Protolytic Equilibria Research Articles

Protolytic equilibrium, light absorption and emission of 2,7-dinitro-4,5-dibromofluorescein and related dyes: Fluorescent indicators sensitive to hydrogen bonds of the solvent

This article reports on the spectral data and protolytic equilibria parameters of a series of fluorescein halogen and nitro-halogen derivatives in solution. Absorption spectra and dissociation constants of 2,7-dinitro-4,5-dibromofluorescein (also called eosin bluish or eosin B), 2,4,5,7-tetrabromofluorescein (eosin or eosin Y), 2,7-dichlorofluorescein, and 3′,4′,5′,6′-tetrachlorofluorescein are determined in DMSO. The state of the tautomeric equilibria of molecules and anions is discussed. The behavior of the nitro-bromo derivative in solution strongly differs from other dyes due to the pronounced influence of the NO2 groups in 2 and 7 positions. As a result, the lactone tautomer predominates not only for the neutral form, H2R, but even for the double-charged anion, R2–. This kind of chain-ring tautomerism is not typical for other dyes under study. The influence of the solvent’s nature on the fluorescent properties of this dye is also of special interest. Whereas the solvatochromism of the HR– form of this compound as well as of the R– ion of its methyl ester is of medium strength, the emission intensity dramatically changes from bright fluorescence in DMSO and other non-HBD solvents to very poor in alcohols and water. Thus, these two dyes join the group of previously studied fluorescein derivatives, the fluorescence quantum yield of which is sensitive to the hydrogen bonds of the medium.

How deprotonation of cohumulone and colupulone influence their UV/Vis and CD spectrum

Cohumulone and colupulone are representatives of α- and β-acids, respectively. These compounds are important antimicrobial hop (Humulus lupulus) constituents, where cohumulone is an important source of the bitter taste of beer. In this study, we examined the pH dependence of UV/Vis spectra of both compounds while CD spectra of cohumulone were also measured at various wavelengths. This facilitated the examination of the protolytic equilibrium of both compounds, where the second pKa value of cohumulone was determined for the first time. Additionally, comparing experimental spectra with spectra calculated using time-dependent density functional theory (TD-DFT) enabled the determination of the most likely deprotonation positions and corresponding species most likely present in the aqueous solution at various pH values. Last but not least, comparing calculated and experimental CD spectra of cohumulone facilitated the determination of the absolute stereoconfiguration of cohumulone.

Ion exchanger‐based electrodes for determination of highly lipophilic physiologically active amines: Peculiarities of functioning and ways for improving analytical characteristics

AbstractNew experimental data are presented demonstrating a much stronger pH dependence of the potential of electrodes with plasticized ion exchange membranes sensitive to cations of highly lipophilic physiologically active amines than would be expected from protolytic equilibrium in solution. Using an example of a vinpocetine‐sensitive electrode, the effectiveness of the dynamic diffusion model of the boundary potential for describing the effect of pH on the form of the electrode function is demonstrated. A simple equation for estimating the effect of distribution of the molecular form of amine between the solution and the membrane phases on the lower detection limit was obtained in the framework of a stationary diffusion model. It has also been shown that the selectivity of electrodes to cations of tertiary physiologically active amines relative to inorganic ions drastically depends on the basicity of the plasticizer (up to 6 orders of magnitude), and among the commercially available ones, ortho‐nitrophenyl octyl ether is the best. Besides, this plasticizer provides the lowest detection limit (1.7×10−7 M) and the best potential reproducibility (0.06 mV) for vinpocetine‐sensitive electrode.

Study on protolytic equilibria and the effects of pH, cyclodextrins and polysorbate 80 on solubility of raloxifene hydrochloride

Study on protolytic equilibria and the effects of pH, cyclodextrins and polysorbate 80 on solubility of raloxifene hydrochloride

Chelation of iron(II) ions by ellagitannins—Effects of hexahydroxydiphenoyl and nonahydroxytriphenoyl groups

Tannins represent secondary plant metabolites that are used to control bacterial populations by chelation of essential metal ions. Their presence in food also affects the bioavailability of iron. This study investigates the influence of ellagitannins (vescalin, castalin, vescalagin, castalagin) structure and pH on the stoichiometry and formation constants of ellagitannin-Fe(II) coordination compounds. We demonstrated that ellagitannins are stable for at least one hour at pH values lower than 7.25. The spectra of neutral compounds were measured and explained with the help of TDDFT calculations. Furthermore, the pH-dependence of the ellagitannins UV–Vis spectra was examined to obtain insight into their protolytic equilibrium. Using Job’s method in the pH range 3.50–5.51, the stoichiometry of the formed ellagitannin-Fe(II) ions complexes was determined. A model explaining interactions between ellagitannins and Fe(II) ions, that took into account the protolytic equilibrium of ellagitannins, was fitted globally to all four Job plots, whereby the corresponding formation constants were obtained.

Protolytic Equilibrium of Glycine and Glycylglycine: Structure of Ionic Forms and Solvent Effect on the Thermodynamic Parameters of Processes

Quantum-chemical calculations of the structure of the molecular forms of glycine and glycylglycine and conjugate ionic forms have been performed. Our own and published data on the thermodynamic characteristics of the reactions of acid–base interactions of glycine and glycylglycine in aqueous organic solutions are summarized, and the influence of the composition of mixed solvents on the acid dissociation constant of the amide group in the copper(II) peptide complex is considered.

Protolytic Equilibrium of Glycine and Glycylglycine: Structure of Ionic Forms and Solvent Effect on the Thermodynamic Parameters of Processes

Protolytic Equilibrium of Glycine and Glycylglycine: Structure of Ionic Forms and Solvent Effect on the Thermodynamic Parameters of Processes

CITRIC ACID–SODIUM CITRATE–WATER SOLUTIONS ACID-BASE AND ELECTROCHEMICAL BEHAVIOR

The protolytic equilibria in the system citric acid (H3Cit)–sodium citrate (Na3Cit)–water at a total content of citrates forms (citric acid, dihydrocitrate (H2Cit-), hydrocitrate (HCit2-) and citrate (Cit3-) anions) 1.0 mol/l have been studied by pH- and conductometric methods in the temperature range 293 ÷ 313 K. The first concentration ranges at which pH – lg(νH3Cit/νNa3Cit) and κ – CNa+/Ccit functions are linear correspond to the H3Cit/H2Cit- buffer system; the second ones – to H3Cit/H2Cit- and H2Cit-/HCit2-; the third ones to – HCit2-/Cit3- buffer system. The investigated solutions ion-molecular composition and ionic strength (m, mol/l) have been calculated using the mathematical model taking into account the law of mass action, material balance for citrates and electrical neutrality principle. The ionic strength concentration dependences have a complex character due to the ion-molecular composition multicomponent nature for the studied solutions. The ionic strength values are directly proportional to the ratio [Na+]/CCit in the concentration intervals 0 ≤ [Na+]/CCit ≤ 1.0 and 2.0 ≤ [Na+]/CCit ≤ 3.0. In the concentration range 1.0 ≤ [Na+]/CCit ≤ 2.0, the values of m = 1.15 ± 0.27 mol/l) weakly depend on the [Na+]/CCit ratio and practically do not depend on temperature. Citric acid concentration and thermodynamic constants for the first, second, and third dissociation stages have been determined. The obtained data on the acid-base and electrochemical characteristics of the solutions HOC3H4(COOH)3 – HOC3H4(COONa)3 – H2O(СCit = 1.0 mol/l; СNa+ = 0 ÷ 1.0 mol/l) can be used in chemical analysis, microbiological and biochemical studies, and the acidity data of the solutions studied can simulate chemisorption of acidic (SO2) and/or basic (NH3) gases.

Protolytic Equilibria of L-Alanyl-L-histidine in Aqueous Solutions

Constants of the stepwise dissociation of L-alanyl-L-histidine (HL) at 298.15 K and ionic strengths of 0.1, 0.5, 1.0, and 1.5 (KNO3) are found potentiometrically. Thermal effects of dissociation are measured for the dipeptide via direct calorimetry. Standard thermodynamic parameters (pK°, ΔdisG°, ΔdisH°, and ΔdisS°) of HL protolytic equilibria are calculated from thermochemical and potentiometric data obtained under identical experimental conditions. The results are compared to those of similar compounds.

Electrochemical Reduction of 1H‐Thioxanthene‐1,4,9‐trione and 3‐Methyl‐1H‐thioxanthene‐1,4,9‐trione – Representatives of a New Class of Heterocycles Related to Thioxanthenone

AbstractA convenient synthetic route was proposed for the synthesis of 3‐methyl‐1H‐thioxanthene‐1,4,9‐trione. This compound and its 3‐H congener are representatives of a new class of thioxanthone‐like heterocycles, which can be used as building blocks for materials used in organic electronics. Electrochemical reduction (ECR) of 1H‐thioxanthene‐1,4,9‐trione and its 3‐methyl derivative in DMF and acetonitrile is a three‐stage process leading to the formation of the corresponding long‐lived radical anions and unstable dianions at the first and second stages, respectively. In a protolytic equilibrium, dianions are converted into 1‐oxy‐4‐hydroxy‐1H‐thioxanthene‐9‐one anions. Their one‐electron reversible ECR leads to the corresponding radical dianions, followed by the formation of (3‐methyl/H)‐1,4‐dihydroxy‐1H‐thioxantene‐9‐ones as the final products. The latter compounds can be converted into the initial (3‐methyl/H)‐1H‐thioxanthene‐1,4,9‐triones. The main intermediates were characterized using a combination of cyclic voltammetry, EPR spectroscopy, 3D spectroelectrochemistry, and DFT calculations at the (U)B3LYP/6‐31+G*/PCM level of theory. A general ECR scheme was proposed for both types of compounds.

Functioning of a Fluorescein pH-Probe in Aqueous Media: Impact of Temperature and Viscosity.

In this work, we considered the influence of viscogenic agents (glycerol, sucrose) as well as the temperature on the fluorescent characteristics of fluorescein at pH 6.5 in order to describe the acid-base status of local environment in terms of a spectrally detectable dianion-anion equilibrium. The protolytic equilibrium of fluorescein was found to depend on the solvent viscosity in a complex way. Whereas in the presence of sucrose the ratiometric signal of fluorescein (I488/I435) remains rather unchanged, the addition of glycerol (up to 40% w/w) results in the increase of the signal (up to 19%), that can be attributed to the different mechanisms of cosolvents effects on dye molecules in the ground state. Molecular dynamics of the dye in the presence of glycerol and sucrose revealed that the cosolvents preferentially interact with fluorescein monoanion and dianion, displacing water molecules from the local environment which in turn reduces the average number of the hydrogen bonds between xanthene ring of the dye and water molecules. The ratiometric signal demonstrates linear growth with the temperature in the range of 10-80 °C regardless of the presence of viscogenic agents. A linear correlation between the temperature sensitivity of the ratiometric signal and the change in the molar enthalpy of the proton dissociation reaction in buffer and viscous media was determined.

Spectrophotometric study of basic properties of glycosylated tetraphenylporphin derivatives

The paper presents the results of spectrophotometric study of the basic properties of glycosylated substituted derivatives of 5,10,15,20-tetraphenylporphyrin, namely 5-(4′- O -galactosylphenyl)-10,15,20-tris( N -methylpyridin-3-yl)porphine, 5,15-bis(4′- O -galactosylphenyl)-10,20-bis( N -methylpyridin-3-yl)porphine, 5,10,15,20-tetra( N -methylpyridin-3-yl)porphin, 5,10,15,20-tetra(4- O -galactosylphenyl)porphin. The basic properties of the studied ligands in acetonitrile at 298 K were studied by spectrophotometric method. The effect of glycosylated fragments, as constituents of a macromolecule, on protolytic equilibria during acid-base interactions of the ligand in basic media and the change in the reactivity of porphyrin ligands depending on the nature of the peripheral substituent was analyzed.

Molecular Characteristics of Water-Insoluble Tin-Porphyrins for Designing the One-Photon-Induced Two-Electron Oxidation of Water in Artificial Photosynthesis

Faced with the new stage of water oxidation by molecular catalysts (MCs) in artificial photosynthesis to overcome the bottle neck issue, the "Photon-flux density problem of sunlight," a two-electron oxidation process forming H2O2 in place of the conventional four-electron oxidation evolving O2 has attracted much attention. The molecular characteristics of tin(IV)-tetrapyridylporphyrin (SnTPyP), as one of the most promising MCs for the two-electron water oxidation, has been studied in detail. The protolytic equilibria among nine species of SnTPyP, with eight pKa values on the axial ligands' water molecules and peripheral pyridyl nitrogen atoms in both the ground and excited states, have been clarified through the measurements of UV-vis, fluorescence, 1H NMR, and dynamic fluorescence decay behaviour. The oxidation potentials in the Pourbaix diagram and spin densities by DFT calculation of the one-electron oxidized form of each nine species have predicted that the fully deprotonated species ([SnTPyP(O-)2]2-) and the singly deprotonated one ([SnTPyP(OH)(O-)]-) serve as the most favourable MCs for visible light-induced two-electron water oxidation when they are adsorbed on TiO2 for H2 formation or SnO2 for Z-scheme CO2 reduction in the molecular catalyst sensitized system of artificial photosynthesis.

New fluorescein dyes with unusual properties: Tetra- and pentanitrofluoresceins

Among various fluorescein dyes, nitro derivatives are less studied. At the same time, these compounds have some unusual properties. This article reports on the behavior of new dyes of this series in solution. Two compounds, 2,4,5,7,4′- and 2,4,5,7,5′-pentanitrofluoresceins, were synthesized, and their molecular structures in the solid state and solution were characterized by X-ray diffraction analysis and NMR spectroscopy. Electronic spectroscopy in dimethyl sulfoxide, DMSO, proves that doubly charged anions of these compounds, R2–, exist in solution in the form of lactone structures, which is not characteristic of other fluorescein dyes. These lactones are yellow (λmax = 420–430 nm) due to the nitrophenolate absorption. On the contrary, singly charged anions are represented predominantly by fluorescent and intensely colored particles with molar absorptivities of (92–96)× 103 M−1cm−1 at λmax = 525 nm, while H2R molecular forms are colorless lactones. The dyes are relatively strong acids; pKa1 and pKa2 values ​​determined in DMSO do not exceed 5 units. It is important to note that in this solvent, as in other media, the pH range of the predominance of HR– monoanions is very narrow. The protolytic equilibrium state of 2,4,5,7-tetranitrofluorescein is similar. The product of the pyran ring rupture of the last-named dye was isolated and identified; this allowed to rationalize the color changes of the parent dye in aqueous solutions within a wide pH range. Of particular interest is the methyl ester of 2,4,5,7-tetranitrofluorescein. The esterification of the carboxyl group avoids lactonization. In contrast to the initial dye, this compound exists as a stable singly charged anion in a wide pH range in all solvents except nonpolar ones, and its fluorescence is very sensitive to the hydrogen bonding ability of the solvent.

Insight into response of ion-exchanger-based electrodes sensitive to highly lipophilic physiologically active amines

The concentration profiles change of the cationic form of verapamil in the sample solution and its molecular form in the membrane in the process of sequential twofold dilution at pH = 6. Dashed lines designate the concentration of verapamil without taking into account the processes of extraction and re-extraction of its molecular form. • An insight on the nature of a verapamil-sensing electrode response is outlined. • Protolytic equilibrium is not the only factor that impairs the electrode response. • Partition of the molecular form of verapamil into the membrane should be avoided. • The partition constant can be estimated from the potentiometric data. • Dynamic diffusion model allows quantitative predicting the electrode response. The influence of electrode conditioning algorithms, as well as the composition of the internal reference solution and measurement conditions on the analytical characteristics of an electrode reversible to cations of highly hydrophobic physiologically active amines, was studied in detail for the first time using the example of a verapamil-sensitive electrode with a polyvinyl chloride membrane containing tetrakis-(4-chlorophenyl) borate as a cation exchanger and plasticized with ortho -nitrophenyloctyl ether. It has been shown that to ensure the Nernstian slope of the electrode response, a low detection limit, and prevent the potential drift, the most important condition is to exclude the extraction of the amine molecular form from both the internal and the test solution into the membrane phase during measurements. The experimental data obtained are well described in terms of the dynamic diffusion model of the boundary potential and make it possible to formulate general principles for working with electrodes sensitive to ions of highly lipophilic physiologically active amines.

Aggregates of cationic calix[4]arenes in aqueous solution as media for governing protolytic equilibrium, fluorescence, and kinetics

Many water-soluble ionic calixarenes readily form micelle-like aggregates in aqueous solution. Presently, it is firmly proved that such aggregates tend to shift the state of the acid-base equilibrium of indicator dyes just in the same manner as cationic surfactant micelles do. In this paper, we further the corresponding studies and report the peculiarities of several kinds of solvatochromic effects, fluorescence, acid-base equilibrium and kinetic processes in aqueous solutions of four cationic amphiphilic calix[4]arenes bearing hydrophilic choline groups at the upper rim: 5,11,17,23-tetra(N,N-dimethyl-N-hydroxyethylammonium)methylene-25,26,27,28-tetraalkyl-oxycalix[4]arene tetrachlorides. n-Propyl, n-hexyl, n-octyl, and n-dodecyl were used as alkyls, and the calixarenes were designated as of Cn-C4A-Ch, where n = 3, 6, 8, and 12, respectively. The principal colloidal properties, i.e., particle size and electrophoretic mobility of aggregates were characterized via dynamic light scattering. In parallel, the results were compared with those obtained in the presence of micelles of the common cationic surfactant cetyltrimethylammonium bromide and of large-sized aggregates of the calix[6]arene with six methoxy groups at the lower rim. As tools, molecular probes of triphenylmethine and fluorescein series, as well as the solvatochromic Reichardt’s dye were used. The obtained block of data sheds light upon the properties of calixarene aggregates as tools for governing chemical processes.

Thermodynamic Characteristics of the Protolytic Equilibria of Glycyl-Alanine and Glycyl-Histidine

Thermodynamic Characteristics of the Protolytic Equilibria of Glycyl-Alanine and Glycyl-Histidine

Protolytic Equilibrium of Excited States of Fluorescein

Protolytic Equilibrium of Excited States of Fluorescein

Determination of ionization constants for sequinidazole in aqueous solutions and mixtures of water with isopropanol, acetonitrile and ethanol

Seknidazole is a drug from the 5-nitroimidazole group with a number of side-effects. This drug is virtually unexplored from the point of view of chemico-toxicological analysis (CTA). Therefore, the development of a set of methods for the detection and quantification of secnidazole in biological fluids for use in CTA is an urgent task. Aim. To study the acid-base equilibrium of secnidazole in aqueous solutions and mixtures of water with amphiphilic solvents and determine the corresponding ionization constants. Materials and methods. Spectrophotometric measurements were performed using a SPEKOL®1500 single-beam spectrophotometer. An EV-74 ionomer was used to control the pH of the solutions. Seknidazole solutions in the concentrationof 1000 μg/ml were used in the experiment. Results and discussion. Absorption spectra of secnidazole in water and mixtures of water and amphiphilic solvents in the pH range studied showed four isobestic points characterizing two protolytic equilibria (equilibrium 1 – 240 and 295 nm, equilibrium 2 – 261 and 297 nm). The position of the isobestic points of equilibrium 2 fluctuated in the range ± 3 nm; it was due to the presence of tautomeric transformations for the molecular form of secnidazole. Conclusions. The acid-base equilibria of secnidazole in aqueous solutions and mixtures of water with amphiphilic solvents have been studied. It has been shown that in a strongly alkaline medium there is an anionic form of secnidazole R–, with further decrease in the pH value the molecular form HR appears, in a strongly acidic medium the final product – a protonated form H2R+ – is formed. The ionization constant 1 has been determined for aqueous solutions of secnidazole, mixtures of water with isopropanol, acetonitrile and ethanol. It is impossible to determine the equilibrium constant 2 by the method specified.

HOCH2CH2NH2 – TROPEOLIN OOO – H2O SYSTEM ACID-BASIC PROPERTIES IN THE PRESENCE OF HCl, HClO4, H2SO4 AND SO2H2O

The influence of hydrochloric, chloric, sulfuric and sulfurous acids (HCl, HClO4, H2SO4 and SO2×H2O, respectively) on protolytic equilibria in the system monoethanolamine (MEA) – tropeolin OOO (TrOOO) – water (CMEA = 0.1 M; pH = 1.0 ÷ 9.5) was studied by pH-metric, spectrophotometric and colorimetric methods. The acid-base behavior of the HOCH2CH2NH2 – TrOOO – H2O system was investigated at CTrOOO = 1,12 ×10-4 М, CMEA = 1,0×10-4 ÷ 1,0 М (pH = 8,25 ÷ 12,05), T = 293 K. TrOOO in this system exists in two tautomeric forms due to acid-base dissociation of the 4-OH group and associates formed by H-bonding with MEA molecules. There is a direct ratio between the pH values ​​of solutions and the total color difference (ΔE76), in contrast to the specific color difference (SCD). The difference in the behavior of the H2SO4 – HOCH2CH2NH2 – TrOOO – H2O system from the systems with HCl and HClO4 is due to the fact that the interaction of the first acid with MEA (8,0 £ pH) forms an ionic associate [HOCH2СH2NH3]2(SO4), which is more stable than ionic pairs [[HOCH2СH2NH3](HSO4), [HOCH2СH2NH3](ClO4), [HOCH2СH2NH3]Cl и [HOCH2СH2NH3](O3S-C­10H6-N=N-C6H4-SO3)[H3NCH2СH2OH]. Acid-base dissociation constants in systems significantly depend on the structure and physicochemical parameters of the mineral acid. In the electronic absorption spectra of the SO2 – MEA – TrOOO – H2O system (pH £ 7,5), there is a pronounced isobestic point at 415 nm, due to the dynamic equilibrium between ion-molecular forms. The coincidence of the maxima on the curves ΔE76 = f(pH) and SCD = f(pH) for the indicated system, in contrast to others studied in this work, was stated. The difference between the spectrophotometric and colorimetric behavior of the SO2 – MEA – TrOOO – H2O system from behavior of the systems with HCl, HClO4, and H2SO4 systems is due to the sulfur(IV)oxyanions with an azo indicator redox interaction.