Acid-base Dissociation Equilibrium Research Articles

Determination of thermodynamic binding constants by affinity capillary electrophoresis

A strategy to study thermodynamic binding constants by affinity capillary electrophoresis (ACE) is presented. In order to simplify mathematical treatment, analogy with acid-base dissociation equilibrium is proposed: instead of ligand concentration [X], negative logarithm of ligand concentration (or activity), pX = -log[X], is used. On this base, and taking into account ionic activities, a general procedure for obtaining thermodynamic binding constants is proposed. In addition, the method provides electrophoretic mobilities of the free analyte and analyte-ligand complex, even when binding constants are low and thus, the complexed analyte fraction is also low. This is useful as a base to rationally analyze a diversity of situations, i.e., different mathematical dependencies are obtained when analytes and ligands with different charges are combined. Practical considerations are given for carrying out a full experimental design.Enantiomeric ACE separation based on the use of chiral selectors is addressed. 2-hydroxypropyl-β-cyclodextrin was chosen as a model ligand, and both enantiomeric forms of four pharmaceutical drugs (propranolol, pindolol, oxprenolol and homatropine methylbromide) were considered as model analytes. Practical aspects are detailed and thermodynamic binding constants as well as free and complexed analytes mobilities are determined.

Determination of the Acid-Base Dissociation Constant of Acid-Degradable Hexamethylenetetramine by Capillary Zone Electrophoresis.

The acid-base equilibrium of hexamethylenetetramine (hexamine) was analyzed with its effective electrophoretic mobility by capillary zone electrophoresis. Although hexamine is degradable in a weakly acidic aqueous solution, and the degraded products of ammonia and formaldehyde can be formed, the effective electrophoretic mobility of hexamine was measured in the pH range between 2.8 and 6.9. An acid-base dissociation equilibrium of the protonated hexamine was analyzed based on the mobility change, and an acid dissociation constant of pKa = 4.93 ± 0.01 (mean ± standard error, ionic strength: 0.020 mol dm-3) was determined. The monoprotic acid-base equilibrium of hexamine was confirmed through comparisons of its electrophoretic mobility with the N-ethylquinolinium ion and with the monocationic N-ethyl derivative of hexamine, as well as a slope analysis of the dissociation equilibrium.

Determination of acid dissociation constants of warfarin and hydroxywarfarins by capillary electrophoresis

In this work the acid dissociation constants – pKa of warfarin and its all important oxidative metabolites have been determined by capillary electrophoresis-based methods. It has resulted in a complete description of two acid–base dissociation equilibria, yet not investigated experimentally for phase I metabolites of warfarin. The capillary electrophoresis (CE) method based on the relation between effective electrophoretic mobilities and pH has proven to be a suitable tool for pKa determination, while the spectrophotometric (CE-DAD) and the internal standard methods (IS-CE), have appeared to be promising alternative approaches. The CE-DAD approach based on the change in absorbance spectra between the acidic and basic forms is a combination between capillary electrophoresis and spectrophotometric titration, and yields very consistent values of pKa1 with CE. The IS-CE, in turn, enables an estimation of pKa1 and pKa2 from only two analytical runs, however, less accurate than CE and CE-DAD. The Debye–Hückel model has been confirmed experimentally as a good predictor of pKa values at various ionic strengths. Therefore, it has been used in determination of thermodynamic pKa1 and pKa2, referring to the zero ionic strength. The results are important from the analytical, pharmacological, and theoretical points of view.

Fluorescence studies on the photophysical properties and encapsulation behavior of acetaminophen in different environments

A systematic study on the spectroscopy and photophysical properties of widely used analgesic and anti-pyretic drug acetaminophen (NAPAP) was presented using steady state and time-resolved fluorescence spectroscopy. The results in homogeneous solvents were compared with those in bio-mimicking environments of cyclodextrin and micellar cavities. Extensive theoretical calculations using time dependent density functional theory (TDDFT) were also done to substantiate the spectral assignment as well as to compare the structure and stability of possible hydrogen bonded conformations of NAPAP in aqueous medium. Facile proton dissociation occurs due to extensive charge redistribution in the excited state. The variation in fluorescence yield and the life time of excited state species in cyclodextrin cavities and micellar medium is due to shift in acid-base dissociation equilibrium in these environments.

Determination of dissociation constants of p-hydroxybenzophenone in aqueous organic mixtures – Solvent effects

The apparent acidity constant of p-hydroxybenzophenone, which is a practically insoluble drug in water but of great pharmaceutical interest, was determined by reversed-phase high-performance liquid chromatography in organic solvent – water mixtures (acetonitrile–water, ethanol–water, and methanol–water), varying the reaction medium permittivity in the interval 56 to 70, at constant ionic strength (0.050) and temperature (30 °C). A combined glass electrode calibrated with aqueous standard buffers was used to obtain pH readings based on the concentration scale (swpH). The pKa values from chromatographic data were obtained using the Hardcastle–Jano equation. Moreover, excellent linear relationships between the pKa values and solvation properties of the reaction medium (relative permittivity and Acity) were used to derive acid dissociation constants in aqueous solution. It has been concluded that the pKa values extrapolated from such solvent–water mixtures are consistent with each other and with previously reported measurements. In addition, the molecular structures of all the chemical species involved in the acid–base dissociation equilibrium studied were calculated with a B3LYP/6–311++G(d,p) method that makes use of the polarizable continuum model (PCM). Taking into account the theoretical pKa values, the conclusions obtained match our experimental determinations.Key words: solvent effects, p-hydroxybenzophenone, acidity constant, solvation parameters, structure, DFT calculation.

Kinetic Study of Radical-Scavenging and Vitamin E-Regenerating Actions of Edaravone (3-Methyl-1-phenyl-2-pyrazolin-5-one)

Abstract A kinetic study was performed for edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) in order to clarify the mechanism of its free-radical-scavenging and vitamin E-regenerating actions. The second-order rate constants for the radical-scavenging reaction of edaravone were measured in several organic solvents and in a water/methanol mixed solvent at various pH. The keto–enol tautomerism and the acid–base dissociation equilibrium of edaravone produce keto, enol, and anion forms in solutions, and their contributions and activities varied depending on the properties of the solutions. From the results of NMR and kinetic studies, it has been clarified that the keto–enol tautomerism of edaravone actually exists, and the keto form has larger radical-scavenging activity than the enol form. Furthermore, the pH dependence of the rate constants suggests that the anion form produced by the acid–base dissociation equilibrium of edaravone has the highest radical-scavenging activity in the keto, enol, and anion forms.

Kinetic Study of the Mechanism of Free-Radical Scavenging Action in Curcumin: Effects of Solvent and pH

Abstract A kinetic study was performed for the free-radical scavenging actions of curcumin and half-curcumin, which has a half-structure of curcumin, in order to clarify the mechanism of free-radical-scavenging in curcumin. The second-order rate constants for radical-scavenging reactions of curcumin and half-curcumin were measured by a stopped-flow spectrophotometer in several organic solvents (methanol, ethanol, acetonitrile, chloroform, and benzene) and in aqueous Triton X-100 (5.0 wt %) micelle solutions at various pH. The difference in the rate constant and solvent dependence between curcumin and half-curcumin suggests that the enol structure with the intramolecular hydrogen-bond of curcumin strongly enhances the radical-scavenging activity. Furthermore, notable pH dependences were observed for the rate constants of curcumin and half-curcumin in micelle solutions, suggesting that the acid–base dissociation equilibrium of phenol-protons in curcumin and half-curcumin affects their radical-scavenging activities.

Synthesis of 1‐amino‐2‐methylindoline by Raschig process: Kinetics of the oxidation of 1‐amino‐2‐methylindoline by chloramine

AbstractThe synthesis of 1‐amino‐2‐methylindoline by the Raschig process was undertaken in aqueous solution. The principal side reaction that occurs in the medium is the oxidation of 1‐amino‐2‐methylindoline formed by chloramine. To increase the yield of 1‐amino‐2‐methylindoline, its oxidation by chloramine was studied by GC and HPLC at various concentrations of reactants and for a pH interval ranging between 9.9 and 13.5. The reaction is bimolecular and exhibits a specific acid catalysis. In alkaline medium, 1‐amino‐2‐methylindole is the principal product. The enthalpy and entropy of activation were determined at pH 12.89. In unbuffered solution, the interaction was autocatalyzed by the ammonium ions formed, which indicates a competitive oxidation of neutral and ionic forms of 1‐amino‐2‐methylindoline by chloramine. A mathematical treatment based on one implicit equation allows a quantitative interpretation of all the phenomena observed over the above pH interval. It takes both acid–base dissociation equilibrium and alkaline hydrolysis of chloramine into account. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 515–523, 2002

Acidity and Dynamic Structure of the Potent Uncoupler SF6847 (2,6-Di-t-butyl-4-(2,2-dicyanovinyl)phenol) and Its Derivatives

Abstract The acid dissociation constant (Ka) of a series of 2,6-di-t-butyl-4-(disubstituted vinyl)phenols was measured in water spectrophotometrically at 25 °C. The electronic substituent effects of the substituted vinyl moiety on the pKa value were examined by dual-electronic-parameter analyses. The electronic effects of the two substituents at the end of the vinyl moiety were found to be not equal in the acid-base dissociation equilibrium. The inductive and resonance effects of the X-substituents trans to the benzene ring and the inductive effect of the Y-substituents cis to the benzene ring were significant factors governing the pKa value. These results and the molecular orbital calculation studies suggest that although the π-bonded X-substituents are coplanar to the vinyl plane, the π-bonded Y-substituent are twisted by a steric repulsion with the benzene ring. The variations in the pKa value of variously substituted analogs were related with the ease of intramolecular rotational motion around the bond connecting the vinyl group and the benzene ring, based on 1H NMR studies.

The use of microelectrodes for studying the process involved in 1-naphthylamine oxidation in dimethyl sulphoxide

The electrochemical oxidation of 1-naphthylamine at platinum electrodes has been investigated in dimethyl sulphoxide. Such a process occurs through a pathway involving a very fast chemical reaction interposed between two one-electron charge transfers. The kinetic constant of this chemical step is inaccessible to voltammetric measurements performed at conventional electrodes. Conversely, microelectrodes are suitable for this purpose when used at both high and low scan rates, thus allowing one to establish that the interposed chemical reaction occurs through a two-step pathway involving an acid-base dissociation equilibrium of the cation radical formed in the primary electrode step followed by a faster radical dimerization. By following this approach, a mean value of 3.3 × 10 3 s −1 is found for the relevant pseudo-first-order rate constant. The competition of this rate-determining step with a second-order dissociation reaction involving a proton transfer from the cation radical to the basic starting species has also been considered and discussed.