Search

The influence of the different side chain residues on the thermodynamic and kinetic parameters for complexation reactions of the Co2+ and Ni2+ ions has been investigated by using the isothermal titration calorimetry (ITC) technique supported by potentiometric titration data. The study was concerned with the 2 common tripodal aminocarboxylate ligands, namely, nitrilotriacetic acid and N-(2-hydroxyethyl) iminodiacetic acid. Calorimetric measurements (ITC) were run in the 2-( N-morpholino)ethanesulfonic acid hydrate (2-( N-morpholino) ethanesulfonic acid), piperazine- N, N′-bis(2-ethanesulfonic acid), and dimethylarsenic acid buffers (0.1 mol L−1, pH 6) at 298.15 K. The quantification of the metal-buffer interactions and their incorporation into the ITC data analysis enabled to obtain the pH-independent and buffer-independent thermodynamic parameters ( K, Δ G, Δ H, and Δ S) for the reactions under study. Furthermore, the kinITC method was applied to obtain kinetic information on complexation reactions from the ITC data. Correlations, based on kinetic and thermodynamic data, between the kinetics of formation of Co2+ and Ni2+ complexes and their thermodynamic stabilities are discussed. [ABSTRACT FROM AUTHOR]

A new oxidovanadium(IV) complex was prepared by the reaction of vanadyl acetylacetonate [VO(acac)2], iminodiacetic acid (H2IDA) and 1,10-phenanthroline (phen) in aqueous solutions. The crystal structure of [VO(IDA)phen]·2H2O was determined by X-ray diffraction crystallographic analysis. The stability of the title compound in aqueous solution was investigated by using potentiometric titration methods. Moreover, the kinetics of substitution reactions of the VO- (IDA) binary complex with 1,10-phenanthroline were investigated in aqueous solutions. These reactions were monitored spectrophotometrically at 249 nm by using stopped-flow methods. Kinetic measurements were performed in the 293-308 K temperature range, at different concentrations (0.1- 0.5 mM) of the binary complex and at a constant concentration of phen (0.05 mmol/L). Moreover, the antioxidant properties of this complex against superoxide radicals were studied by nitrobluetetrazolium (NBT) and cyclic voltammetry (CV) and the antioxidant properties were also investigated against two organic radicals: the 1,1-diphenyl-2- picrylhydrazyl radical (DPPH·) and the 2,2'-azinobis(3-ethylbenzothiazoline- 6-sulfonic acid) radical (ABTS·+) by using spectrophotometric methods. The results of the antioxidant tests show that VO(IDA)phen scavenges superoxide free radicals as well as organic radicals (ABTS·+ and DPPH·). [ABSTRACT FROM AUTHOR]

The aquation reaction of the oxydiacetate cobalt(II) complex, namely [Co(ODA)(H2O)2]•H2O (ODA = oxydiacetate), induced by the Fe(III) ions as the promoter has been studied spectrophotometrically (UV-Vis). Kinetic measurements were carried out in the 283.15 - 303.15 K temperature range and at a constant ionic strength of 1.0 M (NaNO3). The observed rate constants were computed by a program based on global analysis. Furthermore, the reaction activation parameters were determined using the Arrhenius and Eyring equations. The changes in the enthalpy, entropy as well as activation energy barriers were calculated in the range of 0.00125 M - 0.125 M of the Fe(III) ion concentration. Based on kinetic data a mechanism for the aquation of the [Co(ODA)(H2O)3] complex has been proposed. [ABSTRACT FROM AUTHOR]

A potentiometric titration method (PT) and a stopped-flow kinetic technique monitored by a UV−Vis spectroscopy have been used to characterize the stability of series of Co(II)- and Ni(II)-thiodiacetato complexes, M(TDA), in the presence of 1,10-phenanthroline (phen) or 2,2'-bipyridine (bipy) in aqueous solutions. The stability constants of the binary (1:1), ternary (1:1:1) as well as the resulting hydroxo complexes were evaluated and compared to the corresponding oxydiacetate complexes. Based on the species distribution as a function of pH the relative predominance of the species in the system over a pH range was discussed. Furthermore, the kinetic measurements of the substitution reactions of the aqua ligands to phen or bipy in the coordination sphere of the binary complexes M(TDA) were performed in the 288-303 K temperature range, at a constant concentration of phen or bipy and at seven different concentrations of the binary complexes (0.2-0.5 mM). The kinetic stability of the M(TDA) complexes was discussed in relation to the experimental conditions and the kind of the auxiliary ligands (phen/bipy). Moreover, the influence of the type of primary ligand (thiodiacetate/oxydiacetate) on the substitution rate of the auxiliary ligands was also compared. [ABSTRACT FROM AUTHOR]

The purpose of this study was to examine the application of the coordinated cis-[Cr(C2O4)(pm)(OH)2]+ cation where pm denotes pyridoxamine, as a specific sensing ion for the detection of hydrogen peroxide (H2O2). The proposed method for H2O2 detection includes two key steps. The first step is based on the nonenzymatic decarboxylation of pyruvate upon reaction with H2O2, while the second step is based on the interaction of cis-[Cr(C2O4)(pm)(OH2)2]+ with the CO2 released in the previous step. Using this method H2O2 generated during glutamate-induced oxidative stress was detected in HT22 hippocampal cells. The coordination ion cis-[Cr(C2O4)(pm)(OH2)2]+ and the spectrophotometric stopped-flow technique were applied to determine the CO2 concentration in cell lysates, supernatants and cell-free culture medium. Prior to CO2 assessment pyruvate was added to all samples studied. Pyruvate reacts with H2O2 with 1:1 stoichiometry, and consequently the amount of CO2 released in this reaction is equivalent to the amount of H2O2. [ABSTRACT FROM AUTHOR]

Potentiometric (PT) and conductometric (CT) titration methods have been used to determine the stoichiometry and formation constants in water for a series of ternary complexes of Co(II) and Ni(II) involving the oxydiacetate anion (ODA) and 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy) ligands, namely [Co(ODA)(phen)(HO)], [Co(ODA)(bpy)(HO)], [Ni(ODA)(phen)(HO)] and [Ni(ODA)(bpy)(HO)]. The ternary complex formation process was found to take place in a stepwise manner in which the oxydiacetate ligand acts as a primary ligand and the phen or bipy ligands act as auxiliary ones. The stability of the ternary complexes formed is discussed in the relation to the corresponding binary ones. Furthermore, the kinetics of the substitution reactions of the aqua ligands in the coordination sphere of the Ni-ODA and Co-ODA complexes to phen or bipy were studied by the stopped-flow method. The kinetic measurements were performed in the 288-303 K temperature range, at a constant concentration of phen or bipy and at seven different concentrations of the binary complexes (4-7 mM). The influence of experimental conditions and the kind of the auxiliary ligands (phen/bipy) on the substitution rate was discussed. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

This study has demonstrated the direct formation of CO2 during the decomposition of the cis-[Cr(C2O4)(PM)(CO3)]− complex anion after addition of perchloric acid to the aqueous solution of this ion. Thus, the studied coordination species can be successfully used as a source of CO2 under specific conditions. This conclusion has been drawn from results of the kinetic studies of acid hydrolysis of the cis-[Cr(C2O4)(PM)(CO3)]− anion. This reaction was investigated using the stopped-flow technique. The measurements were carried out at temperatures ranging from 5 to 25 °C and in [H+] concentrations 0.01, 0.05, 0.1, 0.3, 0.5, 0.8, 1.1, 1.4, 1.7, and 2.0 M. The decarboxylation mechanism for the studied coordination compound has been proposed. [ABSTRACT FROM AUTHOR]

The direct and accurate estimation of nitric dioxide levels is an extremely laborious and technically demanding procedure in the molecular diagnostics of inflammatory processes. The aim of this work is to demonstrate that a stop-flow technique utilizing a specific spectroscopic biosensor can be used for detection of nanomolar quantities of NO2 in biological milieu. The use of novel compound cis-[Cr(C2O4)(AaraNH2)(OH2)2]+ increases NO2 estimation accuracy by slowing down the rate of NO2 uptake. In this study, an animal model of pancreatitis, where nitrosative stress is induced by either 3g/kg bw or 1.5 g/kg bw dose of L-arginine, was used. Biochemical parameters and morphological characteristics of acute pancreatitis were monitored, specifically assessing pancreatic acinar cell death mode, NO2 generation and cellular glutathione level. The severity of the process correlated positively with NO2 levels in pancreatic acinar cell cytosol samples, and negatively with cellular glutathione levels. [ABSTRACT FROM AUTHOR]

The fluorescence quenching of norfloxacin, danofloxacin, enrofloxacin and levofloxacin, belonging to a group of fluoroquinolone antibiotics, by 4-hydroxy-TEMPO was studied in aqueous solutions with the use of steady-state, time-resolved fluorescence spectroscopy as well as UV-VIS absorption spectroscopy methods. In order to understand the mechanism of quenching the absorption and fluorescence emission spectra of all fluoroquinolone antibiotics studied as well as decreases of their fluorescence were registered as a function of the 4-hydroxy-TEMPO concentration. No deviations from a linearity in the Stern-Volmer plots (determined from both, steady-state and time-resolved measurements) were observed. The fluorescence quenching mechanism was proved to be totally dynamic, what was additionally confirmed by the registration of Stern-Volmer plots at 5 temperatures ranging from 15 to 55°C. On the basis of theoretical calculations of fluoroquinolones' molecular radii and ionization potentials the mechanism of electron transfer was rejected. It seems that the fluorescence quenching is diffusion-limited and is caused by the increase of nonradiative processes, such as internal conversion or intersystem crossing. The Stern-Volmer quenching constants and bimolecular quenching constants were determined at the room temperature for all fluoroquinolone antibiotics studied. [ABSTRACT FROM AUTHOR]