Kinetics and mechanisms of the reactions of S-methyl chlorothioformate with pyridines and secondary alicyclic amines

The pyridinolysis of S-methyl chlorothioformate shows a biphasic Brønsted-type plot, in agreement with a stepwise mechanism and a change in the rate-limiting step, from formation of a zwitterionic tetrahedral intermediate (T^±) at high pK_a to its breakdown at low pK_a. The reaction of the same substrate with secondary alicyclic amines shows a linear Brønsted-type plot of slope 0.23, which is in accordance with a stepwise mechanism where formation of T^± is the rate-determining step for the whole pK_a range examined.     

Large volume sample stacking in capillary electrophoresis of weakly acidic compounds using coated capillaries at high pH

Large volume stacking using the electroosmotic flow (EOF) pump (LVSEP) in capillary electrophoresis under a reverse potential is a convenient and straightforward approach for on-line concentration of dilute anionic sample solutions. LVSEP achieves automatic sample matrix removal and subsequent separation without intermediate polarity switching nor complicated instrumental setup. Since anionic analytes should move against the EOF in LVSEP, EOF needs to be suppressed. We extended the range of LVSEP up to pH 11 using various EOF suppression methods, such as dynamic coating by polymer pretreatment and permanent coating. Weakly acidic organic compounds (pK_a<5.2), chlorinated phenols (pK_a=7-9), and aromatic amino acids (pK_a2∼9.3) were concentrated and separated. By hydrodynamically filling the whole capillary of 27 cm long with the sample solution, fast and reliable injection was achieved and sensitivity enhancement factors as large as 170 were readily obtained in less than 8 min.     

Reactivity of imidazolidin-4-one derivatives of primaquine: implications for prodrug design

In contrast to peptide-based imidazolidin-4-ones, those synthesized from N-(α-aminoacyl) derivatives of the antimalarial drug, primaquine and ketones are unexpectedly stable in pH 7.4 at 37 °C. The kinetics of hydrolysis of primaquine-based imidazolidin-4-ones were investigated in the pH range 0.3-13.5 at 60 °C. The hydrolysis to the parent α-aminoacylprimaquine is characterized by sigmoidal-shaped pH-rate profiles, reflecting the spontaneous decomposition of both unionized and protonated (at N-1) forms of the imidazolidin-4-one. The kinetically determined pK_a values are ca. 3.6-4.0, i.e., 4 pK_a units lower than those of amino acid amides, thus implying that hydrolysis of imidazolidin-4-ones at pH 7.4 involves the unionized form. Reactivity of this form decreases with the steric crowding of the amino acid α-substituent. In contrast, the rate constant for the spontaneous decomposition of the unionized form increases sharply for imidazolidin-4-ones derived from cyclic ketones, an observation that can be explained by the I-strain (internal strain) effect. These results are consistent with a mechanism of hydrolysis involving an S_N1-type unimolecular cleavage of the imidazolidin-4-one C2-N3 bond with departure of an amide-leaving group. The mechanism for the decomposition of the protonated imidazolidin-4-one is likely to involve an amide-carbonyl oxygen protonated species, followed by the C2-N3 bond scission, as supported by computational studies. The results herein presented suggest that imidazolidin-4-ones derived from simple N-alkyl α-aminoamides are too stable and therefore, may be useful as slow drug release prodrugs.     

Charge equilibria and pK values of 5-carboxypyranomalvidin-3-glucoside (vitisin A) by electrophoresis and absorption spectroscopy

Apparent pK values of the wine pigment, 5-carboxypyranomalvidin-3-glucoside (vitisin A), were determined using UV-vis spectroscopy, viz. pK_a1=0.98 (±0.10), pK_H1=4.51 (±0.03) and pK_H2=7.57 (±0.02). An additional ionisation constant at high pH (pK_a4=8.84±0.06) was established by high-voltage paper electrophoresis. These data in conjunction with previously published pK_a values determined by high-voltage electrophoresis suggest that in wine (pH 3.2-3.8), 5-carboxypyranomalvidin-3-glucoside exists as a complex mixture of hydrated and non-hydrated, partially ionised species with the predominant species being the quinonoidal base (λ_max 498 nm).     

Acid-base properties of N-methyl-substituted 1,2-dihydroquinolines in the ground and excited states

Changes in the absorption and fluorescence spectra of 1,2,2,3-tetramethyl-(1), 1,2,2,4-tetramethyl-(2), 6-ethoxy-1,2,2,4-tetramethyl-(3), and 1,2,6-trimethyl-1,2-dihydroquinolines (4) were studied in aqueous solution over a wide pH range from 1.0 to 12.0. The quantum yields of fluorescence and the values of pK _a of dihydroquinolines (DHQs) under study in the ground and excited states were determined, pK _a = 4.5, 3.8, 4.5, and 4.2 for the ground state of compounds 1–4, respectively, and pK _a ∼ 1.7 for the S ₁^* state for all DHQs.     

Development of negligible depletion hollow fiber membrane-protected liquid-phase microextraction for simultaneous determination of partitioning coefficients and acid dissociation constants

A new method based on negligible depletion hollow fiber-protected liquid-phase microextraction coupled with high-performance liquid chromatography (HPLC) was developed for the simultaneous determination of partitioning coefficients (K_OW) and acid dissociation constants (pK_a), by using phenol, 4-chlorophenol and 2,4-dichlorophenol as model compounds. A 37-mm length polypropylene hollow fiber membranes (600 μm inner diameter, 200 μm wall-thickness, 0.2 μm pore size, ∼70% porosity) with two-end sealed were filled with 1-octanol by ultrasonic agitation to prepare the extraction device. The extraction device was deployed in sample solutions, prepared by spiking target analytes in 1-octanol saturated aqueous solutions (500 mL), for negligible depletion extraction. After equilibrium was reached (∼5 h), the 1-octanol in the lumen of the hollow fiber membrane was collected for HPLC determination of the target analytes. As the depletion of the analytes in aqueous samples was negligible, the distribution coefficient (D_OW) could be calculated based on the measured equilibrium concentration in 1-octanol (C_O) and the initial concentration (C_W) in the aqueous sample of the target analyte (D_OW = C_O/C_W). The D_OW values measured at various pH values were nonlinearly regressed with pH to obtain the K_OW and pK_a values of a compound. Results showed that the measured values of the K_OW and pK_a of these model compounds agreed well with literature data.     

Reactions of transition metal carbonyl anions with 2-(1-bromoalkylidene)thiazolidin-4-ones: halogenophilic attack or deprotonation

Bromophilic attack by the transition metal carbonyl anion, [Re(CO)₅]Na (pK_a = 21.1), on 2-(1-bromoalkylidene)thiazolidin-4-ones is significantly faster than abstraction of an acidic lactam hydrogen (pK_a ∼17-18), when the generated carbanion is stabilized by an α-CN or α-PhCO group. The bromophilic reaction of 2-(1-bromoalkylidene)thiazolidin-4-one, having an α-CN electron-withdrawing group, resulted in formation of a new metallacyclic anionic complex. With less reactive vinyl bromides, containing an α-CONHPh or α-CO₂Et group, only deprotonation is observed. The role of the metal carbonyl anion is highlighted by a comparison with the 9-methylfluorenide carbanion (pK_a of 9-methylfluorene is 22.3), which reacts exclusively via a deprotonation pathway.     

1H NMR spectroscopic identification of protonable sites in cryptolepines with C‐11 substituents containing two amino functionalities

Knowledge of protonable sites and acid dissociation constants of cryptolepine derivatives having C‐11 substituents containing two amino functionalities is of great importance to the understanding of the mechanism of their antimalarial action, which may contribute to their further development as drug candidates. In this work, we applied ¹H NMR titration to investigate the acid–base characteristics of these polyprotic compounds in the pH range 3–13. We identified three acid–base equilibria with most acid dissociation constants (pK_a*) being greater than 10.5, which prevented us from using the potentiometric method. Overall, ¹H NMR titration was sensitive and suitable for the determination of pK_a values for these drug leads. Copyright © 2012 John Wiley & Sons, Ltd.     

Redox behavior of juglone in buffered aq.: Ethanol media

The present work reports the redox mechanism of 5-hydroxynaphthalene-1,4-dione (HND), commonly known as juglone, in buffered aqueous media having 50% of ethanol. HND followed different mechanistic routes depending upon the pH of the media and more than one pK_a were evaluated from the changes in the slope of the E_p vs. pH plot. The change of pH from acidic to neutral conditions was found to switch the mechanism from CEC to EE mechanism. Pulse techniques were utilized to determine the number of electrons involved in the oxidation and/or the reduction step and to ensure the nature of the redox process. Based upon the obtained results, an electrode reaction mechanism was proposed. Computational studies of HND supported the experimental results. UV-Visible spectroscopy was also employed for the detailed characterization of the compound in a wide range of pH and for the determination of its pK_a.     

Equilibria of formation and dehydration of the carbinolamine intermediate in the reaction of benzaldehyde with hydrazine

Measurement of polarographic limiting currents at equilibria made it possible at pH 3-7 to simultaneously determine concentrations of benzaldehyde, of its hydrazone and of the carbinolamine derivative. The dependence of concentration of carbinolamine at equilibrium on pH indicated presence of its di-, mono-, and unprotonated forms. Acid dissociation constants of the formation (pK_a¹≈3.2) of the diprotonated form and of the dissociation of the monoprotonated form of carbinolamine (pK_a²≈4.7) were estimated. The equilibrium constants of formation (K₁) and dehydration (K₂) of the carbinolamine intermediate were determined.     

Uncertainty estimation in measurement of pKa values in nonaqueous media: A case study on basicity scale in acetonitrile medium

The difficulties in estimating uncertainty of pK_a values determined in nonaqueous media are reviewed and two different uncertainty estimation approaches are presented and applied to the pK_a values of the compounds on a previously established self-consistent spectrophotometric basicity scale in acetonitrile. One approach is based on the ISO GUM methodology (the “ISO GUM” approach) and involves careful analysis of the uncertainty sources and quantifying the respective uncertainty components. The second approach is based on the standard-deviation-like statistical parameter that has been used for characterization of the consistency of the scale (the “statistical” approach). It is demonstrated that the ISO GUM approach somewhat overestimates the uncertainty. The statistical approach is based on long-term within-laboratory statistical data and it is demonstrated that it underestimates the uncertainty. In particular it neglects the laboratory bias effects that are taken into account at least to some extent by the ISO GUM approach. Thus, together these two approaches allow to “bracket” the uncertainties of the pK_a values on the scale. The uncertainties of the pK_a values are defined in two different ways. Definition (a) includes the uncertainty of the pK_a of the reference base (anchor base of the scale) pyridine. Definition (b) excludes it. It is demonstrated that both definitions have their virtues. Definition (a) leads to the uncertainty ranges of 0.12-0.22 and 0.12-0.14 pK_a units at standard uncertainty level for different bases using the ISO GUM and statistical approach, respectively. Definition (b) leads to the uncertainty ranges of 0.04-0.19 and 0.02-0.08 pK_a units, respectively. The uncertainty of the pK_a of a given base is dependent on the quality of the measurements involved and on the distance from the reference base on the scale. The importance of the correlation between the pK_a values of bases belonging to the same scale is stressed.     

Towards surface acid–base property of the carboxylic multi-walled carbon nanotubes by zero current potentiometry

The surface acid–base property of carboxylic multi-walled carbon nanotubes (MWNTs) is investigated by zero current potentiometry with a new electrochemical measurement system. The pH dependent interface potential variation at the interface of carboxylic MWNTs/solution is investigated by measuring zero current potential E_zcp. In the pH range of 1–11, the pH response of carboxylic MWNTs exhibits two linear relationships according to the following equations: E_zcp = 0.791–0.0535 pH (pH 1–5.1) and E_zcp = 0.643–0.0241 pH (pH 5.1–11), respectively. The intersection at pH 5.1 of two regions indicates the surface pK_a value of carboxylic group terminated MWNTs.     

The mechanism of the sonochemical degradation of benzoic acid in aqueous solutions

The sonolytic degradation of benzoic acid in aqueous solution was investigated at an ultrasonic frequency of 355 kHz. The degradation rate was found to be dependent upon the solution pH and the surface activity of the solute. The degradation rate was favoured at a solution pH lower than the pK _a of benzoic acid. At pH < pK _a, HPLC, GC and ESMS analysis showed that benzoic acid could be degraded both inside the bubble by pyrolysis and at the bubble/solution interface by the reaction with OH radicals. At higher pH (> pK _a) benzoic acid could only react with OH radicals in the bulk solution. During the sonolytic degradation of benzoic acid, mono-hydroxy substituted intermediates were observed as initial products. Further OH radical attack on the mono-hydroxy intermediates led to the formation of di-hydroxy derivatives. Continuous hydroxylation of the intermediates led to ring opening followed by complete mineralization. Mineralization of benzoic acid occurred at a rate of < 40μM/h.     

Tributylphosphine, excellent organocatalyst for conjugate additions of non-nucleophilic N-containing compounds

Conjugate additions of non-nucleophilic N-containing compounds such as amides, thioamides, sulfonamides, and electron-poor anilines with different Michael acceptors can be promoted through the use of tributylphosphine. The range of useful pK_a's of nucleophiles has been established (pK_a<25) and new insights into the mechanism proposed.     

Prediction of the separation of phenols by capillary zone electrophoresis

The effect of pH and ionic strength on the migration of neutral acids in capillary zone electrophoresis (CZE) has been studied for several phenols. The mobilities of the phenols and the efficiency of the capillary have been related to the studied factors. The mobility can be related to the pH of the running buffer through the mobility of the phenolate ion, and the conditional acidity pK value of the phenol at the working ionic strength. This allows prediction of the migration of the phenol, solely from its pK_a^′ value (literature pK_a corrected for the ionic strength of the solution) and mobility of the anion, which can be easily calculated from the mobility at a basic pH value and the pK_a^′ value. Combination of the predicted mobility with the efficiency allows estimation of the resolution of the consecutive peaks obtained for a mixture of phenols. This method has been tested for two groups of phenols of environmental interest.     

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 ).     

Single-point titrations: Part-I. The determination of bases

Equations are derived for the calculation of acid mixtures, which upon titration with base show a linear relation between pH and the amount of base. Three to five weak acids were used and a linearity of better than ±0.02 pH units was obtained. The use of such mixtures for analysis of the base content of samples by means of a single pH measurement is described. A procedure for obtaining conditional pK_a values of the components of the acid mixture is also described. The single-point titration method is advocated for use when better accuracy than that of direct potentiometry is desired but less than that of an ordinary titration can be accepted. It is not necessary to know the pK_b or the number of weak bases.     

Acid dissociation constants of uridine-5′-diphosphate compounds determined by phosphorus nuclear magnetic resonance spectroscopy and internal pH referencing

The acid dissociation constant (pK_a) of small, biological molecules is an important physical property used for investigating enzyme mechanisms and inhibitor design. For phosphorus-containing molecules, the ³¹P nuclear magnetic resonance (NMR) chemical shift is sensitive to the local chemical environment, particularly to changes in the electronic state of the molecule. Taking advantage of this property, we present a ³¹P NMR approach that uses inorganic phosphate buffer as an internal pH reference to determine the pK_a values of the imide and second diphosphate of uridine-5′-diphosphate compounds, including the first reported values for UDP-GlcNAc and UDP-S-GlcNAc. New methods for using inorganic phosphate buffer as an internal pH reference, involving mathematical correction factors and careful control of the chemical shift reference sample, are illustrated. A comparison of the newly determined imide and diphosphate pK_a values of UDP, UDP-GlcNAc, and UDP-S-GlcNAc with other nucleotide phosphate and thio-analogs reveals the significance of the monosaccharide and sulfur position on the pK_a values.     

Ionization of some derivatives of benzamide, oxamide and malonamide in DMF-water mixture

The ionization of six compounds of bis-phenolic amides was studied spectrophotochemically in DMF-water mixture. The compounds showed two pK_a values in the range of 5.97-7.32 for pK_a1 and 7.61-8.44 for pK_a2. The obtained values of K_a were normalized using the distribution diagrams of the different species and found to be in the range of 5.81-7.42 for pK_a1 and 7.48-8.27 for pK_a2.     

Electronic absorption study on acid–base equilibria for some pyrimidine derivatives containing semi- and thiosemicarbazone moiety

The UV–vis spectra of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one, (I), and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione, (II) were studied in aqueous methanol (5%, v/v methanol). The nature of the electronic transitions and the roles of carbonyl oxygen of I and thiocarbonyl sulfur of II on the behavior of UV–vis spectra were discussed.Acid–base equilibria of the compounds against varying pH and pK_a values related equilibria were determined at an ionic strength of 0.10 M by using the Henderson–Haselbalch equation. The mean acidity constants for the protonated forms of the compounds were determined as pK_a1 = 5.121, pK_a2 = 7.929 and pK_a3 = 11.130 for I and pK_a1 = 4.684, pK_a2 = 7.245 and pK_a3 = 10.630 for II. The preferred dissociation mechanisms were discussed based on UV–vis data and a mechanism was proposed for each compound.