Chiral separations using heparin and dextran sulfate in capillary zone electrophoresis

Antimalarial analysis of pharmaceutical formulations and biological samples by capillary electrophoresis: the state of the art and applications

Malaria is a serious public health problem, being an endemic disease in 84 countries, mainly in Africa. This review explores the application of capillary electrophoresis (CE) techniques for analyzing antimalarial drugs, highlighting methods from 2000 to 2023 for the analysis of pharmaceutical formulations and human biological samples. The versatility, selectivity, high efficiency, cost-effectiveness, and high analytical frequency of CE techniques have become attractive choices for pharmaceutical analysis, focusing on quality control and impurity analysis applications. The evolution of achiral and chiral electromigration methods has been described based on the features of each mode of separation: capillary zone electrophoresis (CZE), micellar electrokinetic chromatography, microemulsion electrokinetic chromatography, and capillary electrochromatography. As expected, CZE is reported in most articles owing to its compatibility with drug properties and separation mode. However, it is necessary to perform other separation modes for a few drugs that are present in neutral form. After exhaustive research using different databases and statistical analyses, 27 articles using CE techniques for antimalarial drug analysis were found and are mentioned in this review.

Enantiomeric separation of β-blockers and tryptophan using heparin as stationary and pseudostationary phases in capillary electrophoresis

The separation methods of the enantiomers of two β-blockers and tryptophan were studied using capillary electrochromatography with heparin covalently as well as non-covalently, bonded onto the capillary inner wall as stationary phase and electrokinetic chromatography with heparin as pseudostationary phase. In the case of heparin, used as a stationary phase, the method was unable to resolve enantiomers in both cases β-blockers and tryptophan. On the other hand, when heparin was used as a pseudostationary phase, the resolution of the enantiomers was obtained only with 3-aminopropyltriethoxysilane which were immobilised onto the inner phase of the capillary. The results of this study let us infer that the electrostatic, hydrophobic, and steric interactions were involved in the separation mechanisms. The separation was achieved in less than 10 minutes under the optimized conditions: 30 mM phosphate buffer (pH 2.5) with the adding of 15 mg/mL of heparin at 15°C and 10 kV. The usefulness of heparin as a chiral selector both in electrokinetic chromatography using 3-aminopropyltriethoxysilane attached to the capillary was demonstrated for the first time. The developed method was powerful, sensitive, and fast, and it could be considered an important alternative to conventional methods used for chiral separation.

Layer-by-Layer Assembly of Polysaccharides and 6,10-Ionene for Separation of Nitrogen-Containing Pharmaceuticals and Their Enantiorecognition by Capillary Electrophoresis

Two silica capillaries modified layer-by-layer with 6,10-ionene and N-(3-sulfo-3-carboxy)-propionylchitosan (SCPC) and with 6,10-ionene and dextran sulfate (DS) were prepared and investigated. Dynamic coating of the capillary efficiently reduces the adsorption of the background electrolyte, sample matrix components, and analytes on its inner wall. Such coatings effect good reproducibility and sensitivity of determination. We demonstrate that separation of betablockers, calcium channel blockers, alpha-adrenergic agonists, H1-blockers, and diuretics was the most efficient and rapid separation with a capillary modified with dextran sulfate. Tetrahydrozoline, carbinoxamine, and furacilin, which are commonly employed as treatments for allergic rhinitis, were identified in human urea. Their concentrations, independently verified by HPLC, were found to be5.3±0.8,6.6±0.5, and0.9±0.2 μg mL−1, withLOD=0.07, 0.03, 0.10 μg mL−1, andLOQ=1.0, 0.8, 0.6 μg mL−1, respectively.

Evaluation of sulfated maltodextrin as a novel anionic chiral selector for the enantioseparation of basic chiral drugs by capillary electrophoresis

Introducing a new class of chiral selectors is an interesting work and this issue is still one of the hot topics in separation science and chirality. In this study, for the first time, sulfated maltodextrin (MD) was synthesized as a new anionic chiral selector and then it was successfully applied for the enantioseparation of five basic drugs (amlodipine, hydroxyzine, fluoxetine, tolterodine, and tramadol) as model chiral compounds using CE. This chiral selector has two recognition sites: a helical structure and a sulfated group which contribute to three corresponding driving forces; inclusion complexation, electrostatic interaction, and hydrogen binding. Under the optimized condition (buffer solution: 50 mM phosphate (pH 3.0) and 2% w/v sulfated MD; applied voltage: 18 kV; temperature: 20°C), baseline enantioseparation was observed for all mentioned chiral drugs. When instead of sulfated MD neutral MD was used under the same condition, no enantioseparation was observed which means the resolution power of sulfated MD is higher than neutral MD due to the electrostatic interaction between sulfated groups and protonated chiral drugs. Also, the countercurrent mobility of negatively charged MD (sulfated MD) allows more interactions between the chiral selector and chiral drugs and this in turn results in a successful resolution for the enantiomers. Furthermore, a higher concentration of neutral MD (approximately five times) is necessary to achieve the equivalent resolution compared with the negatively charged MD.

CE frontal analysis based on simultaneous UV and contactless conductivity detection: A general setup for studying noncovalent interactions

CE frontal analysis (CE-FA) has been established as a powerful tool to study noncovalent interactions between macromolecules and small molecules such as drug substances or pharmaceutical excipients. However, when using traditional commercial CE instrumentation, a serious drawback is related to the fact that only UV-active compounds can be studied. In recent years, contactless conductivity detection has become an attractive alternative to UV detection in CE due to its high versatility. In this study, we combine contactless conductivity detection and UV detection in a highly versatile setup for profiling noncovalent interactions between low-molecular-weight molecules and macromolecules. In the case of molecules having a chromophore the setup allows determination of binding constants using two independent detectors. The new contactless conductivity detection cell is compatible with commercial CE instrumentation and is therefore easily implemented in any analysis laboratory with CE expertise.

Chiral recognition of dextran sulfate with d- and l-cystine studied by multiwavelength surface plasmon resonance

A multiwavelength surface plasmon resonance (mwSPR) approach has been developed to study the chiral discrimination between D- and L-cystine (Cys). A monolayer of the two enantiomers was separately assembled on a pair of gold films of about 50 nm in thickness and their resonance wavelength shifts, Deltalambda, were measured under a continuous flow of an identical chiral probe solution. Dextran sulfate (DS) was found to be an excellent chiral probe because it has rich chiral centers and is large enough to produce sensitive mwSPR response. The chiral discrimination was investigated either by Deltalambda(max), the maximum resonance wavelength shift in recognition equilibrium, or by recognition kinetics (Deltalambda vs time). The equilibrium data showed that D-Cys yielded always the smaller Deltalambda(max) as compared to L-Cys at pH 5.0 or above. This differentiation was enlarged by raising the probe content and became naught at pH <4.5. The kinetic results showed that, as pH increased from 5.0 to 7.5, the non-equilibrium Deltalambda for D-Cys rose above the level for L-Cys at the first 30s of recognition but came back gradually to its equilibrium position after about 150 s, with crossing at 50--150 s depending on DS concentration. This phenomenon was thought to be the result of molecular orientation adjustment after DS binding to D-Cys. Both kinetic and thermodynamic mechanisms were thus considered to be deeply involved in the investigated chiral recognition system.

Enantioselective separation of rivastigmine by capillary electrophoresis with cyclodextrines

Different beta-cyclodextrines (beta-cyclodextrin, heptakis (2,3,6-tri-O-methyl)-beta-cyclodextrin, hydroxypropyl-beta-cyclodextrin, and sulfated beta-cyclodextrin) were investigated as additives for the enantioselective separation of the R-form from rivastigmine ((S)-N-ethyl-3-[(1-dimethylamino) ethyl]-N-methyl-phenyl carbamate), contained as impurity in this drug, which is used for the treatment of Alzheimer's disease. Electrophoresis was performed in an acidic background electrolyte (triethanolammonium phosphate, 75 mM, pH 2.5) with various concentrations of the additives. The electrophoretic mobilities measured are typical functions of the additive concentrations, with complex constants (obtained by fitting the appropriate binding curve on the data) ranging between about 180 and 770 M(-1). Best separation was obtained with 7.5 mM beta-cyclodextrin, with the R-enantiomer as impurity migrating before the main S-compound. Intra- and interday reproducibility (n = 6 and 18, respectively) of migration time and peak area was in the low percentage range, linearity of the calibration line for the quantitation of the impurity in the range between 2.3 and 50 microg/ml, expressed by the linear correlation coefficient, was 0.9998. The limits of detection and quantitation, respectively, were 0.7 and 2.3 microg/ml, corresponding to 0.05 and 0.15%, m/m of the R- relative to the S-compound. Analysis can be carried out at 18 degrees C in less than 19 min.

Vesicles as pseudostationary phase for enantiomer separation by capillary electrophoresis

A vesicle-forming single-tailed amino acid derivatized surfactant, sodium N-(4-n-dodecyloxybenzoyl)-L-valinate (SDLV) has been used as a chiral selector in micellar electrokinetic chromatography to study the molecular recognition of sterically hindered atropisomeric compounds (+/-)binaphthol, (+/-)binaphthyl diamine, (+/-)binaphthol phosphate, Tröger's base and the chiral compound benzoin (BZN). The aggregation behavior and microstructure of the surfactant were studied in separation buffer. The amphiphile was found to form bilayer vesicles in dilute aqueous solutions. The chromatographic separation of enantiomers by use of large liposome-like vesicles spontaneously formed by the amphiphile was explored. The separations were optimized with respect to voltage, pH, and surfactant and buffer concentrations. The resolutions obtained for the above mentioned racemates by use of SDLV vesicles as chiral selectors are higher compared to those reported for other chiral surfactant monomers. The results have been discussed in light of the aggregation behavior of the amphiphile in buffered aqueous solutions.

Enantioselective separations in capillary electrophoresis with dextran sulfate as the chiral selector

Dextran sulfate, a polyanionic polysaccharide, was evaluated as a chiral additive in capillary electrophoresis. Structurally related compounds having a variety of functional groups were utilized to probe the selectivity of the chiral selector. The effects of pH, chiral selector concentration, and chiral selector composition on resolution were also studied. At low pH, the reversed polarity mode was employed to achieve separation of the probe compounds. The electrophoretic results provided insight into the chiral recognition of dextran sulfate in capillary electrophoresis. Several factors, including hydrophobic, steric, and electrostatic interactions, appeared to play a role in the observed enantioseparations.

Development of a validated capillary electrophoresis method for enantiomeric purity testing of dexchlorpheniramine maleate

A capillary zone electrophoresis method has been developed for the detection of 0.1% of (R)-levochlorpheniramine maleate in samples of (S)-dexchlorpheniramine maleate. Using 1.5 mM carboxymethyl-beta-cyclodextrin in an acidic background electrolyte, resolution values of more than 10 were obtained. Under these conditions the R-enantiomer is migrating in front of the bulk S-enantiomer. The assay was validated for linearity (2-10 microg/ml; R2 = 0.9992), selectivity [(RS)-pheniramine maleate and (RS)-brompheniramine maleate], limit of detection (0.25 microg/ml), limit of quantification (0.75 microg/ml), analytical precision (intra- and inter-day variability), repeatability of the method (RSD = 5.0%) and accuracy. In samples of dexchlorpheniramine maleate from two different manufacturers, concentrations of, respectively, 0.15% and 1.95% (m/m) of levochlorpheniramine maleate were detected. The method was compared to the HPLC method described in the European Pharmacopoeia III monograph.

Enantiomer separation by capillary electrophoresis utilizing carboxymethyl derivatives of polysaccharides as chiral selectors

Enantiomer separations of various drugs by capillary electrophoresis (CE) were investigated utilizing carboxymethyl (CM) derivatives of some polysaccharides. Three types of CM-polysaccharides, namely CM-dextran, -amylose and -cellulose were employed as chiral selectors in the CE enantiomer separation. Capability of enantiomer separation by these CM-polysaccharides was compared with that by polysaccharides without CM residues (i.e. native or neutral polysaccharides). Among three selectors employed, CM-dextran and -cellulose showed a relatively wide capability of enantiomer separation. Modification of polysaccharides seems to lead to the enhancement of the capability of enantiomer separation. Degree of substitution greatly affected the capability of enantiomer separation of these polysaccharide derivatives as in the beta-cyclodextrins derivatives.

Fundamental aspects of chiral separations by capillary electrophoresis

A review is presented that surveys the basic theory of direct separation of enantiomers by capillary electrophoretic (CE) techniques. These separations are based on the formation of diastereomeric complexes between the enantiomeric analytes and a chiral selector added to the electrolyte solution. The review covers a comprehensive treatment of the equations needed for optimization of selectivity coefficients, resolution and analysis time in the zone electrophoretic mode. In this context, it takes into account combined equilibria of complexation and protonation/deprotonation as well as complexation and paritition into micelles. On the basis of these equations, the benefits of charged selectors and the optimization potential inherent to pH tuning can be documented. In addition, the review deals with some basic aspects of chiral isoelectric focusing and briefly discusses indirect enantioseparation. In a subsequent section a survey is given on particularfeatures of the various types of chiral selectors. Finally, the recent developments in preparative enantioseparation in continuous free-flow system and by use of isoelectric membranes are discussed.

Enantiomer separation by capillary electrophoresis utilizing noncyclic mono-, oligo- and polysaccharides as chiral selectors

Various noncyclic mono-, oligo- and polysaccharides have been successfully used for enantiomer separation in the analytical sciences such as HPLC and capillary electrophoresis (CE). This review presents enantiomer separation by CE utilizing mainly polysaccharides as chiral additives. The operation conditions that affect the enantioselectivity are briefly discussed.

Separation of enantiomers on anion exchangers modified with heparin in liquid chromatography

The separation of enantiomers of chloroquine and other amine derivatives has been investigated by liquid chromatography using anion exchangers modified with heparin as the stationary phase. Modification and mobile phase conditions affected the separation of analytes. Separation of the chloroquine enantiomers could be achieved by using a buffer solution containing acetonitrile as the mobile phase.

Chiral separations in capillary electrophoresis

The marked increase in the number of communications on the utilization of electrophoresis for practical chiral separations within the last three years is the most evident, and the most important fact. It reveals that the basic period of intensive research in the field is finished. The search for chiral selectors discriminating racemates in a reasonably analytical manner and the study of both the mechanism and physicochemical aspects of the chiral discrimination process were the main features of that period. Here, we review the state of the art in the field and state the references of the related literature up to the end of 1998.

Semisynthetic chondroitins as chiral buffer additives in capillary electrophoresis

Chemically oversulfated galactosaminoglycans with potential as therapeutic agents (inhibitors of human leukocyte elastase) were tested as chiral selectors in capillary electrophoresis of basic racemates. The high anionic character of these compounds provides them with anodic mobility in acidic buffer; using uncoated capillaries, the enantioresolution of racemic basic drugs was obtained at pH 2.5. Dimethindene, chloroquine and chlorpheniramine were enantioresolved applying negative voltage (-15 kV) while the other analytes (propranolol, pindolol, tetrahydrozoline and cloperastine) exhibited catodic migration. The addition of organic solvents to the running buffer was evaluated in order to increase the resolution; methanol provides the best results and in general, baseline separation of the analytes was reached. The studied oversulfated mucopolysaccharide, shows the same ionic character of heparin but presents different stereochemistry and sites of sulfation. A comparison with heparin, used in the same acidic conditions, may underline the role of ionic, spatial and steric features of glycosaminoglycans in the enantiorecognition.

Separation of enantiomers by capillary electrophoresis using pentosan polysulfate

Pentosan polysulfate, a semisynthetic polysaccharide, was employed as a chiral run buffer additive in capillary electrophoresis. Twenty-eight racemic analytes were resolved. The separations were successful only at low pH when the analytes were significantly protonated. This suggests that ionic interactions were the dominant associative interactions between the anionic pentosan polysulfate and the positively charged analytes. Compared to other linear, carbohydrate-based chiral selectors (i.e., chondroitin sulfates, heparin and dextran sulfate) pentosan polysulfate has some characteristics common of anionic polysaccharides; yet it has several differences in its structure and properties which account for its unusual enantioselectivity. The effects of pH, concentration of phosphate buffer, concentration of pentosan polysulfate and the type and concentration of organic modifier on the enantiomeric separations were investigated. The optimization of these separations were dependent on the nature of the analytes and could be achieved by the proper choice of experimental conditions.

Enantiomeric separations of terbutaline by CE with a sulfated beta-cyclodextrin chiral selector: a quantitative binding study

Sulfated beta-cyclodextrin, a negatively charged chiral selector, was used for the enantiomeric separation of racemic terbutaline by capillary electrophoresis. Chiral separation was found to increase with decreasing cyclodextrin concentration. Host-guest complex binding constants for this system were determined by UV difference spectroscopy (Kav = 1490 M-1) and by CE under conditions of minimal EOF and reversed polarity (K1 = 1730 M-1, K2 = 1590 M-1, alpha = 1.09). The effect of organic modifiers, methanol, and acetonitrile was also studied over a wide range of modifier concentrations. Binding constants decreased while selectivity increased with increasing organic modifier concentration (10% MeOH: K1 = 1590 M-1, K2 = 1130 M-1, alpha = 1.41. 10% ACN: K1 = 1320 M-1, K2 = 870 M-1, alpha = 1.52). Experimental results are discussed in the context of existing separation models.

Enantioselectivity in chiral capillary electrophoresis with polysaccharides

This review surveys enantiomer separation by capillary electrophoresis (CE) using polysaccharides as chiral selectors. Many ionic or electrically neutral polysaccharides, such as heparin, chondroitin sulfate, dextrin, etc., have been employed successfully for the CE separation of enantiomers. The operational conditions that affect the enantioselectivity of the chiral separation system will be described. The mechanism of enantioseparation will also be discussed briefly.

Chiral capillary electrophoresis with noncyclic oligo- and polysaccharide chiral selectors

A number of noncyclic oligo- and polysaccharides have been used as chiral additives in capillary electrophoresis (CE). This review offers a broad survey of the types of oligo- and polysaccharides which have been investigated and also some of the conditions developed for the enantioseparation of a wide range of drugs and other racemic compounds. Details of enantioseparation mechanisms are also discussed, in addition to some of the parameters required for optimization of the enantioresolution of chiral molecules.

Enantioseparation using selected polysaccharides as chiral buffer additives in capillary electrophoresis

Selected water-soluble, native polysaccharides--such as amylose, laminaran, pullulan--and derivatized polysaccharides--methyl cellulose, hydroxypropyl cellulose, and carboxymethyl amylose sodium salt (CM-Am)--were investigated as chiral selectors in capillary electrophoresis. Effects of degree of polymerization and concentration of amylose on the separation of enantiomers of 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BDHP) and a chiral cardiovascular drug cis-diltiazem were also studied. Pullulan and amyloses used in this study showed the same migration order for enantiomers of BDHP. In contrast, the migration order of BDHP enantiomers for cellulose derivatives and luminaran as well as with beta-cyclodextrin was opposite to that for amylose and pullulan. The enantioseparation of several chiral drugs was also performed using high-molecular-mass amylose (Am-4900) and CM-Am.

Studies of polymerized sodium N-undecylenyl-L-valinate in chiral micellar electrokinetic capillary chromatography of neutral, acidic, and basic compounds

The polymerized surfactant poly(sodium N-undecylenyl amino L-valinate) [poly(L-SUV)] has been used in micellar electrokinetic capillary chromatography for the chiral separation of various acidic and basic drugs, as well as neutral compounds. Under the conditions studied, poly(L-SUV) was shown to be a very versatile anionic chiral selector in the pH range of 5.6-11. The micelle was used for the enantioseparation of coumarinic anticoagulant drugs with various buffers under moderately acidic conditions. Neutral and alkaline buffer conditions were used to successfully separate the neutral atropisomers (+/-)-1,1'-bi-2-naphthol, (+/-)-1,1'-binaphthyl-2,2'-diamine, and Tröger's base. Chiral separation of the cationic paveroline drugs, laudanosine, norlaudanosoline, and laudanosoline, was influenced by pH and the use of coated capillaries. The acquired data focused on optimizing the migration times, capacity and separation factors, and electrophoretic mobilities of the various racemic mixtures.

Comparison and modeling study of vancomycin, ristocetin A, and teicoplanin for CE enantioseparations

The structurally related glycopeptide antibiotics vancomycin, ristocetin A, and teicoplanin can all be used as chiral selectors in capillary electrophoresis (CE). Both experimental and modeling studies were done to elucidate their similarities and differences. There are identifiable morphological differences in the aglycon macrocyclic portions of these three compounds. In addition, there are other structural distinctions that can affect their CE enantioselectivity, migration times, and efficiency. Teicoplanin is the most distinct of the three and is the only one that is surface active. Its aggregational properties appear to affect its enantioselectivity among other things. The similar but not identical structures of the three glycopeptides produce similar but not identical enantioselectivities. This leads to the empirically useful "principle of complementary separations", in which a partial resolution with one chiral selector can be brought to baseline with one of the others. Overall, ristocetin A appears to have the greatest applicability for CE enantioseparations.

Chiral separation of chloroquine using heparin as a chiral selector in high-performance liquid chromatography

The chiral selectivity of a commercially available heparin affinity column was investigated for chloroquine. The (+) enantiomer eluted first, which is consistent with results reported previously using heparin as a chiral additive in capillary zone electrophoresis. The effects of pH, ionic strength, and organic modifier on the enantiodiscrimination were explored. A combination of electrostatic and hydrophobic interactions seems to play an important role in the enantiodiscrimination exhibited by this novel phase.

Application of sulfated cyclodextrins to chiral separations by capillary zone electrophoresis

Mixtures of randomly substituted sulfated cyclodextrins (degree of substitution, approximately 7-10) were successfully used as chiral additives for the enantioseparation of 56 compounds of pharmaceutical interest, including anesthetics, antiarrhythmics, antidepressants, anticonvulsants, antihistamines, antihypertensives, antimalarials, relaxants, and bronchodilators. The separations were accomplished at pH 3.8, with the anode at the detector end of the column. Under these conditions, in which electroosmotic flow is directed toward the injection end of the column and the electrophoretic mobility of the negatively charged cyclodextrin is toward the detector, none of the analytes reached the detector in the absence of the sulfated cyclodextrin. Most (40) of the successfully resolved enantiomers contained basic functionality and a stereogenic carbon. However, the versatility of this sulfated cyclodextrin additive was also demonstrated by the fact that three atropisomers, 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate, 1.1'-binaphthyl-2,2'-diol, and Troger's base, and several neutral analytes were also successfully enantioresolved under these conditions. The separation mechanism seems to involve inclusion complexation.

Sulfated cyclodextrins for the chiral separations of catecholamines and related compounds in the reversed electrophoretic polarity mode

The utility of negatively charged sulfated cyclodextrins (SCD) as chiral additives (CA) in capillary electrophoresis (CE) was studied in the chiral resolution of several compounds of pharmaceutical interest, including catecholamines such as norepinephrine, epinephrine, DOPA and their precursors, phenylalanine, and tyrosine. Experiments were conducted using 10 mM sodium phosphate monobasic solution and 2% SCD adjusted to pH 3.2 with phosphoric acid. Chiral recognition mechanisms were explored using structurally related analytes including basic, acidic, and neutral compounds as well as 3,5-dinitrobenzoyl phenylglycine, phenylalanine, and homophenylalanine. The advantage of the reversed electrophoretic polarity mode for the enantioresolution of these compounds is also discussed.