This chapter reviews the most important features of the buffers commonly employed in capillary electrophoresis. Effect of the buffer properties such as pH, concentration, or ionic strength, on the electro-osmotic flow and on acid–base... more
This chapter reviews the most important features of the buffers commonly employed in capillary electrophoresis. Effect of the buffer properties such as pH, concentration, or ionic strength, on the electro-osmotic flow and on acid–base analytes migration is discussed. Other parameters that can affect buffer properties or analyte migration during CE separations, such as temperature, electrolysis time, or buffer-analyte interactions, are also reviewed.
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The progressive addition of cosolvent into any particular solution of an electrolyte causes a gradual change in the dissociation constant (pK) of the electrolyte. The changes in pK values of picric acid and tetrabutylammonium picrate in... more
The progressive addition of cosolvent into any particular solution of an electrolyte causes a gradual change in the dissociation constant (pK) of the electrolyte. The changes in pK values of picric acid and tetrabutylammonium picrate in isopropyl alcohol-cosolvent mixtures have been related with the dielectric constant (e) and the polarity of the mixed solvent, the latter measured by means of Dimroth-Reichardt's parameter. Theoretical equations, which show linear relationships between these magnitudes and also with the solvent composition, have been developed. A variety of cosolvents of different polarity have been used
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... Como resultado, se destacan las dificultades con que se encuentra un profesor al inicio de su carrera en cuanto al contenido de la materia que han de impartir, y en cuanto a la vertiente pedagógica de su labor. Details der... more
... Como resultado, se destacan las dificultades con que se encuentra un profesor al inicio de su carrera en cuanto al contenido de la materia que han de impartir, y en cuanto a la vertiente pedagógica de su labor. Details der Publikation. ...
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A wide set of well-known drugs, most of them included in the Abraham´s reference database, covering a wide variety of chemical structures and therapeutical functionalities were chosen in order to determine some molecular properties from... more
A wide set of well-known drugs, most of them included in the Abraham´s reference database, covering a wide variety of chemical structures and therapeutical functionalities were chosen in order to determine some molecular properties from solvent/water partition measurements. Partition data from aqueous solutions and four different solvents (n-dodecane, toluene, chloroform and n-octanol) were measured and reported. From them, Abraham´s molecular descriptors of selected compounds (A, B and S, accounting for hydrogen bond donor, hydrogen bond acceptor and dipolarity/polaritzability, respectively) were estimated. A and B values derived from the experimental measurements strongly agree with the tabulated ones showing the suitability of the used procedure to achieve reliable values for new molecules. However, obtained S values differ from those previously reported for several compounds. Moreover, values for a new indicator of the propensity to form intramolecular hydrogen bonds (Δlog Poct-tol) were estimated from the experimental data and also calculated according to both, the Abraham´s model and the molecular structures (SMD). The quality of both series of calculated descriptors was evaluated by contrast with the experimental values and satisfactory results were obtained in both instances. Thus, the Abraham´s way is useful when molecular descriptors are available but very good estimations can be achieved by SMD, which only requires the drug´s molecular structure.
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Several micellar electrokinetic chromatography (MEKC) systems (sodium dodecyl sulfate, lithium dodecyl sulfate, lithium perfluorooctanesulfonate, sodium cholate, sodium deoxycholate, tetradecyltrimethylammonium bromide and... more
Several micellar electrokinetic chromatography (MEKC) systems (sodium dodecyl sulfate, lithium dodecyl sulfate, lithium perfluorooctanesulfonate, sodium cholate, sodium deoxycholate, tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide) have been characterized by means of the solvation parameter model. It has been observed that the coefficients of the correlation equations depend strongly on the particular set of compounds analyzed. Principal component analysis has been used to
Research Interests: Engineering, Chemistry, Technology, Chromatography, Principal Component Analysis, and 11 moreMicelle, Solvation, Partition Coefficient, Electrokinetic Phenomena, Hydrogen Bond, CHEMICAL SCIENCES, Bromide, Solvent, Pulmonary Surfactant, Sodium Dodecyl Sulfate, and micellar electrokinetic chromatography
Research Interests: Engineering, Chemistry, Technology, Chromatography, Capillary electrophoresis, and 15 moreMedicine, Amines, PKA, CHEMICAL SCIENCES, Phenols, Electrophoresis, Buffers, Chemical Properties, Acids, Alkalies, Dissociation Constant, Amine, Reference standards, Mean Deviation, and capillary zone electrophoresis
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The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of sodium dodecyl sulfate and Brij 35 as surfactant. The variation in MEKC surfactant... more
The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of sodium dodecyl sulfate and Brij 35 as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute–solvent and solute–micelle interactions. Since the same solvation model can
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The autoprotolysis of binary aqueous mixtures of dimethyl sulfoxide, acetonitrile, acetone, tetrahydrofuran and 1,4-dioxan is studied. It is demonstrated that except for solvent mixtures very rich in water, the autoprotolysis is produced... more
The autoprotolysis of binary aqueous mixtures of dimethyl sulfoxide, acetonitrile, acetone, tetrahydrofuran and 1,4-dioxan is studied. It is demonstrated that except for solvent mixtures very rich in water, the autoprotolysis is produced by proton transfer from water to the dipolar organic solvent. Only in water rich mixtures the autoionization of water contributes appreciably to autoprotolysis. It is also suggested that
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The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair... more
The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK(a). The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK(a) values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.
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The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair... more
The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK(a). The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK(a) values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.
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... Acta 1993,276, 211-221. (8) Rosbs, M. Anal. Chim. Acta 1993,276, 223-234. (9) Mob, C.; ROB&, M.; Bosch, E. Anal. Chim. Acta 1993,280,75-83. ... 422. (20) Longhi, P.; Mussini,T.; Veleva, MG Anal.Quim. 1975,71,1043-(21)... more
... Acta 1993,276, 211-221. (8) Rosbs, M. Anal. Chim. Acta 1993,276, 223-234. (9) Mob, C.; ROB&, M.; Bosch, E. Anal. Chim. Acta 1993,280,75-83. ... 422. (20) Longhi, P.; Mussini,T.; Veleva, MG Anal.Quim. 1975,71,1043-(21) Georgieva, M.; Velinov, G.; Budevsky, 0. Anal. Chim. ...
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Research Interests: Engineering, Algorithms, Chemistry, Analytical Chemistry, Chromatography, and 15 moreMedicine, Potentiometry, Methanol, Experimental Study, High Performance Liquid Chromatography, CHEMICAL SCIENCES, Ionization, Buffers, Column chromatography, Acids, Acetonitrile, Solvents, Selectivity, prediction equation, and Elution
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The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate as surfactant. The variation in... more
The solvation parameter model has been applied to the characterization of micellar electrokinetic chromatographic (MEKC) systems with mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate as surfactant. The variation in MEKC surfactant composition results in changes in the coefficients of the correlation equation, which in turns leads to information on solute-solvent and solute-micelle interactions. Lithium perfluorooctanesulfonate is more dipolar and hydrogen bond acidic but less polarizable and hydrogen bond basic than lithium dodecyl sulfate. Therefore mixtures of lithium dodecyl sulfate and lithium perfluorooctanesulfonate cover a very wide range of polarity and hydrogen bond properties, which in turn results in important selectivity changes for analytes with different solute properties.
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A wide study of the compounds and procedures mostly used to determine the electroosmotic flow (EOF) and micelle elution times has been done in seven different micellar electrokinetic chromatography (MEKC) systems. These systems are formed... more
A wide study of the compounds and procedures mostly used to determine the electroosmotic flow (EOF) and micelle elution times has been done in seven different micellar electrokinetic chromatography (MEKC) systems. These systems are formed from mixtures of an aqueous buffer with the surfactants sodium dodecyl sulfate, lithium dodecyl sulfate, lithium perfluorooctane sulfonate, sodium cholate, sodium deoxycholate, tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide. The solvation parameter model has been used to evaluate the usefulness of the compounds studied as EOF or micellar markers in each of the seven MEKC systems. It is demonstrated that methanol, acetonitrile and formamide are the best EOF markers, and that dodecanophenone is the best micellar marker.
Research Interests: Chemical Engineering, Chemistry, Analytical Chemistry, Chromatography, Micelle, and 15 moreMethanol, Electrokinetic Phenomena, Electrophoresis, Bromide, Diluent, Micellar Solutions, Organic Chemicals, Pulmonary Surfactant, Acetonitrile, Biochemistry and cell biology, Molecular probes, Reference standards, formamide, micellar electrokinetic chromatography, and electroosmotic flow
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The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair... more
The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK(a). The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK(a) values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.
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ABSTRACT
Research Interests: Chemistry, Organic Chemistry, Solvation, Methanol, Ethanol, and 4 moreSolvatochromism, Benzene, N Hexane, and Solvent
The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair... more
The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK(a). The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK(a) values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.
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The development of methods to increase the bioavailability of drugs is of great interest, especially for those which are poorly soluble or permeable. One of the strategies to enhance the solubility (which in turn has the potential of... more
The development of methods to increase the bioavailability of drugs is of great interest, especially for those which are poorly soluble or permeable. One of the strategies to enhance the solubility (which in turn has the potential of increase bioavailability) of drugs is the use of additives in the formulation process, so that the drug can stay supersaturated in biological fluids for a period of time long enough to allow absorption. The use of polymers as pharmaceutical excipients in order to stabilize the supersaturation of drugs is common practice. In this work, the ability of different polymers of vinylpyrrolidone (K-12, K-17, K-25, K-29/32, K-90) and a copolymer of vinylpyrrolidone and vinylacetate (S-630) have been tested for their impact on the supersaturation of drugs. Sixteen drugs of different chemical nature have been selected, and analyzed using the Cheqsol method. The results of the drug alone, and of physical mixtures with the different polymers at several polymer:drug ...
Research Interests: Chemistry, Kinetics, Powder Diffraction, Drug development, Medicine, and 9 moreMacromolecular X-Ray Crystallography, Dissolution, Solubility, Pharmaceutical Formulation Technology, Excipients, Drug Compounding, Hydrogen-Ion Concentration, Pharmaceutical preparations, and Pharmacology and pharmaceutical sciences
Common drugs intended for action in plasma (antibacterials, antiallergics, diuretics...) often display both acidic and basic behavior, and some of these amphoteric compounds can appear as zwitterions. In such cases, accurate profiling of... more
Common drugs intended for action in plasma (antibacterials, antiallergics, diuretics...) often display both acidic and basic behavior, and some of these amphoteric compounds can appear as zwitterions. In such cases, accurate profiling of lipophilicity vs. pH, which plays a fundamental role in drug pharmacokinetics, might be complex. In the present work two common lipophilicity determination methods based on the drug distribution between 1-octanol and aqueous buffer i.e. phase equilibration (shake-flask) and two-phase titration (potentiometry), were compared with a high-throughput lipophilicity index, the Chromatographic Hydrophobicity Index (CHI). The results were also compared with log Do/w pH-profiles calculated by different algorithms from ACD/Labs. Accurate and similar results were obtained for both octanol-water approaches but, due to the lower determination times and the absence of different ion-pairing buffers, potentiometry was shown to be the most convenient method. CHI vs....
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Nowadays, pharmaceutical research needs to deal with sparingly soluble compounds and requires high-throughput technology to rapidly and accurately determine the p K a values of drug candidates. In this chapter, on the one hand, the... more
Nowadays, pharmaceutical research needs to deal with sparingly soluble compounds and requires high-throughput technology to rapidly and accurately determine the p K a values of drug candidates. In this chapter, on the one hand, the challenge of p K a determination in hydroorganic solvents and the most widely used approaches to obtain the aqueous extrapolated value are presented. On the other hand, high-throughput methods based on potentiometry, spectrophotometry, and capillary electrophoresis are discussed and compared.
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In this article, we describe a recently developed capillary-electrophoresis method for the determination of acidity constants and compare it with other existing methods. The new method is based on the use of an internal standard (compound... more
In this article, we describe a recently developed capillary-electrophoresis method for the determination of acidity constants and compare it with other existing methods. The new method is based on the use of an internal standard (compound similar in nature and pKa value to the analyte), and offers several benefits, since it has all the advantages of capillary electrophoresis. In addition, it is very fast, because the exact measure of the pH of the separation electrolytes is not needed, and only a few electrophoretic runs are required to perform a pKa determination. The acidity constants of some monoprotic weak acids and bases were determined by this fast method, yielding a very good agreement with literature values.
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A fast and accurate lipophilicity determination is fundamental in the drug discovery process, as long as it is a relevant property in the absorption, distribution, metabolism, excretion and toxicity (ADMET) of a potential drug substance.... more
A fast and accurate lipophilicity determination is fundamental in the drug discovery process, as long as it is a relevant property in the absorption, distribution, metabolism, excretion and toxicity (ADMET) of a potential drug substance. In the present work, different models based on chromatographic retention values for a large set of compounds and some of their molecular descriptors (calculated by ACD/Labs or CODESSA programs) have been examined in order to establish reliable equations for logPo/w determination from fast chromatographic hydrophobicity index (CHI) measurements. This appears to be a very interesting high-throughput methodology for screening purposes, since CHI values can be measured by UHPLC in very short runs (<4min) and molecular descriptors can be easily computed from the structure of any compound. The selected final descriptors were Abraham's hydrogen-bond acidity (A) and excess molar refraction (E) from ACD/Labs, and hydrogen-bond acidity HDCA-1/TMSA and ...
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This chapter reviews the most used classical methods, potentiometry and spectrophotometry, for acidity constant determination. For this purpose, the theoretical and operational definitions of pH and the equations to calculate acidity... more
This chapter reviews the most used classical methods, potentiometry and spectrophotometry, for acidity constant determination. For this purpose, the theoretical and operational definitions of pH and the equations to calculate acidity constants from potentiometric and spectrophotometric data are explained. In addition, capillary electrophoresis is presented as a very convenient technique for p K a determination, especially in the case of low-purity samples.
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In the present study three different procedures have been compared for the determination of the lipophilicity of the unionized species (log P) of neutral, acidic, basic, amphoteric, and zwitterionic drugs. Shake-flask, potentiometric and... more
In the present study three different procedures have been compared for the determination of the lipophilicity of the unionized species (log P) of neutral, acidic, basic, amphoteric, and zwitterionic drugs. Shake-flask, potentiometric and chromatographic approaches have been assayed in a set of 66 representative compounds in different phases of advanced development. An excellent equivalence has been found between log P values obtained by shake-flask and potentiometry, while the chromatographic approach is less accurate but very convenient for screening purposes when a high-throughput is required. In the case of zwitterionic and amphoteric compounds, either for shake-flask and chromatographic methods, the pH has to be accurately selected in order to ensure the compound to be in its neutral form.
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The most commonly used mobile phases in reversed-phase high-performance liquid chromatography (RP-HPLC) are hydro-organic mixtures of an aqueous buffer and an organic modifier. The addition of this organic solvent to buffered aqueous... more
The most commonly used mobile phases in reversed-phase high-performance liquid chromatography (RP-HPLC) are hydro-organic mixtures of an aqueous buffer and an organic modifier. The addition of this organic solvent to buffered aqueous solutions involves a variation of the buffer properties (pH and buffer capacity). In this paper, the pH variation is studied for acetic acid-acetate, phosphoric acid-dihydrogenphosphate-hydrogenphosphate, citric acid-dihydrogencitrate-citrate, and ammonium-ammonia buffers. The proposed equations allow pH estimation of acetonitrile-water buffered mobile phases up to 60% (v/v) of organic modifier and initial aqueous buffer concentrations between 0.001 and 0.1 mol L(-1), from the initial aqueous pH. The estimated pH variation of the mobile phase and the pKa variation of the analytes allow us to predict the degree of ionisation of the analytes and from this and analyte hydrophobicities, to interpret the relative retention and separation of analyte mixtures.
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ABSTRACT Potassium hydrogen phthalate (KHP) is a primary titrimetric standard, and KHP solutions are also used as standards for pH reference values, in pure water or in hydrorganic solvent mixtures. However, the pH of these solutions can... more
ABSTRACT Potassium hydrogen phthalate (KHP) is a primary titrimetric standard, and KHP solutions are also used as standards for pH reference values, in pure water or in hydrorganic solvent mixtures. However, the pH of these solutions can be affected by any shift in the pKa1 or pKa2 of the substance due to changes in the solvent composition or temperature. In this work, the two dissociation constants of o-phthalic acid were independently determined for different compositions of acetonitrile/water mixtures within the range from 0 % to 70 % by weight, in steps of 10 %, and at different temperatures from (15 to 50) °C, in steps of 5 °C. The dependence of ln Ka on the inverse of temperature has been examined by using statistical methods, and the corresponding ionization thermodynamic functions have been calculated for different acetonitrile compositions.
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Interaction thermodynamics between warfarin, a very popular anticoagulant, and Sudlow I binding site of human (HSA) or bovine (BSA) serum albumin have been examined in strictly controlled experimental conditions (HEPES buffer 50 mM, pH... more
Interaction thermodynamics between warfarin, a very popular anticoagulant, and Sudlow I binding site of human (HSA) or bovine (BSA) serum albumin have been examined in strictly controlled experimental conditions (HEPES buffer 50 mM, pH 7.4 and 25 °C) by means of isothermal titration calorimetry (ITC), fluorescence spectrometry (FS) and frontal analysis capillary electrophoresis (FA/CE). Each technique is based on measurements of a different property of the biochemical system, and then the results allow a critical discussion about the suitability of each approach to estimate the drug-protein binding parameters. The strongest interaction step is properly evaluated by the three assayed approaches being the derived binding constants strongly consistent: from 4 × 104 to 7 × 104 for HSA and from 0.8 × 105 to 1.2 × 105 for BSA. Binding enthalpy variations also show consistent results: -5.4 and -5.6 Kcal/mol for HSA and -4.3 and -3.7 Kcal/mol for BSA, as measured by ITC and FS, respectively...