In this paper, the dissociation constants (pKa1, pKa2) of five anthraquinones were determined from the relation between the effective mobility at different pH values and the buffer pH value, which was derived from the basic electrophoresis theory and the dissociation equilibrium of a binary acid. In addition, the changes of pKa values of the five compounds were also investigated when organic modifiers were added to the buffer system.

Determination of pKa values of anthraquinone compounds by capillary electrophoresis.

We combined theoretical and experimental studies to elucidate the important deprotonation process of Emodin in water. We used the UV/Visible spectrophotometric titration curves to obtain its pK a values, pK a 1  = 8.0 ± 0.1 and pK a 2  = 10.9 ± 0.2. Additionally, we obtained the pK a values of Emodin in the water–methanol mixture (1:3v/v). We give a new interpretation of the experimental data, obtaining apparent pK a 1  = 6.2 ± 0.1, pK a 2  = 8.3 ± 0.1 and pK a 3  > 12.7. Performing quantum mechanics calculations for all possible deprotonation sites and tautomeric isomers of Emodin in vacuum and in water, we identified the sites of the first and second deprotonation. We calculated the standard deprotonation free energy of Emodin in water and the pK a 1 , using an explicit model of the solvent, with Free Energy Perturbation theory in Monte Carlo simulations obtaining, Δ G aq = 12.1 ± 1.4 kcal/mol and pK a 1  = 8.7 ± 0.9. With the polarizable continuum model for the solvent, we obtained Δ G aq = 11.6 ± 1.0 kcal/mol and pK a 1  = 8.3 ± 0.7. Both solvent models gave theoretical results in very good agreement with the experimental values.

Protonation/deprotonation process of Emodin in aqueous solution and pKa determination: UV/Visible spectrophotometric titration and quantum/molecular mechanics calculations

The montmorillonite dissolution in saline solutions that mimic synthetic lung fluids (SLF) was investigated to gain knowledge on the clearance mechanisms of inhaled clay particles. Dissolution rates were measured at pH 4 (macrophages) and 7.5 (interstitial fluids) at 37 °C in flow-through reactors. The effect of organic acids was investigated through the addition of lactate, citrate, and glycine (0.15, 1.5, and 15 mmol/L). Lactate or glycine does not markedly affect the montmorillonite dissolution rates at pH 4, but at pH 7.5 there exists a slight inhibitory effect of lactate on the dissolution, probably due to a reduction in the number of reactive surface sites caused by lactate adsorption. Citrate enhances the dissolution rate by 0.5 order of magnitude at pH 4 and more than 1 order of magnitude at pH 7.5, thus indicating the prevalence of the ligand-promoted over the proton-promoted dissolution mechanism under these experimental conditions. The kinetic data were used to estimate the reduction in size of an inhaled clay particle. At pH 7.5, a particle 500 nm in diameter could be reduced 25% in the presence of citrate, whereas the reduction in saline solution would only be 10% after 10 years. Ligand adsorption was measured in batch experiments at pH 2–11 and EQ3NR was used to model the capacity of the ligands to form soluble species of Al. Citrate, glycine, and lactate adsorb onto montmorillonite under acidic conditions, up to 23, 26, and 60 μmol/g, respectively. However, only citrate can complex the released aqueous Al at pH 4 and 7.5, which contributes to enhance dissolution rate and prevents precipitation of gibbsite at pH 7.5. The enhancement of the dissolution rate in acidic citrate solution very likely comes from the formation of surface complexes between the ligand and the edge surface of montmorillonite. In neutral conditions the effect may be also due to the decrease of the activity of Al 3+ by formation of aqueous Al-citrate complexes.

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Effect of lactate, glycine, and citrate on the kinetics of montmorillonite dissolution

Accurate determination of pK(a) values is important for proper characterization of newly synthesized molecules. In this work we have used CZE for determination of pK(a) values of new compounds prepared from intermediates, 2, 3 and 4-(2-chloro-acetylamino)-phenoxyacetic acids, by substituting chloride for 2-oxo-pyrrolidine, 2-oxo-piperidine or 2-oxo-azepane. These substances are expected to have a cognition enhancing activity and free radicals scavenging effect. Measurements were performed in a polyacrylamide-coated fused-silica capillary of 0.075 mm ID using direct UV detection at 254 nm. Three electrolyte systems were used for measurements to eliminate effects of potential interactions between tested compounds and components of the BGE. In the pH range 2.7-5.4, chloride, formate, acetate and phosphate were used as BGE co-ions, and sodium, beta-alanine and epsilon-aminocaproate as counterions. Mobility standards were measured simultaneously with the tested compounds for calculations of correct electrophoretic mobilities. Several approaches for the calculation of the pK(a) values were used. The values of pK(a) were determined by standard point-to-point calculation using Henderson-Hasselbach equation. Mobility and pH data were also evaluated by using nonlinear regression. Three parameter sigmoidal function fitted the experimental data with correlation coefficients higher than 0.99. Results from CZE measurements were compared with spectrophotometric measurements performed in sodium formate buffer solutions and evaluated at wavelength where the highest absorbance difference for varying pH was recorded. The experimental pK(a) values were compared with corresponding values calculated by the SPARC online calculator. Results of all three used methods were in good correlation.

Determination of dissociation constants of compounds with potential cognition enhancing activity by capillary zone electrophoresis.

A method is proposed for the determination of dissociation constants of amino acids by capillary zone electrophoresis. According to the dissociation equilibrium of amino acids and the basic theory of electrophoresis, the nonlinear relationship between the pH value of the buffer and the effective electrophoretic mobilities of the analyte was obtained. The dissociation constants can be calculated from the pH values and the corresponding effective electrophoretic mobilities using the program written in C++. The dissociation constants, pKa1 and pKa2, of 11 kinds of amino acids were determined successfully by the proposed method. The determined dissociation constants were compared with values in the literature; the differences between them are in the range of -0.03 to 0.06. No significant differences were observed between the determined dissociation constants and the corresponding literature values.

Determination of dissociation constants of amino acids by capillary zone electrophoresis.

Stability constants of adipate complexes of copper(II), nickel(II), zinc(II), cobalt(II), uranyl(II) and thorium(IV) determined by electrophoresis

Determination of dissociation constants of α-amino acids by ionophoretic technique

Formation de complexes Ni(G − ) + , Ni(G 2 − ), Pb(G − ) + , Pb(G − ) 2 , Cr(G − ) 2+ , Cr(G − ) 2 + , Cr(G − ) 3 , Al(G − ) 3 , Th(G − ) 3+ et Th(G − ) 2 2+ . Constantes de stabilite

Stability constants of glycinate complexes of nickel(II), lead (II), chromium(III), aluminium(III) and thorium(IV) by paper electrophoresis

A new method, involving the use of paper electrophoresis is described for the study of the equilibria in mixed-ligand complex systems in solution. This technique is based on the movement of a spot of metal ion under an electric field with the complexants added in the background electrolyte at pH 8.5. The concentration of primary ligafnd (NTA) was kept constant while that of secondary ligand glycylglycine was varied. The plots of log [glycylglycine] against mobility were used to obtain information on the formation of the mixed complex and to calculate its stability constants. The binary equilibria M-glycylglycine and M-NTA have also been studied since it is prerequisite for the investigation of mixed complexes. The stability constants of the complexes, metal-nitrilotriacetate-glycylglycinate have been found to be 5.39, 5.15, 2.69,2.22 (log K values) for Ni(II), U02(II), Co(II),and Zn(II) complexes respectively at n = 0.1 and temperature 35°C.

A New Technique In the Study of Mixed Complexes (M-Nitrilotriacetate-Glycylglycinate Systems

Stability constants of Zinc(II), Cobalt(ll), Uranyl(II) and Thorium(IV) have been determined by paper electrophoresis. Salicylic acid (0-005 mol dm ) was added to the background electrolyte: 0-1 m o l d m 3 H C I O * . T h e p ropor t ions of C 6 H T O H C O O \" a n d C 6 H » O C O O = were var ied b y c h a n g i n g the pH of the electrolyte. These anions yielded the complexes, ZnC6H<OHCOO, COC6HIOHCOO, ThiCsHiOHCOO)* and U0 2 (C 6 H 4 0C00) | whose stability constants are found to be 10, 10 J , 10 6 and 10\"respectively (/J = 01, temp. 40°Q.

P. C. Yadava

Papers

Stability constants of adipate complexes of copper(II), nickel(II), zinc(II), cobalt(II), uranyl(II) and thorium(IV) determined by electrophoresis

Determination of dissociation constants of α-amino acids by ionophoretic technique

Stability constants of glycinate complexes of nickel(II), lead (II), chromium(III), aluminium(III) and thorium(IV) by paper electrophoresis

A New Technique In the Study of Mixed Complexes (M-Nitrilotriacetate-Glycylglycinate Systems

Stability constants of salicylate of zinc(II), cobalt(II), uranyl(II) and thorium(IV) by paper electrophoresis