Determination of acid-base dissociation constants of amino- and guanidinopurine nucleotide analogs and related compounds by capillary zone electrophoresis

Determination of the binding constants and ionic mobilities of diquat complexes with randomly sulfated cyclodextrins by affinity capillary electrophoresis

The enantiomers of diquats (DQs), a new class of functional organic molecules, were recently separated by capillary electrophoresis (CE) with high resolution up to 11.4 within 5-7 min using randomly sulfated α-, β-, and γ-cyclodextrins (CDs) as chiral selectors. These results indicated strong interactions between dicationic diquats and multiply negatively charged sulfated CDs (S-CDs). However, the binding strength of these interactions was not quantified. For that reason, in this study, affinity CE was applied for the determination of the binding constants and ionic mobilities of the complexes of DQ P- and M-enantiomers with CD chiral selectors in an aqueous medium. The non-covalent interactions of 10 pairs of DQ enantiomers with the above CDs were investigated in a background electrolyte (BGE) composed of 22 mM NaOH, 35 mM H3PO4, pH 2.5, and 0.0-1.0 mM concentrations of CDs. The average apparent binding constant and the average actual ionic mobility of the DQ-CD complexes were determined by nonlinear regression analysis of the dependence of the effective mobility of DQ enantiomers on the concentration of CDs in the BGE. The complexes were found to be relatively strong with the averaged apparent binding constants in the range 13 600-547 400 L/mol.

Determination of acidity constants, ionic mobilities, and hydrodynamic radii of carborane-based inhibitors of carbonic anhydrases by capillary electrophoresis

Capillary electrophoresis (CE) has been applied for determination of the thermodynamic acidity constants (pK_a ) of the sulfamidoalkyl and sulfonamidoalkyl groups, the actual and limiting ionic mobilities and hydrodynamic radii of important compounds, eight carborane-based inhibitors of carbonic anhydrases, which are potential new anticancer drugs. Two types of carboranes were investigated, (i) icosahedral cobalt bis(dicarbollide)(1-) ion with sulfamidoalkyl moieties, and (ii) 7,8-nido-dicarbaundecaborate with sulfonamidoalkyl side chains. First, the mixed acidity constants, pK_a ^mix , of the sulfamidoalkyl and sulfonamidoalkyl groups of the above carboranes and their actual ionic mobilities were determined by nonlinear regression analysis of the pH dependences of their effective electrophoretic mobility measured by capillary electrophoresis in the pH range 8.00-12.25, at constant ionic strength (25 mM), and constant temperature (25°C). Second, the pK_a ^mix were recalculated to the thermodynamic pK_a s using the Debye-Hückel theory. The sulfamidoalkyl and sulfonamidoalkyl groups were found to be very weakly acidic with the pK_a s in the range 10.78-11.45 depending on the type of carborane cluster and on the position and length of the alkyl chain on the carborane scaffold. These pK_a s were in a good agreement with the pK_a s (10.67-11.27) obtained by new program AnglerFish (freeware at https://echmet.natur.cuni.cz), which provides thermodynamic pK_a s and limiting ionic mobilities directly from the raw CE data. The absolute values of the limiting ionic mobilities of univalent and divalent carborane anions were in the range 18.3-27.8 TU (Tiselius unit, 1 × 10^-9 m² /Vs), and 36.4-45.9 TU, respectively. The Stokes hydrodynamic radii of univalent and divalent carborane anions varied in the range 0.34-0.52 and 0.42-0.52 nm, respectively.

Determination of thermodynamic acidity constants and limiting ionic mobilities of weak electrolytes by capillary electrophoresis using a new free software AnglerFish

Thermodynamic acidity constants (acid or acid-base dissociation constants, sometimes called also as ionization constants) and limiting ionic mobilities (both of them at defined temperature, usually 25°C) are the fundamental physicochemical characteristics of a weak electrolyte, that is, weak acid or weak base or ampholyte. We introduce a novel method for determining the data of a weak electrolyte by the nonlinear regression of effective electrophoretic mobility versus buffer composition dependence when measured in a set of BGEs with various pH. To correct the experimental data for zero ionic strength we use the extended Debye-Hückel model and Onsager-Fuoss law with no simplifications. Contrary to contemporary approaches, the nonlinear regression is performed on limiting mobility data calculated by PeakMaster's correction engine, not on the raw experimental mobility data. Therefore, there is no requirement to perform all measurements at a constant ionic strength of the set of BGEs. We devised the computer program AnglerFish that performs the necessary calculations in a user-friendly fashion. All thermodynamic pKa values and limiting electrophoretic mobilities for arbitrarily charged substances having any number of ionic forms are calculated by one fit. The user input consists of the buffer composition of the set of BGEs and experimentally measured effective mobilities of the inspected weak electrolyte.

An aggregation-induced emission star polymer with pH and metal ion responsive fluorescence

A one-pot strategy from ring-opening metathesis polymerization was employed to prepare AIE-active star polymers, which were designed to have multi-responsive fluorescence to varied stimuli including pH, CO₂, and metal ions.

Application of capillary electrophoresis in determination of acid dissociation constant values

The chemical groups undergoing protonation or deprotonation in solution are described by the acid dissociation constant value, the key parameter for physicochemical characterization of biologically- and pharmacologically-important compounds. Capillary electrophoresis (CE) proved to be suitable technique for its determination: it enables automated and accurate measurements even for minute amount of sample, does not require the information about concentration, and handle both the impure and complex samples. In this review, a number of contributions reporting on the application of CE in pKa prediction has been summarized and critically discussed. The reader will find herein the brief introduction of theory, summary of all works published in the last decade, considerations on the most important innovations and achievements, and the discussion of pKa-related issues as e.g. the role of pKa-shifts in the chiral separation mechanism or the elucidation of migration order reversals observed during CE-mediated separations.

Determination of acidity constants and ionic mobilities of polyprotic peptide hormones by CZE

CZE has been applied to determination of thermodynamic acidity constants (pKa ) of ionogenic groups and actual ionic mobilities of polyprotic peptides-synthetic human and salmon gonadotropin-releasing hormones and their derivatives and fragments. First, the mixed acidity constants, pKa,imix, of ionogenic groups, and actual ionic mobilities, mi , of gonadotropin-releasing hormone peptides were determined by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by CZE in a series of BGEs within a broad pH range (1.80-12.10), at constant ionic strength (25 mM) and reference temperature (25°C). Second, the pKa,imix values were recalculated to thermodynamic pKa s using the Debye-Hückel theory. Thermodynamic pKa of carboxyl groups was estimated to be in the range of 2.5-3.3 for C-terminal amino acids of the above peptides, and 5.2 for glutamic acid in the middle of peptide chain; pKa of imidazolyl group of histidine residues was in the range of 5.7-6.8, pKa of N-terminal amino group of the peptide with free N-terminus was equal to 6.2, pKa of phenol group of tyrosine residues was in the range of 9.8-10.8, and pKa of guanidinyl group or arginine residues reached values 11.1-11.3, depending on the position of the residues in the peptide and on the amino acid sequence of the peptide. Absolute values of actual ionic mobilities of peptides with charge number ±2 were in the range (14.6-18.6) × 10(-9) m(2) V(-1) s(-1) , and ionic mobilities of peptides with charge number ±1 reached values (6.5-12.9) × 10(-9) m(2) V(-1) s(-1) .

Chiral analysis of anti-acquired immunodeficiency syndrome drug, 9-(R)-[2-(phosphonomethoxy)propyl]adenine (tenofovir), and related antiviral acyclic nucleoside phosphonates by CE using beta-CD as chiral selector

A new CZE method has been developed for chiral analysis of an important anti-acquired immunodeficiency syndrome drug, 9-(R)-[2-(phosphonomethoxy)propyl]adenine ((R)-PMPA, tenofovir), and six related antiviral acyclic nucleoside phosphonates using beta-CD as a chiral selector. The influence of the composition, concentration and pH of the BGE and the type and concentration of chiral selector on enantiomer resolution was investigated. Complete separations of (R,S)-enantiomers of PMPA with very good resolution (R(s)=1.50-3.64) were achieved within a short time (4-15 min) in 20-50 mM sodium borate or sodium tetraborate BGEs, pH 10.0, at 20 mg/mL concentration of beta-CD. (R,S)-enantiomers of five similar PMPA analogs containing purine bases (adenine, diaminopurine or guanine) and hydroxyl or fluor substituents at C3 carbon atom of propyl chain were baseline separated within 10-17 min in 35 mM sodium tetraborate BGE, pH 10.0, at 20 mg/mL beta-CD concentration. Another important antiviral used by acquired immunodeficiency syndrome patients, derived from pyrimidine base cytosine, 1-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (cidofovir), and the (R)-enantiomer of this drug were successfully separated in 50 mM sodium tetraborate BGE, pH 10.5, at 20 mg/mL beta-CD concentration within 45 min. Using the UV-absorption detection at 206 nm, the concentration detection limits of the analyzed acyclic nucleoside phosphonates were determined in the submicromolar to micromolar range (0.15-2.51 microg/mL level).

Characterization of acid–base properties of unstable drugs using a continuous-flow system with UV–vis spectrophotometric detection

In this paper, we propose a continuous-flow system for the study of the acid-base characteristics of unstable drugs. 5-Azacytidine has been selected as a first model of unstable compound, which progressively decomposes in aqueous solutions. Besides, other compounds undergoing hydrolysis and oxidation side reactions have been also analyzed to explore the performance of the method. In comparison with conventional batch titrations, the drug decomposition can be minimized by the continuous renewal of the analyte solution. The composition of the buffer mixture is varied on-line during the process from successive changes in the flow rates of acid and basic stock solutions. As a result, the pH value of the test solution is varied in a controlled manner in the range of 1-13. Multivariate curve resolution based on alternating least squares has been used to extract relevant information concerning the acid-base properties of analytes. Results from the continuous-flow system have been compared with those obtained, using batch spectrophotometric titrations, and in the case of fast degradations, the performance of the proposed procedure has been superior.

Synthesis and Properties of 2-Guanidinopurines

2-Guanidinopurines were prepared as derivatives of 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]-9H-purine (PMEDAP) (1), which shows an important antiviral activity. It completes earlier described synthesis of 6-guanidinopurine derivatives. The title compounds were obtained by the reaction of the corresponding 2-chloropurines with guanidine. 2- And 6-guanidinopurines were used as model compounds for determination of dissociation constants (pKa) of their ionogenic groups by capillary zone electrophoresis. The pKa values of ionogenic groups of the above compounds were compared with those of the corresponding aminopurines. The pKa of guanidino group at the purine moiety varies from 7.77 to 10.32. There is no protonation of N1-position in contrast to aminopurines. None of these compounds showed any antiviral activity.