).

Complexation Equilibria and Determination of Stability Constants of Some Divalent Metal Ion Complexes of L-Cysteine and Diphenylamine in Aqueous Media

<i>Background. </i>Metal complexes of biologically active ligands had considerable interests. L-cysteinate residue, L-Cys, is a biologically abundant and important versatile binding site of proteins. Diphenylamine, DPA, is an important aromatic amine containing two phenyl groups. Complexation equilibria of the divalent metal ions, Ca²⁺ and Zn²⁺ with the bio- relevant α-amino acid, L-cysteine and the nitrogen-containing diphenylamine ligand were investigated by means of the potentiometric technique at 25.0 ± 0.1°C and constant ionic strength of 0.200 ± 0.001 mol·dm^-3 NaNO₃. <i>Objective.</i> The stability constants and standard free energy changes of the α-amino acid, L-cysteine and diphenylamine complex species were determined at 0.200 ± 0.001 mol·dm^-3ionic strength. <i>Methods.</i> The formation of the different 1:1 and 1:2 binary complexes and 1:1:1 ternary complexes were inferred from the potentiometric titrations. <i>Results.</i> The concentration distribution of L-cysteine species formed in solution was evaluated. The dissociation constants of the α-amino acid and diphenylamine were determined at different ionic strength. The stability constants of these binary and ternary systems were calculated. The values of ∆ log₁₀ K, percent relative stabilization, %R.S. and log₁₀ X for the ternary systems were evaluated and discussed. <i>Concussion.</i> The ternary complex formation occurred in a stepwise manner with L-cysteine acting as the primary ligand. The obtained values of ∆ G⁰ indicated that Complex formation reactions are spontaneous. Also, for all systems studied, the ternary complexes formed are more thermodynamically stable than the binary complexes.

Determination of Stability Constant of The Complexes of Metal Ion Fe(III), Mn(II) And Cr(III) With Some Substituted 2-Oxo-2H-Chromene-3-Carbohydrazide Derivatives at 0.1 M Ionic Strength at 42oC

Proton ligand and metal ligand stability constant are measured for Fe(III) and 1, 2-dihydroxy benzene and 1, 5-disulphonic acid complexes . Determination of stability constant of substituted of pyrazoles with rare earth metals form complexes are reported . Effect of temperature on formation constants of 2 acetylpyridine(N benzoyl)glycine hydrazone with lanthanide(III) ions at different ionic strengths are reported. Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Bjerrum-Calvin titration technique . Kabadi , Jahagirdar and Narwade have determined pK values of salicylaldehyde, salicylic acid and sulphonic acid respectively by similar procedure. The metal ligand stability constant of some β-diketones are reported . Stability constants have investigated for some substituted pyrazolines, isoxalline and diketone . The method most frequently applied for study of complex equilibria is pH-metric titration technique . Stepwise formation of mononuclear binary complexes is described by set of equilibrium constants. For pH-metric measurements an electrode must be selected. According to Bjerrum, Martell and Calvin , the formation of complex MLN is stepwise process and one has to deal with a series of equilibria of the type: Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Calvin-Bjerrum titration technique . The value of nA,n, pK, log K1 and log K2 are evaluated by Irving and Rossotti's equation. The stability constant and thermodynamic parameters of complexes containing lighter lanthanides , copper(II) , calcium and magnesium are reported. It is also important in environmental studies , medicinal and industrial chemistry . Effects of transition metal on stability constant of complexes have studied by pH metric method . Bendi , Janrao and Ramteke have studied stability constant of ligands with lanthanide metals complexes.The present work involves pH metric study of substituted 2-oxo-2H-chromene-3-carbohydrazide derivatives with metal ions Fe(III), Mn(II), Cr(III) and Ti(III) at temperature42oC. The Bjerrum-Calvin titration technique as modified by Irving and Rossotti has been employed in the present study. The following three sets of titrations are carried out in sequence.

Determination of Stability Constants and Thermodynamic Parameters for Ciprofloxacin-HCl With Some Transition Metal Ions by Potentiometric Titration Technique

This study included calculating the stability constants and stoichiometries of some binary complexes formed from the interaction of seven of metal ions (Cr+3,Mn+2,Co+2,Ni+2,Cu+2,Zn+2 and Cd+2) with Ciprofloxacin-HCl. This study was carried out using Potentiometric Titration Technique at different concentrations of the drug and the metal ion and at temperatures (288,298,310,318 and 328 K±0.1) in aqueous solution. Although, there are many methods available to study the stability of metal-ligand complexes, pH-metric is most frequently used to determine the stability constants. Hence, (Calvin - Bjerrum pH-metric titration technique) as adopted by Irving–Rossotti was used to calculate of stability constants. The results of the concentration effect study showed that the formation of complexes were more stable in dilute solutions, and through study of the temperature effect it was concluded that the stability of the complexes can be increased with raising temperature, and the stability constants value obtained were used in the calculation of thermodynamic parameters 〖(S〗^o,H,〖G〗^o). The results of this study also showed that the ciprofloxacin tends to form two types of metal complexes in the solution (ML1),(ML2) and this indicates that the reaction ratio between the drug and the metal ion was (1:1) and (1:2)(M:L) respectively for all metal ions. These ratios depended on the nature of the ligand or the metal ion, and the stability of the complexes formed increases according to the following arrangement (Mn+ 2< Co+ 2< Ni+ 2< Cu+ 2> Zn+ 2) and this arrangement is compatible with Irving Williams’s arrangement for the stability of the elements.

Spectrophotometric Investigation of Complexation of Ferrous Ion with 3,5-Dintrosalicylic Acid

The determination of stoichiometry and stability constant of metal-ligand complex holds great significance for chemical, physical, environmental and biological studies. In the present work, the stoichiometric ratio and stability constant of iron complex of 3,5-dinitrosalicylic acid was determined using Job’s method of continuous variation. Job’s method can be easily applied to investigate a wide range of colored complexes including mononuclear as well as polynuclear complexes of various species, therefore showing its usefulness for practical application in undergraduate University curriculum. The stoichiometry of the major complex formed was determined to have a 1:1 ratio of Fe+2 and 3,5-dinitrosalicylic acid. Stability constant value was determined and found to be 3 x 105. The resulting high stability of the complex can be attributed to resonance stabilization of metal-ligand П-bonding electron density. From the thermodynamic parameter, the negative value of overall change in Gibb’s free energy shows the complex formation between Fe+2 and 3,5-dinitrosalicylic acid is spontaneous and feasible.

Potentiometric determination of stability constants of ternary complexes of first row transition metal ions with ditopic azelaic acid dihydrazide and aspartic acid in aqueous medium

A Chemical speciation study of ternary complexes containing transition metal ions (Mn2+,Co2+,Ni2+, Cu2+ and Zn2+) with a ditopic novel ligand L ​= ​Azelaic acid dihydrazide (AZDH) and X ​= ​l-Aspartic acid of type [MmLlX xHh], (where m ​= ​1, 2; l ​= ​1; x ​= ​1, 2 and h ​= ​−2 to 5) in different ratios of M:L:X ​= ​1:1:1, 1:2:1, 2:1:1and 2:1:2 was carried out using pH -potentiometric method of data acquisition and applied to chemometric modelling methods of analysis. MINIQUAD-75 program was used for the resolution of the plausible species and their corresponding formation constants at 303 ​± ​0.1 ​K, and 0.01 ​M ionic strength. Species concentration distribution diagrams were generated using HySS program. Best-fit chemical models were selected on the basis of statistical parameters like Standard deviation (SD), U (sum of the squares of the residuals in mass balance equations), and chi-square test etc. Relatively stable ternary complexes were evaluated quantitatively using Δlog K formulation. All the formation constants for the species were compared for Irving-Williams order of stability.

Simultaneous determination of cyclodextrin stability constants as a function of pH and temperature – A tool for drug formulation and process design

The stability constant for complex formation is an important physicochemical property used to guide drug formulation development with cyclodextrins. Complex formation depends on both pH and temperature, factors often altered during pharmaceutical processing and manufacturing. Hence, their influence on the stability constant must be considered during drug formulation to define a robust formulation and manufacturing process. This work demonstrated how a mechanistic model, based upon physical chemical laws, for the apparent stability constant as a simultaneous function of pH and temperature accurately could estimate the apparent stability constant for cyclodextrin complexes. The model required multiple thermodynamic inputs, i.e. the pKa value of guest and buffer, measured stability constants for the cyclodextrin complex, the change in enthalpies and the change in heat capacities. A comprehensive set of isothermal titration calorimetry data for β-CD:ibuprofen at six different pH values and four temperatures were compared to the model calculations. A good agreement was observed between the experimental data and the modelled data, indicating that the model could be used to predict the stability constant at any pH and temperature. The model can be used as an important tool during drug formulation and process design to optimize formulation work with cyclodextrins.

Determination of stability constants of mercury(II) by garlic organosulfur ligands with differential pulse voltammetry

Determination of stability constants of mercury(II) by garlic organosulfur ligands with differential pulse voltammetry

Three cobalt(II) porphyrins ligated with pyridyl-containing nanocarbon/gold(III) porphyrin for solar cells: Synthesis and characterization

Three new donor–acceptor systems, synthesized by axial coordination of 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero[60]pyrrolidine (PyC60)/2,5-di-(pyridin-2-yl)-3,4-fullero[70]pyrrolidine (Py2C70) and meso-tetra(4-pyridyl)porphyrinato)gold(III) chloride (ClAuP) with (meso-tetra(4-octyloxyphenyl)porphyrinato)cobalt(II) (CoP) were observed. In order to optimize the reaction conditions, the formation mechanism and composition of the complexes dependent on the structure of axial molecular ligand were explored. The triad, (PyC60)2CoP/(Py2C70)2CoP, and dyad, (ClAuP)CoP, were formed respectively in the two-stage and one-stage process. The confirmation of their chemical structure and determination of stability constants was performed using the UV–vis, IR, 1H NMR, MS (MALDI-TOF) spectrometry and chemical thermodynamics/kinetics, respectively. Photoelectrochemical studies of the porphyrin-fullerene/porphyrin-porphyrin systems were carried out compared with corresponding precursors by the amperometry methods. The high values of the photocurrent density and incident photon energy conversion to current (IPCE365nm) of the titanium electrode modified by (PyC60)2CoP/(Py2C70)2CoP/(ClAuP)CoP were demonstrated, which points to good perspectives in the application of the coordination complexes synthesized as a photoactive molecular material for solar cells.

Phytochelatins as a Dynamic System for Cd(II) Buffering from the Micro- to Femtomolar Range.

Phytochelatins (PCs) are short Cys-rich peptides with repeating γ-Glu-Cys motifs found in plants, algae, certain fungi, and worms. Their biosynthesis has been found to be induced by heavy metals—both biogenic and toxic. Among all metal inducers, Cd(II) has been the most explored from a biological and chemical point of view. Although Cd(II)-induced PC biosynthesis has been widely examined, still little is known about the structure of Cd(II) complexes and their thermodynamic stability. Here, we systematically investigated glutathione (GSH) and PC2–PC6 systems, with regard to their complex stoichiometries and spectroscopic and thermodynamic properties. We paid special attention to the determination of stability constants using several complementary techniques. All peptides form CdL complexes, but CdL2 was found for GSH, PC2, and partially for PC3. Moreover, binuclear species CdxLy were identified for the series PC3–PC6 in an excess of Cd(II). Potentiometric and competition spectroscopic studies showed that the affinity of Cd(II) complexes increases from GSH to PC4 almost linearly from micromolar (log K7.4GSH = 5.93) to the femtomolar range (log K7.4PC4 = 13.39) and additional chain elongation does not increase the stability significantly. Data show that PCs form an efficient system which buffers free Cd(II) ions in the pico- to femtomolar range under cellular conditions, avoiding significant interference with Zn(II) complexes. Our study confirms that the favorable entropy change is the factor governing the elevation of phytochelatins’ stability and illuminates the importance of the chelate effect in shifting the free Gibbs energy.

Spectrophotometric Determination of the Overall Stability Constants of Complexes of Calcium(Ii) Ions with Glycine, L-Methionine, and L-Tryptophan Using Multiple Linear Regression

Spectrophotometric Determination of the Overall Stability Constants of Complexes of Calcium(Ii) Ions with Glycine, L-Methionine, and L-Tryptophan Using Multiple Linear Regression