Uranyl(VI) complexes of ethylene-1,2-dioxydiacetic acid and ethylene-1,2-diaminodiacetic acid in aqueous solution: a potentiometric and calorimetric study

The stability constants and the changes in enthalpy and entropy for the formation of uranyl(VI) complexes with dicarboxylate ligands of the type (-CH₂RCH₂COO)₂²⁻, where RO or NH, have been determined by potentiometric and calorimetric titrations at 25.0 °C in 1.0 mol dm⁻³ aqueous solution of sodium perchlorate.The ethylene-l,2-dioxydiacetate ligand forms either 1:1 and 1:2 chelated complexes or unchelated protonated complexes, owing to the low stability of the chelate ring. Because of precipitation of solid compounds only one complex of 1:1 stoichiometry was observed in the uranyl(VI)—ethylene-l,2-diaminodiacetate system.Factors influencing the stability constants and the enthalpy and entropy changes in the uranyl(VI) complexes with these potentially tetradentate ligands are discussed in comparison with analogous complexes involving bi- and tridentate dicarboxylate ligands.     

An investigation into the genotoxic species generated during reduction of chromium(VI) by catechol(amine)s

Abstract

Chromium(VI) is a common occupational carcinogen.¹ The major carcinogenic and mutagenic species are proposed to be Cr(V) and Cr(IV) intermediates formed during the reduction of Cr(VI) to stable Cr(III) compounds,² although indirect evidence suggests that reactive oxygen species (ROS) may also be important.³ The reductions of Cr(VI) by some biological reductants (e.g. ascorbate) have been studied previously, and genotoxic Cr(IV/V) species have been detected.⁴ Another potential reductant in vivo is protein-bound DOPA, which is present on oxidised proteins at low steady-state concentrations prior to enzymatic breakdown.⁵ Recently, we have shown, by EPR spectroscopy, that the reactions of Cr(VI) with model DOPA compounds (catechol(amine)s), and with oxidised proteins themselves, generate several reactive intermediates, including Cr(V) complexes and organic radicals.⁶ Previous studies have proposed that ROS may also be produced during catechol(amine) oxidation.⁷ Here we describe studies of the interaction of DNA with the reactive species produced during the reductions of K₂Cr₂O₇ by catechol(amine)s.     

Lactase-phlorizin hydrolase complex from monkey small intestine: stimulation of phlorizin hydrolase activity by organic acids

Lactase-phlorizin hydrolase complex from monkey small intestine reveals a new phlorizin hydrolase activity at pH 3.3 in the presence of certain organic acids in addition to the normal activity at optimum pH 5.4. The highest stimulation (10–15 fold) was obtained with tartaric acid. Lactase activity at pH 5.3 is unaffected but its activity at pH 3.3 is inhibited by organic acids. Tartaric acid-stimulated phlorizin hydrolase activity is inhibited by a number of organic acids which have no effect on the unstimulated enzyme. Pyruvic acid inhibits the unstimulated activity as well. SO₄^2? and Cl^? ions are potent inhibitors of the tartaric acid-stimulated phlorizin hydrolase.     

Metal Ion Catalysis in the Hydrolysis of Esters of 2-Hydroxy-1,10-phenanthroline: The Effects of Metal Ions on Intramolecular Carboxyl Group Participation

Rate constants have been determined for hydrolysis of the acetate, glutarate, and phthalate monoesters of 2-hydroxy-1,10-phenanthroline in water at 30°C and μ = 0.1 M with KCl. The hydrolysis reactions of the esters are hydroxide ion catalyzed at pH > 9. The phthalate and glutarate monoesters have in addition pH-independent reactions from pH 5.5 to 9 that involve intramolecular participation by the neighboring carboxylate anion. The pH-independent reaction of the glutarate monoester is 5-fold faster than that of the phthalate monoester. The plots of log k_obsd vs pH for hydrolysis of the carboxyl substituted esters are bell shaped at pH < 5, which indicates a rapid reaction of the zwitterionic species (carboxyl anion and protonated phenanthroline nitrogen). The divalent metal ions, Cu²⁺, Ni²⁺, Zn²⁺, and Co²⁺, complex strongly with the esters; saturation occurs at metal ion concentrations less than 0.01 M. The 1:1 metal ion complexes have greatly enhanced rates of hydrolysis; the second-order rate constants for the OH⁻ reactions are increased by factors of 10⁵ to 10⁸ by the metal ion. The pH-rate constant profiles for the phthalate and glutarate ester metal ion complexes have a sigmoidal region below pH 6 that can be attributed to a metal ion-promoted carboxylate anion nucleophilic reaction. The carboxyl group reactions are enhanced 10² - to 10³ -fold by the metal ions, which allows the neighboring group reaction to be competitive with the favorable metal ion-promoted OH⁻ reaction at pH < 6, but not at pH > 6. The half-lives of the pH-independent neighboring carboxyl group reactions of the Cu(II) complexes at 30°C are l2 s. The other metal ion complexes are only slightly less reactive (half-lives vary from 2.5 to 40 s). These are the most rapid neighboring carboxyl group reactions that have been observed in ester hydrolysis.     

Spectroscopic speciation and structural characterisation of uranyl(VI) interaction with pyridine carboxylic acid N-oxide derivatives

Spectroscopic speciation of U(VI) solutions holding pyridine carboxylic acid N-oxides in a range pH 2.3-4.5 results in the single component spectrum of the U(VI) isonicotinic acid N-oxide complex. The molar absorption is 14 ± 2 L mol⁻¹ cm⁻¹ at 415.4 nm. The formation constant lg K_UL = 2.1 ± 0.2 (k = 2) is derived from solution modelling and by multivariate chemometric analysis. The first crystal structure analysis of a U(VI) pyridine carboxylic acid N-oxide revealed a sheet-like structure where the isonicotinic acid N-oxide binds to the uranyl(VI) both bidentately by the carboxylate group and monodentately by the N-O group. The single component spectrum of the [UO₂L]⁺ (where L⁻ is isonicotinate N-oxide) is compared to the small number of other U(VI) single ligand species. The comparison revealed the possible pitfalls of U(VI) spectroscopic speciation close to the pH region where U(VI) hydrolysis starts to interfere. On basis of the results for U(VI)-L coordination and physicochemical properties of the pyridine carboxylic acid N-oxides some conclusions could be drawn on the likely behaviour of nicotinic acid N-oxide and picolinic acid N-oxide. For the former, complex formation in a narrow range of pH and U(VI) concentrations close to the hydrolysis range of U(VI) might reveal thermodynamic data. In the case of picolinic acid N-oxide, additional experimental evidence is required to characterize suitable conditions.     

Preparation and properties of mono, homo- and heterobinuclear complexes with a new heptadentate Schiff base ligand

A new heptadentate Schiff base, containing an inner N₃O₂ and an outer O₂O₂ site, has been obtained by the reaction of 3-formylsalicylic acid and diethylenetriamine. By reaction of this ligand with copper(II), nickel(II) or uranyl(VI) salts, mononuclear and dinuclear complexes have been synthesized. The mononuclear complexes can act as ligands towards a second metal ion giving rise to homodinuclear or heterodinuclear complexes. The enlargement of the inner coordination chamber allows the synthesis of dinuclear uranyl(VI) species, impossible to obtain with the inner N₂O₂ site of the ligands previously employed. The equatorial pentacoordination of the UO₂²⁺ group in the outer O₂O₂ chamber is reached with the coordination of a solvent molecule to the central metal ion. The electrochemical behaviour of some complexes prepared is also reported.     

Infrared spectroscopic studies of aqueous solutions of dioxouranium(VI) and its hydrolysed products and of in situ electro-generated dioxouranium(V)

Aqueous solutions of dioxouranium(VI) (pH range 0 to 4) give rise to bands at 954 and 938 cm⁻¹ attributable to the v₃(MO₂) stretching modes of the UO₂²⁺ and (UO₂)₂(OH)_2²⁺ cations, respectively. A shoulder at 916 cm⁻¹ is assigned to the v₃(MO₂) mode of hydrolysed dioxouranium(VI) species of higher nuclearity. Infrared spectro-electrochemical studies using a thin-layer reflection-absorption cell have facilitated the study of the reduction of aqueous solutions of dioxouranium(VI) to yield dioxouranium(V) which may be further reduced to uranium(IV). The electrogeneration of dioxouranium(V) is monitored by following the increase in intensity of a band at 914 cm⁻¹ which is present in the spectra at potentials between −0.2 and −0.8 V. The dioxouranium(V) species is predominantly in the form UO₂⁺, which may be in solution or incorporated into an insoluble phase of uranium oxides which deposit onto the working electrode. The U^VO bond length is estimated to be 1.76 Å, 0.03 Å longer than the U^VIO bond in aqueous solution. The maximum concentration of UO₂⁺able to be achieved is highly dependent on the pH and is optimum at pH 3.4. Changes in the pH of the solution under study can be monitored by infrared spectroscopy during the course of the reduction by determining the relative concentrations of hydrolysed dioxouranium(VI) species.     

The thermodynamics of the complexation of lanthanides by 1-hydroxy-4,7-disulfo-2-naphthoic acid

The thermodynamic parameters, log β, ΔH and ΔS, for formation of lanthanide-1-hydroxy-4,7- disulfo-2-naphthoic acid complexes have been determined at 25 °C in 0.10 M NaClO₄ solutions by potentiometric and calorimetric titrations. Under the experimental conditions, the data can be explained with the formation of LnL⁻, LnL₂⁵⁻ and LnHL complexes (H₂L²⁻ = 1-hydroxy-4,7-disulfo-2- naphthoic acid anion). At pH < 3 the LnHL complex is the major species, whereas by increasing pH the formation of LnL_n³⁻⁴ⁿ complexes becomes more important. The data are compared to the comparable data for complexing by aromatic carboxylic acids.     

Complexes of hydroxamates III.: Equilibrium and kinetic studies on the reactions of iron(III) with monohydroxamic acids

Spectral analysis of iron(III) complexes with acetohydroxamate (AX) and histidinehydroxamate (HX) in the UV-visible region revealed that many species may exist in pH range 1.0–7.5. The solution spectra were unstable in pH range ~2.7–4.0. Different species were obtained from fresh solutions and overnight solutions. The difference was rationalized due to hydrolysis and/or polymerization of complexes in solution, especially in pH range 2.7–4.0. The kinetics of the reactions of Fe(III) with AX and HX were accomplished, and mechanisms were suggested for both systems. In both cases, Fe³⁺ and FeOH²⁺ species were found to be the active species in the complex formation of 1:1 complex.     

Chromium(III) complexes and their relationship to the glucose tolerance factor. Part II. Structure and biological activity of amino acid complexes

A number of octahedral chromium complexes with amino acids are ligands have been prepared and their structures assigned on the basis of their chromatographic and spectral properties. These include complexes with the general structure Cr(AA)₂(H₂O)₂ where the amino acids glycine, glutamic acid and glutamine act as bidentate ligands. The analogous compound with cysteine as ligand is stable at low pH, but at high pH a terdentate cysteine complex, Cr(cysteine)₂^?, is formed. These complexes, as well as a solution of monodentate glycine aquo complexes, and Cr-nicotinic acid-glycine and Cr-nicotinic acid-cysteine complexes of undetermined structure, have been assayed for glucose tolerance factor activity using a yeast assay. Only Cr(glutamine)₂- (H₂O)₂⁺, Cr-nicotinic acid-glycine and the mixture of complexes Cr(glycine)_n(H₂O)_6-n⁺³ showed significant activity. It is proposed that a trans arrangement of the non-coordinated nitrogen atoms in the ligands of these complexes can mimic the structural features of the glucose tolerance factor which are essential for biological activity.     

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

Abstract

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.     

The development of an 191Os →191mIr generator using an osmium chelate parent complex—I. Trans-dioxobismalonatoosmate(VI)

A ¹⁹¹Os→ ^191mIr generator has been developed that has higher ^191mIr yield and lower ¹⁹¹Os breakthrough than previous designs. These improvements have been realized through the use of the osmium chelate complex trans-dioxobismalonatoosmate(VI) as the parent species on the generator. The new generator provides an initial ^191mIr yield of 40%/mL and ¹⁹¹Os breakthrough of 2–3 × 10⁻³% when eluted with a solution of 0.05 M malonic acid/0.10 M sodium chloride at pH 4. Other advantages of the new design include faster clearance of the ¹⁹¹Os breakthrough products and simpler assembly.     

Coordination abilities of tetrapeptides containing proline and tyrosine—A spectrophotometric and potentiometric study

The synthesis of three tetrapeptides, Gly-Pro-Gly-Tyr. Gly-Pro-Tyr-Gly. and Tyr-Pro-Gly-Gly, are described. All contain proline as the second amino acid subunit to act as a break point in metal complex formation.The proton and copper(II) complex formation constants have been measured at 22°C and l = 0.10 mol dm^?3 (KNO₃). The copper(II) complexes have also been studied spectrophotometrically over the pH range of 6–11 by absorption spectroscopy (800–200 nm), circular dichroism spectroscopy, and electron paramagnetic resonance spectroscopy. The experimental data have been combined to determine the complex species present as a function of pH and the coordination centers used.     

Copper(II) binding of prion protein’s octarepeat model peptides

The complexes between copper(II) and the synthetic octapeptide fragments of the prion protein Ac-GWGQPHGG-NH₂ (1), Ac-PHGGGWGQ-NH₂ (3) and the cyclic analogue c-(GWGQPHGG) (2) have been comparatively investigated by circular dichroism (CD), absorption (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopic methods.The results suggest a similar copper(II) coordination behaviour of the two linear peptides. In both cases two major complex species were spectroscopically detected. The first one, existing in the range of pH 7-9, showed spectroscopic parameters attributable to a 3N complex species, while the 4N complex was the main species at strongly alkaline pH values. Copper(II) binding appears to be confined within the aminoacid sequence HGG.Cyclisation of the main chain, as in the peptide 2, was found to have remarkable effects on the copper(II) complex speciation especially at pH 7-8 where the 3N species predominated in the linear counterparts. By contrast the spectroscopic data obtained at pH 11 provided evidence of the restoration of the same set of donor atoms as in the linear peptides.     

Zinc(II) and cadmium(II) metal complexes of thiamine pyrophosphate and 2-(α-hydroxyethyl)thiamine pyrophosphate: models for activation of pyruvate decarboxylase

Metal complexes of thiamine pyrophosphate (TPP) of the general formula [M₂(TPPH)₂Cl₂].4H₂O (M =Zn²⁺, Cd²⁺) were isolated from methanolic solutions and characterized by elemental analysis, FT-IR, and multinuclear NMR spectroscopies. The data provide evidence for the bonding of the metals to the N(1') atom of the pyrimidine ring and to the pyrophosphate group. The stability constant measurements of TPP and 2-(α-hydroxyethyl)thiamine pyrophosphate (HETPP) metal complexes in aqueous solution imply the formation of dimeric complex species similar to the isolated solid products. They indicate also that HETPP forms more stable metal complexes than does TPP. To evaluate the coenzyme action of TPP and HETPP metal complexes, enzymic studies have been done using pyruvate decarboxylase apoenzyme. TPP metal complexes do not bind to the apoenzyme, unlike the Zn(II)-HETPP complex which can act as coenzyme. Considering these results, possible functional implications for thiamine involvement in catalysis are discussed. Received 13 September 1999 / Accepted 4 January 2000     

Interaction of Cu(II)with His-Val-Gly-Asp and of Zn(II) with His-Val-His,Two Peptides at the Active Site of Cu,Zn-Superoxide Dismutase

His-Val-His and His-Val-Gly-Asp are two naturally occurring peptide sequences, present at the active site of Cu,Zn-superoxide dismutase (Cu,Zn-SOD). We have already studied the interaction of His-Val-His=A (copper binding site) with Cu(II) and of His-Val-Gly-Asp=B (zinc binding site) with Zn(II). As a continuation of this work and for comparison purposes we have also studied the interaction of Zn(II) with His-Val-His and Cu(II) with His-Val-Gly-Asp using both potentiometric and spectroscopic methods (visible, EPR, NMR). The stoichiometry, stability constants and solution structure of the complexes formed have been determined. Histamine type of coordination is observed for/ZnAH/²⁺, /ZnA/⁺, /ZnA²H/⁺ and/ZnA²/ in acidic pH while deprotonation of coordinated water molecules is observed at higher pH. /CUB/ species is characterized by the formation of a macrochelate and histamine type coordination. Its stability results in the suppression of amide deprotonation which occurs at high pH resulting in the formation of the highly distorted from square planar geometry 4N complex/CuBH_-3/³.     

Synthesis, structure and DNA cleavage activity of two 4,4′-dimethyl-2,2′-bipyridyl manganese(II) complexes

Two new mononuclear Mn(II) complexes, Mn(dmbpy)₂(OCN)₂ (1) and Mn(dmbpy)₂(dca)₂ (2) (dmbpy = 4,4′-dimethyl-2,2′-bipyridine, dca = dicyanamide), have been synthesized and characterized by IR, elemental analysis, and single crystal X-ray analysis. Both complexes have similar molecular structures. The coordination sphere of the Mn(II) ion in 1 or 2 is a seriously distorted octahedron formed by two dmbpy ligands and two OCN⁻ or dca anions in cis positions. For both complexes, the most striking feature is that the mononuclear molecules are linked together by plentiful weak C-H?N hydrogen bonds into a compact 3D supramolecular structure. DNA cleavage studies show that the complexes can promote plasmid DNA cleavage in the presence of H₂O₂ under physiological conditions, and their cleavage activities are obviously both pH value and complex concentration-dependent. The cleavage mechanism between the complexes and plasmid DNA is likely to involve hydroxyl radicals as reactive oxygen species.     

Characterization of reactions of antimoniate and meglumine antimoniate with a guanine ribonucleoside at different pH

It has been shown previously that Sb^V forms mono- and bis-adducts with adenine and guanine ribonucleosides, suggesting that ribonucleosides may be a target for pentavalent antimonial drugs in the treatment of leishmaniasis. In the present work, the reactions of antimoniate (KSb(OH)₆) and meglumine antimoniate (MA) with guanosine 5′-monophosphate (GMP) have been characterized at 37 °C in aqueous solution and two different pH (5 and 6.5), using ESI(−)-MS and ¹H NMR. Acid and base species for both 1:1 and 1:2 Sb^V–GMP complexes were identified by ESI(−)-MS. The ¹H NMR anomeric region was explored for determining the concentrations of mono- and bis-adducts. This allows for the determination of stability constants for these complexes (5900 L mol⁻¹ for 1:1 complex and 370 L mol⁻¹ for 1:2 complex, at pD 5 and 37 °C). Kinetic studies at different pH indicated that formation and dissociation of both 1:1 and 1:2 Sb–GMP complexes are slow processes and favored at acidic pH (2150 L mol⁻¹ h⁻¹ for the rate constant of 1:1 complex formation and 0.25 h⁻¹ for the rate constant of 1:1 complex dissociation, at pD 5 and 37 °C). When MA was used, instead of antimoniate, formation of 1:1 Sb–GMP complex occurred, but with a slower rate constant. Assuming that MA consists essentially of a 1:1 Sb–meglumine complex, a stability constant for MA could also be estimated (8600 L mol⁻¹ at pD 5 and 37 °C). Thermodynamic and kinetic data are consistent with the formation of 1:1 Sb–ribonucleoside complexes in vertebrate hosts, following treatment with pentavalent antimonial drugs.     

NMR study of the complexation of D-gulonic acid with tungsten(VI) and molybdenum(VI)

By using multinuclear (1H, 13C, 17O, 95Mo, 183W) magnetic resonance spectroscopy (1D and 2D), D-gulonic acid is found to form ten and seven complexes, respectively, with tungsten(VI) and molybdenum(VI), in aqueous solution, depending on pH and metal-ligand molar ratios. Two isomeric 1:2 (metal-ligand) complexes involving the carboxylate and the adjacent OH group are present in the pH range 2-9. At intermediate and high pH, molybdate forms a 2:1 tetradentate complex involving the four secondary hydroxyl groups, whereas tungstate forms one 2:1 terdentate species. At low and intermediate pH values, three 2:1 complexes are found for both metals, involving the carboxylate group and three secondary hydroxyl groups, as well as a 5:2 species involving the carboxylate group and all the secondary hydroxyl groups; the concentration of this species increases in time mainly at the expense of 2:1 and 1:2 complexes. Tungstate can also form two additional species, probably a 5:2 species involving the carboxylate group and all the hydroxyl groups, and a 2:1 pentadentate species involving the carboxylate group and all the secondary hydroxyl groups. In alkaline solutions, tungstate is able to form an additional 2:1 pentadentate complex involving all the hydroxyl groups.     

Complexes of polyadenylic acid and 7-methyl-xanthine and their optical rotatory dispersion

Polyadenylic acid forms a 2:1 complex with 7-methyl-xanthine at pH 7.0, 0.15 M Na⁺, and a 1:1 complex at pH 6.0, 0.15M Na⁺. The complexes have been characterized by equilibrium dialysis and by ORD measurements. Both complexes are laevorotatory at wavelengths greater than 286 nm, and the 1:1 complex is remarkably optically active: [m] = ?154,000° at 292 nm. The properties and structures of the complexes are discussed.