V. A. Isaeva

The gibbs energies of transfer of glycine and glycinate ion from water into water-dimethyl sulfoxide mixtures

The Gibbs energies of transfer of glycine from water into water-dimethyl sulfoxide mixtures were determined at 298 K by measurements of substance distribution between immiscible phases. The enthalpy and entropy contributions to ΔG° of glycinate ion resolvation in water-dimethyl sulfoxide solvents were analyzed. The destabilization of glycinate ion was found to be largely determined by the energy component mainly. There was a correlation between the enthalpies of transfer of glycinate ion and the autoprotolysis constants of water-dimethyl sulfoxide solvents. The suggestion was made that changes in the Gibbs energies of glycinate ion resolvation in water-dimethyl sulfoxide and water-alcohol mixtures were largely determined by changes in the acid-base properties of the solvents.

Nickel(II) complexing with glycylglycine and acid-base equilibria of the ligand in aqueous ethanol solvents

The protonation and dissociation constants of glycylglycine and the stability constants of its complexes with nickel(II) in aqueous ethanol of a variable composition with the ethanol mole fraction ranging from 0.00 to 0.55 are determined potentiometrically at a 0.1 M (NaClO4) ionic strength and temperature of 298 K. The stability of nickel(II) complexes of glycylglycine is found to grow with an increase in ethanol concentration in a solution. The solvation contributions of reagents into the displacement of complexing equilibria are analyzed. The change in the stability of nickel(II) glycylglycinate complexes is explained by the weakening of ligand solvation in aqueous ethanol.

Effect of Water-Acetone Solvent on the Stability of Copper(II) and Nickel(II) Nicotinamide Complexes

Complexation of nicotinamide with Cu2+ and Ni2+ ions in a water-acetone solvent is studied. The equilibrium constants of these reactions are determined by potentiometric titration at the ionic strength 0.25 mol/l NaClO4 and 298 K in the interval of mixed solvent compositions from 0 to 0.45 molar fraction of acetone. The Cu(II) complex with nicotinamide becomes stronger in a water-acetone mixture. The dependence of the stability constant of the Ni(II) complex with nicotinamide on the concentration of the organic component in the solution has extremum with a minimum at 0.1–0.2 molar fraction of acetone. The effect of a change in the solvate state of the reactants on the stability of the complexes is discussed.

Gibbs energy of the resolvation of glycylglycine and its anion in aqueous solutions of dimethylsulfoxide at 298.15 K

Gibbs energies for the transfer of glycylglycine and glycylglycinate ions from water to water-dimethylsulfoxide solvents are determined from the interface distribution of substances between immiscible phases in the composition range of 0.00 to 0.20 molar fractions of DMSO at 298.15 K. It is shown that with a rise in the concentration of nonaqueous components in solution, we observe the solvation of dipeptide and its anion, due mainly to the destabilization of the carboxyl group.

The influence of a water-acetone solvent on the stability of nickel(II) complexes with glycylglycine

The stability constants of nickel(II) complexes with glycylglycine in water-acetone mixtures (mole fraction of acetone X = 0–0.40) were determined by potentiometric titration at 298 K and an ionic strength of 0.1 M (NaClO4). With an increase in the content of the organic cosolvent, nickel(II) glycylglycinates become more stable, mainly because of the better coordination of the ligand through the carboxy group.

Dependence of the enthalpies of formation of glycylglycinate complexes of nickel(II) on the composition of a mixed water-dimethylsulfoxide solvent

The heat effects of the complexation reactions of nickel(II) with a glycylglycinate ion in a water-dimethylsulfoxide solvent in a range of compositions of 0.00–0.60 molar parts of dimethylsulfoxide (DMSO) (an ionic strength of 0.1 was maintained using sodium perchlorate) were determined by means of calorimetry at 298.15 K. It is established that the exothermicity of complexation reactions rises by the first two steps and falls upon the addition of a third glycylglycinate anion with an increase in the concentration of DMSO. It is shown that the formation of mono- and bis-glycylglycinate complexes of nickel(II) in a water-DMSO solvent is determined mostly by the enthalpic contribution. It is concluded that the formation of tris-ligand complexes is more associated with the entropic contribution.

Stability of nickel(II) glycylglycinate complexes in aqueous solutions of dimethylsulfoxide at 298.15 K

Stability constants of nickel(II) glycylglycinate complexes in aqueous solutions of dimethylsulfoxide of variable composition (from 0.00 to 0.60 mole fractions DMSO) are determined according to potentiometry at 298.15 K and an ionic strength of 0.1 M (NaClO4). It is determined that with a rise in the concentration of an organic cosolvent in solution, the stability of nickel(II) complexes with glycylglycinate ion on the whole increases, but the logKstability = f(XDMSO) dependences are of a critical character with a maximum of 0.3 mole fractions DMSO. It is demonstrated that the rise in the stability of complexes is related to the destabilization of ligands in the low concentration range of the organic component, while the presence of a maximum is due to the different dynamics of the solvation contributions from reagents during changes in the Gibbs energy of reaction.

Constants of acid-base equilibria of glycylglycine in water-dimethyl sulfoxide solutions at 298 K

Constants of acid-base equilibria of glycylglycine were obtained by potentiometry at a temperature of 298 K and an ionic strength of 0.1 M (NaClO4) in water-dimethyl sulfoxide solutions. It was determined that upon an increase in the content of the organic component in solution, the dissociation constant of the glycylglycinium ion declines stepwise. It was concluded that the pK of the dissociation of glycylglycine versus the composition of a mixed solvent is of a critical character with a minimum in the water-organic solvent composition range of 0.1 to 0.3 molar fractions of DMSO. It is suggested that upon an increase in the concentration of DMSO, the zwitter-ion form of glycylglycine transitions to the molecular form.

Enthalpies of glycylglycinate ion transfer from water to a water–ethanol solvent

The heat effects of mixing a sodium glycylglycinate water solution with a solvent containing from 0.0 to 0.8 mole fraction of ethanol are measured by means of calorimetry at 298.15 K. The enthalpies of sodium glycylglycinate and glycylglycinate ion transfer from water to water–ethanol solutions of different compositions are calculated. The increase of the concentration of nonaqueous component in solution leads to higher endothermicity of glycylglycinate ion transfer, resulting in weaker solvation. The contribution from the enthalpy of glycylglycinate ion resolvation to the heat effects of its complexation reactions with transition metal ions is assessed.

Stability of silver(I) glycinato complexes in water-acetone and water-isopropanol solvents

AbstractThe stability constants of silver(I) glycinato complexes in water-acetone and water-isopropanol solvents of various compositions have been studied potentiometrically at 298 K. The contributions of the ΔG ° of reagent resolvation into the Gibbs free energy change of the complexing reaction have been estimated with reference to the literature data. The change in the solvation state of the ligand is decisive for the strengthening of silver(I) glycinato complexes in the water-organic solvents studied.