Capillary zone electrophoresis of basic drugs in non-aqueous acetonitrile with buffers based on a conventional pH scale

Summary The pK _a ^* values of 10 nitrogen-containing basic drugs in non-aqueous acetonitrile were determined from the pH^* dependence of their electrophoretic mobilities. The pH^* scale in the organic solvent was established using background electrolytes with known conventional pK _a ^* values, making further calibration with reference pH electrodes unnecessary. In acetonitrile the pK _a ^* values of analytes (or their conjugated cation acids, BH⁺, respectively) were 5.2±8.9 pK units>those in water. The observed change in pK _a ^* values of cationic analytes was, however, much less than the known respective change for neutral acids type HA. From the pK _a ^* values and the actual mobilities, it is possible to predict pH^* conditions to enable separation of analytes, and this was demonstrated for two pairs of common drugs.     

Determination of acidity constants of pyridines,imidazoles, and oximes by capillary electrophoresis

The acidity constant in the form of pK_a is one of the most important physicochemical quantities. There are prediction tools available for calculating the pK_a, but they only deliver precise calculated values for a relatively small set of chemicals. For complex structures with multiple functional groups in particular, the error in the predicted pK_a is high due to the application domain of the corresponding models. Thus, we aim to enlarge the dataset of experimentally determined pK_a values using capillary electrophoresis. We, therefore, selected various pyridines, imidazoles, and oximes to determine the pK_a values using the internal standard approach and the classical method. Especially oximes were not investigated in the past, and predictions for them include larger errors. Thus, our experimentally determined values could contribute to an improved understanding of various functional groups impacting the pK_a values and serve as additional datasets to develop improved pK_a prediction tools.     

Determination of pKa values of some novel benzimidazole salts by using a new approach with 1H NMR spectroscopy

Benzimidazoles and their derivatives including imidazole are studied widely because they exist in the structure of natural products and different drugs. pK_a values are extremely important for drug discovery and improvement in order to determine pharmacokinetic and pharmacodynamic features such as permeation through biological barriers, interactions with the target area or side effects. Acid–base features (pK_a) have great importance not only for physiological characteristics but also for being used as a ligand or changing physico‐chemical features by turning benzimidazoles into salts. Within the scope of this study, a variety of new benzimidazole salts were synthesized, and their characterizations were made by NMR spectroscopy, FTIR spectroscopy and element analysis techniques. The pK_a values of synthesized benzimidazole salts were determined by inflection point approach using integration values obtained with ¹H NMR spectroscopy and Henderson–Hasselbalch analysis. pK_a values of some benzimidazole salts were also determined by potentiometric methods in order to compare those of NMR spectroscopy results. Copyright © 2015 John Wiley & Sons, Ltd.     

Basicity of some aliphatic amines in mixed H2O?CH3CN solvents

The values of pK_a^ms (K_a^ms represents ionization constant of conjugate acid of amine base in mixed water–acetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1–0.4 pK units) with the increase in CH₃CN content from 2 to ∼60% v/v. However, the pK_a^ms values at 70% v/v CH₃CN become nearly equal or slightly larger (by ≤0.7 pK units) than the corresponding pK_a^ms at 2% v/v CH₃CN for all neutral and charged amines. The values of pK_a^ms for phenol increase from 10.17 to 13.38 with the increase in the content of CH₃CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, ρ*, obtained from the plots of pK_a^ms against ∑σ* for primary and secondary amines decrease by ca. 0.8 ρ* units with the increase in the CH₃CN content from 2 to 70% v/v. The values of pK_a^ms show an empirical linear relationship with the corresponding values of pK_a^w (where pK_a^w represents the pK_a obtained in aqueous solvent containing 2% v/v CH₃CN), which allows the estimation of a pK_a in mixed H₂O CH₃CN solvents from that in water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 146–152, 2000     

Determination of pKa values using 15N and 13C nuclear magnetic resonance spectroscopy. The case of apramycin

By comparison of pK^a values derived from ¹⁵N and ¹³C nuclear magnetic resonance (NMR) spectroscopies, the assignment of the ¹⁵N resonances of apramycin is completed. ¹³C NMR spectra appear to provide accurate pK_a determinations with this aminoglycoside. Hydroxylation adjacent to one of the basic nitrogens of apramycin appears to change the pK_a values of all five amines of the molecule.     

Thermodynamic Estimate of pKa Values of the Carboxylic Acids in Aqueous Solution with the Density Functional Theory

The 96 pK_a values of 85 carboxylic acids in aqueous solution were calculated with the density functional theory method at the level of B3LYP/6‐31+G(d,p) and the polarizable continuum model (PCM) was used to describe the solvent. In the calculations of pK_a values, the dissociation Gibbs free energies were directly calculated using carboxylic acid dissociation reactions in aqueous solution, i. e., no thermodynamic cycle was employed, which is different from the previous literatures. A highly significant correlation of R²=0.95 with a standard deviation (SD) of 0.36 between the experimental pK_a values and the calculated dissociation Gibbs free energies [ΔG(calc.)] was found. The slope of pK_a vs. (G(calc.)/(20303RT) is only 47.6% of the theoretically expected value, which implies that the ΔG(calc.) value from the theoretical calculation is larger than the actual one for all 85 carboxylic acids studied. Thus, by adding the 0.476 scaling‐factor into the slope, we can derive a reliably procedure that can reproduce the experimental pK_a values of carboxylic acids. The pK_a values furnished by this procedure are in good agreement with the experimental results for carboxylic acids in aqueous solution.     

Determination of thermodynamic pKa values of pharmaceuticals from five different groups using capillary electrophoresis

A determination of the thermodynamic acid dissociation constants (pK_a) of 22 frequently used pharmaceuticals using capillary electrophoresis in aqueous media is presented in this work. The investigated pharmaceuticals belong to different pharmacological groups: macrolides, fluoroquinolones, sulfonamides, β‐lactams, tetracyclines, and other miscellaneous pharmaceuticals. The electrophoretic mobilities of the investigated analytes were monitored in a pH range from 2.00 to 10.82. The data were fitted with an appropriate mathematical model using a nonlinear regression analysis to obtain pK_a values. Experimentally obtained data were well described by the mathematical model chosen for each analyte that was confirmed by r² values higher than 0.99 for most of the investigated analytes. Extrapolations to zero ionic strength were used to determine the thermodynamic pK_a values. Experimentally obtained acid dissociation constants were interpreted using structural formulae of investigated analytes and the moieties corresponding to specific pK_a were identified.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

Quantum-chemical study on the relation between thermodynamic parameters in the iodination of benzene with N-I and O-I reagents and p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of the corresponding NH and oh acids

Thermodynamic parameters of direct iodination of benzene with several iodinating agents were calculated using semiempirical (PM3), ab initio (3–21G**), and DFT (B3LYP/LanL2DZ) methods, as well as in terms of the polarization continuum model (PCM or Tomasi model). A close to linear correlation was found between the calculated thermodynamic parameters (ΔH ^≠, ΔG ^≠) and pK _T and experimental pK _a values of acids whose anions are incorporated into iodine-containing intermediates.     

Benchmarking and validating algorithms that estimate p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of drugs based on their molecular structures

The REGDIA regression diagnostics algorithm in S-Plus is introduced in order to examine the accuracy of pK _a predictions made with four updated programs: PALLAS, MARVIN, ACD/pKa and SPARC. This report reviews the current status of computational tools for predicting the pK _a values of organic drug-like compounds. Outlier predicted pK _a values correspond to molecules that are poorly characterized by the pK _a prediction program concerned. The statistical detection of outliers can fail because of masking and swamping effects. The Williams graph was selected to give the most reliable detection of outliers. Six statistical characteristics (F _exp, R ², , MEP, AIC, and s(e) in pK _a units) of the results obtained when four selected pK _a prediction algorithms were applied to three datasets were examined. The highest values of F _exp, R ², , the lowest values of MEP and s(e), and the most negative AIC were found using the ACD/pK _a algorithm for pK _a prediction, so this algorithm achieves the best predictive power and the most accurate results. The proposed accuracy test performed by the REGDIA program can also be applied to test the accuracy of other predicted values, such as log P, log D, aqueous solubility or certain physicochemical properties of drug molecules.     

Determination of acid dissociation constants of flavin analogues by capillary zone electrophoresis

Acid dissociation constants (pK_a) of nine kinds of flavin analogues as molecular catalyst candidates were determined by CZE. Although some of the analogues are instable and degradable under the light exposure or in alkaline aqueous solutions, the effective electrophoretic mobility of the flavin analogue of interest has been measured with the residual substance. The pK_a values of the flavin analogues were analyzed through the changes in the effective electrophoretic mobility with varying pH of the separation buffer. One or two steps pK_a values were determined by the analysis. One of the degraded species from the flavin analogues, lumichrome, was also detected in the CZE analysis, and its pK_a values were also determined. While coexisting impurities generated over the storage conditions were found in some analogues, the pK_a values of the target analogues were successfully determined with the help of the CZE separations. A pressure-assisted CZE was utilized for the determination or the estimation of the pK_a values of such analogues as possessing carboxylic acid moiety.     

Theoretical Study of Firefly Luciferin pKa Values—Relative Absorption Intensity in Aqueous Solutions

Ground‐state vibrational analyses of firefly luciferin and its conjugate acids and bases are performed. The Gibbs free energies obtained from these analyses are used to estimate pK_a values for phenolic hydroxy and carboxy groups and the N–H⁺ bond in the N‐protonated thiazoline or benzothiazole ring of firefly luciferin. The theoretical pK_a values are corrected using the experimental values. The concentrations of these chemical species in solutions with different pH values are estimated from their corrected pK_a values, and the pH dependence of their relative absorption intensities is elucidated. With the results obtained we assign the experimental spectra unequivocally. Especially, the small peak near 400 nm at pH 1–2 in experimental absorption spectra is clarified to be due to the excitation of carboxylate anion with N‐protonated thiazoline ring of firefly luciferin. Our results show that the pK_a values of chemical species, which are contained in the aqueous solutions, are effective to assign experimental absorption spectra.     

pKa prediction for acidic phosphorus‐containing compounds using multiple linear regression with computational descriptors

Ninety‐six acidic phosphorus‐containing molecules with pK_a 1.88 to 6.26 were collected and divided into training and test sets by random sampling. Structural parameters were obtained by density functional theory calculation of the molecules. The relationship between the experimental pK_a values and structural parameters was obtained by multiple linear regression fitting for the training set, and tested with the test set; the R² values were 0.974 and 0.966 for the training and test sets, respectively. This regression equation, which quantitatively describes the influence of structural parameters on pK_a, and can be used to predict pK_a values of similar structures, is significant for the design of new acidic phosphorus‐containing extractants. © 2016 Wiley Periodicals, Inc.     

Polynuclear Branched Tetrazole Systems. 2. New 2-(5-Tetrazolyl)ethylpodands and Their NH-Acidity

Nine new polynuclear 2-(5-tetrazolyl)ethyl podands have been obtained by the azidation of the corresponding nitriles. Using Bjerrum distribution functions, the values of pK _a ¹, pK _a ², pK _a ³, and pK _a ⁴ have been determined by a potentiometric method for 14 polynuclear tetrazoles in aqueous and aqueous methanolic solution. The found values lie in the range from 3.5 to 7.5 pH units. The overall rules and the sequence of the ionization of the spatially separated tetrazole fragments in these podand systems are discussed.     

A reliable and efficient first principles‐based method for predicting pKa values. 4. organic bases

The ionization (dissociation) constant (pK_a) is one of the most important properties of a drug molecule. It is reported that almost 68% of ionized drugs are weak bases. To be able to predict accurately the pK_a value(s) for a drug candidate is very important, especially in the early stages of drug discovery, as calculations are much cheaper than determining pK_a values experimentally. In this study, we derive two linear fitting equations (pK_a = a × ΔE + b; where a and b are constants and ΔE is the energy difference between the cationic and neutral forms, i.e., ΔE = E_neutral?E_cationic) for predicting pK_as for organic bases in aqueous solution based on a training/test set of almost 500 compounds using our previously developed protocol (OLYP/6‐311+G**//3‐21G(d) with the the conductor‐like screening model solvation model, water as solvent; see Zhang, Baker, Pulay, J. Phys. Chem. A 2010 , 114, 432). One equation is for saturated bases such as aliphatic and cyclic amines, anilines, guanidines, imines, and amidines; the other is for unsaturated bases such as heterocyclic aromatic bases and their derivatives. The mean absolute deviations for saturated and unsaturated bases were 0.45 and 0.52 pK_a units, respectively. Over 60% and 86% of the computed pK_a values lie within ±0.5 and ±1.0 pK_a units, respectively, of the corresponding experimental values. The results further demonstrate that our protocol is reliable and can accurately predict pK_a values for organic bases. © 2012 Wiley Periodicals, Inc.     

Rapid determination of p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of 20 amino acids by CZE with UV and capacitively coupled contactless conductivity detections

A rapid and universal capillary zone electrophoresis (CZE) method was developed to determine the dissociation constants (pK _a) of the 20 standard proteogenic amino acids. Since some amino acids are poorly detected by UV, capacitively coupled contactless conductivity detection (C⁴D) was used as an additional detection mode. The C⁴D coupling proved to be very successful on a conventional CE-UV instrument, neither inducing supplementary analyses nor instrument modification. In order to reduce the analysis time for pK _a determination, two strategies were applied: (i) a short-end injection to reduce the effective length, and (ii) a dynamic coating procedure to generate a large electroosmotic flow (EOF), even at pH values as low as 1.5. As a result, the analysis time per amino acid was less than 2 h, using 22 optimized buffers covering a pH range from 1.5 to 12.0 at a constant ionic strength of 50 mM. pK _a values were calculated using an appropriate mathematical model describing the relationship between effective mobility and pH. The obtained pK _a values were in accordance with the literature. Figure a UV (1) and C⁴D (2) detectors placed on-line on the CE capillary. b Curve of effective mobility as a function of pH for histidine     

Enantioselective Synthesis and Physicochemical Properties of Libraries of 3‐Amino‐ and 3‐Amidofluoropiperidines

The enantioselective syntheses of 3‐amino‐5‐fluoropiperidines and 3‐amino‐5,5‐difluoropiperidines were developed using the ring enlargement of prolinols to access libraries of 3‐amino‐ and 3‐amidofluoropiperidines. The study of the physicochemical properties revealed that fluorine atom(s) decrease(s) the pK_a and modulate(s) the lipophilicity of 3‐aminopiperidines. The relative stereochemistry of the fluorine atoms with the amino groups at C3 on the piperidine core has a small effect on the pK_a due to conformationnal modifications induced by fluorine atom(s). In the protonated forms, the C?F bond is in an axial position due to a dipole–dipole interaction between the N?H⁺ and C?F bonds. Predictions of the physicochemical properties using common software appeared to be limited to determine correct values of pK_a and/or differences of pK_a between cis‐ and trans‐3‐amino‐5‐fluoropiperidines.     

Microscopic Protonation/Deprotonation Equilibria of the Anti-Inflammatory Agent Piroxicam

The microscopic ionization behavior of piroxicam was investigated using two different approaches, i.e., direct UV spectroscopy and an indirect analogue approach (deductive method). The best microscopic pK_a values (pK_a12 = 4.60, pK_a21 = 5.40, pK_a22 = 2.72, and pK_a11 = 1.92) were obtained by the deductive method using as pK_a22 the pK_a of the enolic O-methylated piroxicam 2 . The results show remarkable electrostatic effects in the protonation/deprotonation equilibria, a marked increase in the acidity of the enolic function (2.68 pK_a units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on ¹H-NMR chemical shifts at various ionization states, indicating an extended electron conjugation through the molecule. The partition measurements in octan-1-ol/H₂O of zwitterionic compound 3 (the pyridyl N-methyl derivative of piroxicam ( 1 )) suggest that the two opposite charges in zwitterionic piroxicam are indeed in a close intramolecular proximity.     

CE determination of the thermodynamic pKa values and limiting ionic mobilities of 14 low molecular mass UV absorbing ampholytes for accurate characterization of the pH gradient in carrier ampholytes-based IEF and its numeric simulation

Fourteen low molecular mass UV absorbing ampholytes containing 1 or 2 weakly acidic and 1 or 2 weakly basic functional groups that best satisfy Rilbe's requirement for being good carrier ampholytes (ΔpK_a = pKa_monoanion ‒ pKa_monocation < 2) were selected from a large group of commercially readily available ampholytes in a computational study using two software packages (ChemSketch and SPARC). Their electrophoretic mobilities were measured in 10 mM ionic strength BGEs covering the 2 < pH < 12 range. Using our Debye-Hückel and Onsager-Fuoss laws-based new software, AnglerFish (freeware, https://echmet.natur.cuni.cz/software/download ), the effective mobilities were recalculated to zero ionic strength from which the thermodynamic pK_a values and limiting ionic mobilities of the ampholytes were directly calculated by Henderson-Hasselbalch equation-type nonlinear regression. The tabulated thermodynamic pK_a values and limiting ionic mobilities of these ampholytes (pI markers) facilitate both the overall and the narrow-segment characterization of the pH gradients obtained in IEF in order to mitigate the errors of analyte ampholyte pI assignments caused by the usual (but rarely proven) assumption of pH gradient linearity. These thermodynamic pK_a and limiting mobility values also enable the reality-based numeric simulation of the IEF process using, for example, Simul (freeware, https://echmet.natur.cuni.cz/software/download ).