Search

Isothermal titration calorimetry, circular dichroism (CD) techniques, and

The metal cations-polysaccharides complexes play crucial roles in the process of immune cell recognitions. Here, we create simpler biomimetic ligands, which emulate binding of carbohydrates in living organisms. We present a detailed study on the interactions of Cu2+, Ni2+ and VO2+ ions with t-butyl 3-azido- and 3-amino-2,3-dideoxy-α-d-arabino-hexopyranosides. NMR, IR and UV–Vis spectroscopies as well as single crystal X-ray diffraction were used for the ligand structure determinations and showed their binding affinity of ions selected. The formation of stable complexes of Cu(II), Ni(II), VO(IV) ions and saccharides with 1:1, 1:2 and 1:3 molar ratios was proved. The coordination sites proposed are N-donor (amino or azido groups), O-donor (hydroxyl group) and endo-oxygen atoms. Additionally, parameters like shape, charge density distribution, and sites of chemical reactivity of the sugars studied were obtained by mapping electron density isosurface with electrostatic potential (MESP).

Pyrazine‑2‑amidoxime (PAOX) is a prototypical sp2-hybridized amidoxime. Although, we described first time in details PAOX structure in solid state, the profile of PAOX behavior in aqueous solution still needs to be improved. While, acid-base properties of PAOX have been tried to define just once, the presented description was not complete and based only on the results of potentiometry during complexometric studies. The selection of method gives rise problems to observation the protonation or deprotonation processes of compound studied. Due to above, the presented studies show first time the complete description of PAOX acid-base properties by obtaining its pKa, solubility, partition (P) and distribution (D) coefficients using various methods. Among them, experiments: potentiometry; UV spectrophotometry by using Hammett's function and pH-metric titrations; cyclic voltammetry (CV) with glassy carbon electrode (GCE) and with GCE electropolymerized by poly‑ l ‑glycine (PGGCE); 1H NMR and theoretical calculations (DFT). Generally, combined four types of experimental and theoretical methods were used to estimate PAOX macro acidic-dissociation constants values. Moreover, log P and log D were determined spectrophotometrically and calculated also by DFT methods. No previous reports were found in the presented form regarding the complete drug-like properties, including negative together with positive values of pKa. Additionally, we proved by our electrochemical results that the quantitative analysis of PAOX do not need the special conditions of the electrode used in research. Hence, the comparison of our results to those described in the literature about analysis made for others, amidoxime derivatives.

Based on the two anomeric methyl 3-amino-2,3-dideoxy- d -arabino-hexopyranosides ten new sugar-modified Schiff base ligands have been synthesized after monocondensation reaction with five o-hydroxyaromatic aldehydes, i.e. salicylaldehyde and its five para-substituted derivatives. The acidity constants and acid-base equilibria processes in polar solution were calculated from data obtained after spectrophotometric titrations at room temperature. The first acidity constant is found to be associated with the deprotonation of an enamine nitrogen and the second constant correspond to the deprotonation of the phenolate oxygen. The contribution of the enol-imine and keto-enamine tautomeric forms and prototropic 1,5-rearrangement has been discussed with the help of the spectroscopic IR, UV–Vis and NMR techniques. Finally, in situ generated catalysts were carried out by mixing separately all synthesized sugar-based Schiff bases with VO(acac)2 or MoO2(acac)2, and have been tested in asymmetric sulfoxidation of thioanisole and the epoxidation of styrene, cyclohexene and S(−)-limonene using tert-butyl hydroperoxide (TBHP) as the terminal oxidant.

Two coordination compounds and their structural characterization of [Ni(PM) 2 ]Cl 2 and [Ni(PL) 2 ]Cl 2 have been studied. Pyridoxamine (PM) and pyridoxal (PL) denote individual vitamers of vitamin B 6 and both demonstrated bidentate nature inside the complexes. The acid-base behavior of the mentioned complexes and their possible interactions with biomolecules in solutions have been never reported. Here, UV-spectroscopic, potentiometric, cyclic voltammetric studies have been carried out on the interaction of the Ni(II)-vitamer complexes with protons as well as with calf thymus -DNA (CT-DNA) in aqueous medium. The acidity constants values of coordination compounds studied have been determined and compared. The potentiometric and pHmetric-spectrophotometric microtitrations were carried out at the same conditions of temperature and pH range, 25 °C and 1.72–11.68, respectively. The procedural decomposition temperatures of the complexes were given and considered in relation to the structures of the examined compounds to complete the structural description of [Ni(PM) 2 ]Cl 2 and [Ni(PL) 2 ]Cl 2 . The CT-DNA binding potential of Ni(II) complexes was carried out by absorption spectroscopy and cyclic voltammetry methods. Mechanism investigation has demonstrated that examined compounds could interact to CT-DNA by grooving binding mode, which was stabilized by hydrogen bond and van der Waals forces.

This article reports the detailed study concerning the mode of binding of three forms of vitamin B6, pyridoxamine (pm), pyridoxine (pn), and pyridoxal (pl), with Cu(II) in aqueous solutions using three independent methods: potentiometry, conductometry, and UV–vis spectroscopy. The stability constants of complexes formed between copper(II) and vitamin B6 were investigated by potentiometric titration in 0.1 M KNO3 ionic medium at 25 °C. While drawing the relations between molar conductance and the ratio of metal to ligand concentrations, different types of lines were obtained indicating the formation of 1 : 1 and 1 : 2 stoichiometric compounds. The stability constants have been determined using EQUID and CVEQUID computer programs and the obtained results were in agreement. The relatively high values of stability constants of Cu(II)-vitamin B6 complexes obtained from three independent methods in comparison to those with other competing cations suggest that the complexes studied are relatively stable in aqueo...

This contribution presents a selection of results obtained using spectrophotometric and potentiometric titrations. For several reasons, the investigated equilibria present particular challenges to traditional analysis techniques. Equilibrium constants and UV–vis absorption spectra for different ligands in the complexation process of Ni(II) with pyridoxamine (pm), pyridoxal (pl) and pyridoxine are reported. The gradual and cumulative stability constants occurring in aqueous solution are presented for all complexes studied. Additionally, crystal-field parameters were calculated for two nickel(II) complexes synthesized, [Ni(pm)2]Cl2 and [Ni(pl)2]Cl2, respectively. The minimum inhibitory concentration and minimal bactericidal/fungicidal concentration values for Ni(II) complexes studied were obtained at 25 °C for 24–48 h. The activity data show that the complexes are more potent antimicrobials than the parent ligands.

The detailed studies concerning the binding mode of bioactive ligands: pyrazine-2-carboxamide (PZA), pyrazine-2-thiocarboxamide (PTCA) and pyrazine-2-amidoxime (PAOX) have been performed by using three independent techniques: UV–Vis spectroscopy, potentiometry and conductometry. The composition and stability constants of new series Ir(III) complexes with antituberculosis drug and its structural analogues have been determined in aqueous and nonaqueous solutions. The hypsochromic shifts of bands and appearance of new peaks during UV–Vis spectrophotometric titration confirmed the interaction of Ir(III) ion with bioligands. In the studied pH range activation of pyrazine derivatives began complexation process. The sharply changed of conductimetric curves slope clearly demonstrated that the ML2 type complexes formation with relatively high stability are favored. Under the experimental conditions, it was found that metal ion interact with ligands leading to the formation of positively charged complexes. The gradual and cumulative stability constants of Ir(III) coordination compounds have been determined. The stability order of Ir(III)– PY systems studied is as follows: PAOX > PZA > PTCA. The analysis of formation equilibria revealed that mononuclear Ir(III) coordination compounds with metal to ligand molar ratio equals 1:2 are preferable with prominently high solution stability. Comparison of stability constants values showed that an increase in the efficiency of the reaction between metal ion and bioligands was observed with elongation of substituent chain. In the binary complexes, the bidentate nature of bioactive ligands and participation of amine and heterocyclic nitrogens in coordination of central ion were confirmed.

The title compound, trans-K2[Co(C2O4)2(H2O)2]·4H2O, was synthesised, and characterised by elemental analysis. Acid dissociation constants for the complex were determined by potentiometric titration and calculated by STOICHIO program. The crystal structure of trans-K2[Co(C2O4)2(H2O)2]·4H2O was determined by X-ray diffraction studies. The asymmetric part of the unit cell contains one symmetric anion of oxalate and water molecule bound with Co(II) ion in crystallographic special position, one potassium cation and two molecules of water. Thermal properties of the complex were examined by thermogravimetric analysis (TGA). A decomposition mechanism is proposed on the basis of the results.

The synthesised complex cis-[Cr(C2O4)(AaraNH2)(OH2)(OSO2)]− anion with SO32− as a ligand in the inner coordination sphere, where AaraNH2 denotes methyl 3-amino-2,3-dideoxy-α-d-arabino-hexopyranoside, was hydrolysed in the presence of acid at H+ concentrations from 0.01 to 2.7 m (HClO4). The reaction kinetics was studied with the stopped-flow spectrophotometric (u.v.–vis.) technique at temperatures of 5, 10, 15, 18 and 20 °C. This hydrolysis turned out to be a single-step process. Determined for this reaction were the rate constant k1 for the removal of SO2 from the coordination sphere of the cis-[Cr(C2O4)(AaraNH2)(OH2)(OSO2)]− ion and the constant pK1 of the protonation of this species in the reaction preceding the hydrolysis. The final product of this reaction – a new complex of CrIII, cis-[Cr(C2O4)(AaraNH2)(OH2)2]+, was obtained. A mechanism for the acid hydrolysis reaction is put forward based on the analysis of the rate constants obtained.