NMR spectra and potentiometry studies of aluminum(III) binding with coenzyme NAD+ in acidic aqueous solutions.

Complexation and conformational studies of coenzyme NAD+ with aluminum were conducted in acidic aqueous solutions (pH 2-5) by means of potentiometry as well as multinuclear (1H, 13C, 31P, 27Al) and two-dimensional (1H, 1H-NOESY) NMR spectroscopy. These led to the following results: (1) Al could coordinate with NAD+ through the following binding sites: N7' of adenine and pyrophosphate free oxygen (O(A)1, O(N)1,O(A)2) to form various mononuclear 1:1 (AlLH23+, AlLH2+) and 2:1 (AlL2-) species, and dinuclear 2:2 (Al2L22+) species. (2) The conformations of NAD+ and Al-NAD+ depended on the solvents and different species in the complexes. The results suggest the occurrence of an Al-linked complexation, which causes structural changes at the primary recognition sites and secondary conformational alterations for coenzymes. This finding will help us to understand role of Al in biological enzyme reaction systems.     

Potentiometric and multinuclear NMR studies on the interaction of aluminum with ascorbic acid in acidic aqueous solution

The complex-formation equilibria between aluminum(III) ion and L-(+)-ascorbic acid (AA) in 0.1 M KCl ionic medium at 25 degrees C and 0.15 M NaCl ionic medium at 37 degrees C were studied by glass electrode pH-metric measurements. The obtained experimental results were explained by the formation of the following complexation species: a weak mononuclear 1:1 species AlL(2+) together with two trinuclear mixed-hydroxo species Al(3)H(-5)L(4) and Al(3)H(-5)L(3+) in acidic aqueous solutions. Meanwhile, the formation of the complexes and structures of Al with AA were proved by multinuclear (1H, 13C, 27Al) NMR spectra in the pH range 2.0-5.0. It is supposed that Al directly coordinates with AA at O-3 moiety; also, Al can coordinate with the O-1 and O-2 moieties of ascorbate ion through the weakly binding and the intramolecular hydrogen bonding in acidic aqueous solutions.     

Potentiometric studies of indium(III) azide complexes in aqueous medium

The stability constants of indium-azide complexes were determined by the potentiometric method (glass electrode). The effect monitored was the change in pH of a solution of azide and hydrazoic acid (N(-)(3)/HN(3)) when indium(III) cations were added. The azide concentration was varied from close to zero to 90mM, the ionic strength being kept at 2.000 M with sodium perchlorate and the temperature at 25.0 degrees . Evaluation of experimental data showed only mononuclear species, and the global constants found were beta(1) = (2.0 +/- 0.1) x 10(3), beta(2) = (7 +/- 2) x 10(5), beta(3) = (5 +/- 1) x 10(7) and beta(4) = (7 +/- 3) x 10(8).     

Photochemistry of the iron(III) complex with pyruvic acid in aqueous solutions

Photochemistry of the 1: 1 Fe^pIII complex with pyruvic acid (PyrH) in aqueous solutions was studied by stationary photolysis and nanosecond laser flash photolysis with the excitation by the 3rd harmonics of an Nd:YAG laser. The quantum yield of [FeIIIPyr]2+ under the excitation at 355 nm is 1.0±0.1 and 0.46±0.05 in the absence and in the presence of dissolved oxygen, respectively. In experiments on laser flash photolysis, a weak intermediate absorption in the region 580–720 nm was found. The absorption was ascribed to the [FeII…MeC(O)COO•]^p2+ radical complex. Laser flash photolysis of [FePyr]^p2+ in the presence of methyl viologen dications (MV^p2+) resulted in the formation of the MV•+ radical cations. The proposed reaction mechanism includes the inner-sphere electron transfer in the light-excited complex accompanied by the formation of the [Fe^pII…MeC(O)COO•]^p2+ radical complex followed by its transformation into the reaction products.     

Potentiometric and spectral studies of complex formation of La(III), Pr(III) and Lu(III) with aspartic acid and asparagine

The composition and stability of La³⁺, Pr³⁺ and Lu³⁺ complexes with aspartic acid and asparagine were analysed. The formation of complexes of the typeML andMHL was determined for La³⁺ and Pr³⁺ with aspartic acid, and of the typeMHL for Lu³⁺ with aspartic acid. For La³⁺, Pr³⁺ and Lu³⁺ with asparagine the formation ofML(OH) complexes was observed. By means of¹H NMR and¹³C NMR studies the participation in the coordination of both -COOH groups was determined for aspartic acid, whereas for asparagine the participation of the -COOH group was determined in complexes with La³⁺, Pr³⁺, and of the -COOH and the -NH₂ groups in the complex with Lu³⁺.

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Potentiometrische und spektroskopische Untersuchungen an La(III), Pr(III) und Lu(III)-Komplexen von Asparaginsäure und Asparagin

Zusammenfassung Die Zusammensetzung und die Stabilität von La³⁺, Pr³⁺ und Lu³⁺-Komplexen mit Asparaginsäure und Asparagin wurden untersucht. Es wurde die Bildung von La³⁺ und Pr³⁺-Komplexen des TypsML undMHL, und ein Lu³⁺-Komplex des TypsMHL mit Asparaginsäure festgestellt. Für diese drei Lanthaniden wurde auch die Bildung von Komplexen des TypsML(OH) mit Asparagin beobachtet. Mit Hilfe von¹H-NMR und¹³C-NMR-Untersuchungen wurde für Asparaginsäure die Teilnahme der beiden -COOH-Gruppen, für Asparagin die Teilnahme der -COOH-Gruppe in den Komplexen mit La³⁺, Pr³⁺ und der-COOH und -NH₂-Gruppen in dem Komplex mit Lu³⁺ an der Koordinierung festgestellt.

     

Mechanistic studies on the oxidation of pyruvic acid by an oxo-bridged diiron(III,III) complex in aqueous acidic media

In aqueous solution [Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe₂(μ-O)(phen)₄(H₂O)(OH)]³⁺ (2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3). In the presence of excess phen and in the pH range 2.5–5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen)₃]²⁺. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D₂O in comparison to that in H₂O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H⁺), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.     

The interpretation of X-ray diffraction by aqueous solutions of aluminum(III) nitrate and chromium(III) nitrate

Aqueous solutions of the nitrates of aluminum (0.5 M) and chromium (0.5 M) are investigated with X-ray diffraction at t = 25° C. The experimental intensity is interpreted in terms of an electron product-function which is a sum of various pair contributions. The contribution containing the information about cationic hydration has been evaluated by combining the results of separate experiments. The method is in several aspects quite different from what is usual. Still the results obtained are in general agreement with the data reported elsewhere. That is, sixfold coordination at 1.90 Å and 1.98 Å and a second shell containing about 12 water molecules appearing at 4.10–4.15 Å and 4.20–4.25 Å for Al³⁺ and Cr³⁺ respectively.     

Inner-sphere oxidation of cis-diaquabis(oxalato)chromate(III) by periodate in aqueous weakly acidic solutions

The kinetics of oxidation of cis-[Cr^III(ox)₂(H₂O)₂]^– (ox = C₂O₄ ^2–) by IO₄ ^– showed a first-order dependence on the initial Cr^III complex concentration in the presence of a vast excess of [IO₄ ^–]. The dependence of the pseudo-first-order rate constant on [IO₄ ^–] is complex and is consistent with the formation of a precursor complex. It is proposed that this complex is formed through the coordination of the two carbonyl oxygens of the ox ligand with the IO₄ ^– ion, forming a cyclic intermediate. The kinetics are consistent with the hydroxo form of the Cr^III complex being the reactive species, whereas the aqua species forms an unreactive complex.     

Mechanistic studies on oxidation of hydrazine by a mu-oxo diiron(III,III) complex in aqueous acidic media-proton coupled electron transfer

[Fe2(mu-O)(phen)4(H2O)2]4+ (1), one of the simplest mu-oxo diiron(III) complexes, quantitatively oxidises hydrazine to dinitrogen and itself is reduced to two moles of ferroin, [Fe(phen)3]2+ in presence of excess phenanthroline. The weak dibasic acid, 1 (pKa1= 3.71 +/- 0.05 and pKa2= 5.28 +/- 0.10 at 25.0 degrees C, I= 1.0 mol dm(-3)(NaNO3)) and its conjugate bases, [Fe2(mu-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(mu-O)(phen)4(OH)2]2+ (3) are involved in the redox process with the reactivity order 1 > 2 > 3 whereas N2H4 and not N2H5+ was found to be reactive in the pH interval studied 3.45-5.60. Cyclic voltammetric studies indicate poor oxidizing capacity of the title substitution-labile diiron complex, yet it oxidizes N2H4 with a moderate rate--a proton coupled electron transfer (1e, 1H+) drags the energetically unfavourable reaction to completion. The rate retardation in D2O media is substantially higher at higher pH due to the increasing basicity of the oxo-ligand in the order 3 > 2 > 1. Marcus calculations result an unacceptably high one-electron self-exchange rate for the iron center indicating an inner-sphere nature of the electron-transfer.     

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Multimethod characterization of the interaction of aluminum ion with alpha-ketoglutaric acid in acidic aqueous solutions.

It has recently been reported that aluminum plays a very important role in reducing the activity of Krebs-cycle enzymes and glutamate dehydrogenase in rat brain homogenate. Therefore, it is necessary to identify the aluminum binding ability with the pivotal substrate alpha-ketoglutarate in biological systems. The interactions of aluminum with alpha-ketoglutarate were studied with pH-potentiometry, cyclic voltammetry, UV-vis, 1H, 27Al-NMR and Raman spectra multi-analytical techniques in acidic aqueous solution to measure the stoichiometries and stability constants of the complexes and its keto-enol tautomerism. The alpha-ketoglutarate was found to bind Al in a bidentate manner at the carboxylate and carbonyl moieties. The mononuclear 1:1 (AlLH(-1), AIL+, AlHL2+) and 2:1 (AlL2-, AlL2H(-2)3-) species, and dinuclear 2:1 (Al2L4+) species were found in acidic aqueous solution. Meanwhile, Al can promote alpha-KG tautomerize to its enolic-structure compounds in solutions. These findings may help to further understand the influence of Al on GDH enzyme reactions in biological systems.     

Redox equilibria in the In(III)-In(I)-In system in aqueous KBr solutions,I. Potentiometric and coulometric study

Summary The reproportionation of In(I) ions in acidic potassium bromide solutions was studied by means of the coulometric and potentiometric method. The formal potentials of the In(III)/In(I), In(III)/In and In(I)/In redox couples as well as the equilibrium constants of the reaction In(III)+2 In=3 In(I) were determined at a background electrolyte concentration ranging from 1 to 4 mol dm^–3. Based on the experimental results, a scheme of the reproportionation reaction was proposed with regard to the participation of bromide ions. In addition, the diffusion coefficient of In(I) ions was determined using the chronopotentiometric technique.

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Das Redox-Gleichgewicht im In(III)-In(I)-In-System in wäßrigen KBr-Lösungen, I. Potentiometrische und coulometrische Untersuchungen

Zusammenfassung Die Reproportionierung von In(I)-Ionen in sauren Kalium-bromid-Lösungen wurde mittels der coulometrischen und potentiometrischen Meßmethode untersucht. Die Formal-Standardpotentiale der In(III)/In(I)-, In(III)/In- und In(I)/In-Redoxpaare sowie die Gleichgewichts-konstanten der Reaktion In(III)+2 In=3 In(I) wurden im Konzentrationsbereich von 1–4 mol dm^–3 ermittelt. Mit der Steigerung der KBr-Konzentration verschiebt sich das untersuchte Gleichgewicht erheblich zugunsten Indium(I). Aufgrund der potentiometrischen Meßergebnisse wurde ein Schema der Reproportionierung-Reaktion unter Berücksichtigung der Teilnahme von Bromidionen aufgestellt. Zusätzlich wurde der Diffusionskoeffizient von In(I)-Ionen mit Hife chronopotentiometrischer Messungen bestimmt.

     

Phosphonium ionic liquids as extractants for recovery of ruthenium(III) from acidic aqueous solutions

The aim of this work is to investigate extraction of ruthenium(III) from acidic aqueous solutions with phosphonium ionic liquids such as trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101), trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) and tributyl(tetradecyl)phosphonium chloride (Cyphos IL 167) as extractants. The influence of HCl content in the feed solutions on extraction of Ru(III) was investigated. The research was performed for model solutions containing Ru(III) and a mixture of waste solutions containing Ru(III) and Rh(III). In addition, investigation of the type of extractant and its concentration in the organic phase on extraction of Ru(III) was carried out. Co-extraction of protons to the organic phase was determined. To the best of our knowledge, the extraction of Ru(III) with Cyphos IL 167 (tributyl(tetradecyl)phosphonium chloride) as an extractant has not yet been described in the scientific literature.     

Insight into the degradation of a manganese(III)-citrate complex in aqueous solutions

Kinetics of the electron transfer process between citrates and manganese(III) ions has been studied in acidic aqueous solutions. Acidification of the reaction mixture increased the reaction rate. The reaction is dependent on pH because there are two main protolytic forms of the Mn(III)-citrate complex in the studied pH range (4.5–6.5). Reduction potentials of Mn(III)/Mn(II) system in acidic and basic solutions as well as protolytic equilibria play a crucial role in understanding the pH profile of the studied system. The rate constants for Mn(III)citH and Mn(III)citH₂⁺ species degradation processes are presented (citH³⁻ and citH₂²⁻ are trivalent and divalent anions of citric acid, citH₄, respectively). Protolytic constant (expressed as pK′_a) for Mn(III)citH protonation is estimated and discussed.     

Multi-NMR and fluorescence spectra study the effects of aluminum(III) on coenzyme NADH in aqueous solutions

The interactions of dihydronicotinamide adenine dinucleotide (NADH) with Al(III) in near neutral aqueous solutions were studied by means of multinuclear (31P, 27Al, 1H and 13C)-NMR and fluorescence spectra techniques. The results suggested that Al(III) interacts with NADH by occupying the binding sites of pyrophosphate oxygen atoms and locks the adenine moiety of coenzyme in an anti folded conformation Meanwhile, the weak attractive interactions ('association') may occur between Al(III) and the hydroxyl groups of ribose rings through the intramolecular hydrogen bonding. Furthermore, at biologically relevant pH and concentrations of Al(III) and NADH (pH 6.5, C(Al)=10(-6)-10(-5) M), Al(III) could increase the amount of folded forms of NADH, which will result in reducing the coenzyme NADH activity in hollow-dehydrogenases reaction systems. However, in the presence of possible competing organic acids such as citrate, oxalate and tartate, could detoxify these Al(III) toxic effect.     

Behaviour of the thiodipropionic complex of In(III) and U(VI) at the DME in aqueous and aqueous methanolic solutions

The complexation of In(III) and U(VI) with thiodipropionic acid has been investigated polarographically in water and water-methanol solutions at 30 +/- 0.1 degrees . All the chelates belong to polaro-graphically reversible systems. With indium(III), complexes with metal to ligand ratios of 1:1, 1:2, 1:3 and 1:4 are found at pH 4.8. Uranium(VI) is found to form three successive complexes with metal to ligand ratios of 1:1,1:2 and 1:3 in 0.1M HCl, with 0.1M KCl as supporting electrolyte.     

Complex formation equilibria between aluminum(III), gadolinium(III) and yttrium(III) ions and some fluoroquinolone ligands. Potentiometric and spectroscopic study

Complex formation equilibria of aluminum(III), gadolinium(III), and yttrium(III) ions with the fluoroquinolone antibacterials moxifloxacin, ofloxacin, fleroxacin, lomefloxacin, levofloxacin, and ciprofloxacin were studied in aqueous solution by potentiometric and spectroscopic methods. The identity and stability of metal–fluoroquinolone complexes were determined by analyzing potentiometric titration curves (310 K, μ = 0.15 M NaCl, pH range = 2–11, C_L/C_M = 1?:?1 to 3?:?1, C_M = 1.0 mM) with the aid of Hyperquad2006 program. The main species formed in the system may be formulated as M_pH_qL_r (p = 1, q = ?2 to 2, r = 1–3, L = fluoroquinolone anion, logarithm of overall stability constant, log β_p,q,r = in the range ca. ?10 to 45). The stability of complexes is mostly influenced by metal ion properties (ionization potential, ionic radius) indicating partial ionic character of the coordination bond. The complexes were also characterized by spectroscopic measurements: spectrofluorimetry, ¹H-NMR, and ESI-MS. Fluorimetric data were evaluated with the aid of HypSpec2014 and indicated the formation of ML_r (r = 1–3) complexes with cumulative conditional stability constants significantly lower than the thermodynamic ones. NMR and MS data corroborate potentiometrically determined speciation. Calculated plasma mobilizing capacity of the ligands generally follows the order levofloxacin > moxifloxacin > ciprofloxacin at concentration levels of the ligands higher or equal to ca. 10^?4 M.