Capillary electrophoresis with chiral selectors: optimization of separation and determination of thermodynamic parameters for binding of tioconazole enantiomers to cyclodextrins

Review of Applications of β-Cyclodextrin as a Chiral Selector for Effective Enantioseparation

The significance and necessity of separating enantiomers in food, pharmaceuticals, pesticides, and other samples remains constant and unrelenting. The successful chiral separation usually includes the application of a chiral auxiliary compound, known also as a chiral selector (CS), that forms complexes with enantiomers of different physicochemical properties, enabling efficient separation. While both native and substituted cyclodextrins (CDs) are commonly used as CSs, β-CD is undoubtedly the most popular one among them. This review includes recent advancements in the application of β-CD as a CS. While the theoretical background behind the enantioseparation is also part of this work, the main emphasis is put on the factors that affect the efficacy of this process such as temperature, pH, solvent, and the choice of other additives. Also, the different analytical methods: Nuclear Magnetic Resonance (NMR) spectroscopy, Capillary Electrophoresis (CE), fluorescence spectroscopy (FS), High-Performance Liquid Chromatography (HPLC), Isothermal Titration Calorimetry (ITC), and UV-vis spectroscopy, used for enantioseparation with the aid of β-CD as CS, are thoroughly compared. Also, since some of the chiral compounds have been studied in the context of their enantioseparation more than once, those works are compared and critically analyzed. In conclusion, while β-CD can be in most cases used as CS, the choice of the experimental conditions and method of analysis is crucial to achieve the success.

Effect of Substitution Degree and Homogeneity on Cyclodextrin-Ligand Complex Stability: Comparison of Fenbufen and Fenoprofen Using CD and NMR Spectroscopy

The stability of host-guest complexes of two NSAID drugs with similar physicochemical properties, fenbufen and fenoprofen, was investigated by comparing induced circular dichroism and ¹H nuclear magnetic resonance methods using eight cyclodextrins of different degrees of substitution and isomeric purity as guest compounds. These cyclodextrins include native β-cyclodextrin (BCyD), 2,6-dimethyl-β-cyclodextrin 50 (DIMEB50), 80 (DIMEB80) and 95% (DIMEB95) isomerically pure versions, low-methylated CRYSMEB, randomly methylated β-cyclodextrin (RAMEB) and 4.5 and 6.3 average substitution grade hydroxypropyl-β-cyclodextrin (HPBCyD). The stability constants obtained by the two methods show good agreement in most cases. For fenbufen complexes, there is a clear trend that the stability constant increases with the degree of substitution while isomer purity has a smaller effect on the magnitude of stability constants. A significant difference was found in the case of DIMEB50 when compared to DIMEB80/DIMEB95, while the latter two are similar. In the fenbufen-fenoprofen comparison, fenbufen, with its linear axis, gives a more stable complex, while fenoprofen shows lower constants and poorly defined trends.

Characterization of the hydrochloride salt hemihydrate as a new salt of the antifungal agent tioconazole

Tioconazole is an effective antifungal agent, which has a very low solubility in aqueous media, that limits its bioavailability and efficacy. In an effort to overcome the drug limitations by improving its solubility, the hydrochloride salt was prepared in methanolic 1M HCl and obtained as the hemihydrate, as demonstrated by elemental analysis. Single crystals were grown by slow evaporation from an aqueous 1M HCl solution and their structure was determined using single-crystal X-ray diffraction at 302 K. The structures resulting from dehydration and further rehydration were also assessed, at 333 and 283 K, respectively. The morphology of the crystal, which exhibited birefringence under polarized light, was verified by hot stage microscopy. The solid was characterized by additional means, including thermal analysis (melting point, differential scanning calorimetry and thermogravimetry), spectroscopic methods (mid infrared, near infrared, ¹H, ¹³C and ¹⁵N nuclear magnetic resonance in solution, as well as ¹³C and ¹⁵N solid state with spinning at the magic angle) and X-ray diffraction techniques. Functional evaluation tests, including the intrinsic dissolution rate and the dissolution of powders were also performed. In the intrinsic dissolution rate test, the salt proved to dissolve over 2000 times faster than tioconazole. The results suggest that the new salt has physicochemical and performance properties which may support its use as a replacement of the free base in certain applications, especially where improved dissolution rate, solubility or bioavailability of the drug would be desired.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Simple circular dichroism method for selection of the optimal cyclodextrin for drug complexation

Abstract

Cyclodextrins are very important excipients in the pharmaceutical industry. Given the multitude of native and semisynthetic cyclodextrin derivatives, there is a need for a rapid and reliable method for the selection of the optimal cyclodextrins for further pharmaceutical testing. During our research, circular dichroism (CD) spectroscopy has been successfully used to describe the qualitative and quantitative complexation of model compounds with different cyclodextrins. For the appearance of a circular dichroism signal, either a chiral or a chirally perturbed chromophore is required. Achiral or racemic compounds do not have corresponding circular dichroism spectra and neither do chiral cyclodextrins due to the absence of a chromophore group. During complexation of a chromophoric guest molecule, its absorption transition becomes chirally perturbed in the proximity of a cyclodextrin molecule and an induced circular dichroism (ICD) signal appears. This phenomenon gives an inherent selectivity to the method. The sign and intensity of the induced circular dichroism signal in case of different cyclodextrins provides information about the approximate structure of the complex as well as their stability relative to each other. In this study, we report a straightforward induced circular dichroism -based approach for the rapid preselection of the optimal cyclodextrin. The distinctive features of the method were demonstrated using five azole-type antifungal drug molecules (fluconazole, miconazole, clotrimazole, bifonazole and tioconazole) along with native α-, β-, and γ-cyclodextrins, as well as dimethyl-, trimethyl-, carboxymethyl-, hydroxypropyl- and sulfobuthylether-β-cyclodextrins. In addition, with the aid of this method, 27 stability constants were determined, amongst which 16 have been unavailable in the literature previously.

Graphic abstract

Determination of thermodynamic binding constants by affinity capillary electrophoresis

A strategy to study thermodynamic binding constants by affinity capillary electrophoresis (ACE) is presented. In order to simplify mathematical treatment, analogy with acid-base dissociation equilibrium is proposed: instead of ligand concentration [X], negative logarithm of ligand concentration (or activity), pX = -log[X], is used. On this base, and taking into account ionic activities, a general procedure for obtaining thermodynamic binding constants is proposed. In addition, the method provides electrophoretic mobilities of the free analyte and analyte-ligand complex, even when binding constants are low and thus, the complexed analyte fraction is also low. This is useful as a base to rationally analyze a diversity of situations, i.e., different mathematical dependencies are obtained when analytes and ligands with different charges are combined. Practical considerations are given for carrying out a full experimental design. Enantiomeric ACE separation based on the use of chiral selectors is addressed. 2-hydroxypropyl-β-cyclodextrin was chosen as a model ligand, and both enantiomeric forms of four pharmaceutical drugs (propranolol, pindolol, oxprenolol and homatropine methylbromide) were considered as model analytes. Practical aspects are detailed and thermodynamic binding constants as well as free and complexed analytes mobilities are determined.

Multivariate analysis of tioconazole - TCNQ charge transfer interaction: Kinetics, thermodynamics and twofold response optimization

Charge-transfer complex (CTC) formation between tioconazole (TCZ) as an n-electron donor and 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) as a π-acceptor was studied spectrophotometrically with an accompanying kinetic and thermodynamic investigation. Multivariate data analysis via a set of experimental designs was executed for this purpose. A 2³ - two-level full factorial design (FFD) was used for inspecting the proposed variables while a face-centered central composite design (FCCCD) was used to adjust the levels of variables proved to be significant. Two responses were quantified as a result of this interaction; complex I (Y1, measured at 743 nm) and complex II (Y2, measured at 842 nm). Derringer's function and overlaid contour plots were used to concurrently optimize both responses. Benesi-Hildebrand equation was applied to determine of formation constant (K), and the molar absorptivity (Ɛ) of the formed complex. Different thermodynamic parameters; the standard Gibbs free energy change (∆G°), the standard enthalpy of formation (∆H°) and the standard entropy change (∆S°) were determined for the reaction product. The proposed method was validated regarding the linearity, intra-, and inter-day precision and accuracy, limit of detection, limit of quantification and following the ICH standards. The proposed method was also applied for the determination of TCZ in its pharmaceutical preparations. Having a higher molar absorptivity and higher formation constant, complex II was of choice for all subsequent measurements. Application of Benesi-Hildebrand equation supported the formation of 1: 1 CTC. Thermodynamic study revealed the endothermic characters and the spontaneity of formation of the CTC at high temperature.

Contemporary theory of enantioseparations in capillary electrophoresis

The first separation of enantiomers in capillary electrophoresis (CE) counts slightly longer than three decades. Fast development of the practice and theory of chiral CE occurred in the past 30 years and today one can consider this technology to have a solid and mature theoretical background. The goal of the present review is not only to summarize the history and contemporary theory of enantioseparations by using CE but also to present the authors personal view where shall we head to with this attractive technology not only from the viewpoint of separation of enantiomers but also for better understanding the mechanisms of non-covalent (enantioselective) interactions in chemistry, biology, medicine and related disciplines.

Molecular modeling study of chiral separation and recognition mechanism of β-adrenergic antagonists by capillary electrophoresis

Chiral separations of five β-adrenergic antagonists (propranolol, esmolol, atenolol, metoprolol, and bisoprolol) were studied by capillary electrophoresis using six cyclodextrins (CDs) as the chiral selectors. Carboxymethylated-β-cyclodextrin (CM-β-CD) exhibited a higher enantioselectivity power compared to the other tested CDs. The influences of the concentration of CM-β-CD, buffer pH, buffer concentration, temperature, and applied voltage were investigated. The good chiral separation of five β-adrenergic antagonists was achieved using 50 mM Tris buffer at pH 4.0 containing 8 mM CM-β-CD with an applied voltage of 24 kV at 20 °C. In order to understand possible chiral recognition mechanisms of these racemates with CM-β-CD, host-guest binding procedures of CM-β-CD and these racemates were studied using the molecular docking software Autodock. The binding free energy was calculated using the Autodock semi-empirical binding free energy function. The results showed that the phenyl or naphthyl ring inserted in the hydrophobic cavity of CM-β-CD and the side chain was found to point out of the cyclodextrin rim. Hydrogen bonding between CM-β-CD and these racemates played an important role in the process of enantionseparation and a model of the hydrogen bonding interaction positions was constructed. The difference in hydrogen bonding formed with the -OH next to the chiral center of the analytes may help to increase chiral discrimination and gave rise to a bigger separation factor. In addition, the longer side chain in the hydrophobic phenyl ring of the enantiomer was not beneficial for enantioseparation and the chiral selectivity factor was found to correspond to the difference in binding free energy.

About the role of enantioselective selector-selectand interactions and the mobilities of diastereomeric associates in enantiomer separations using CE

It is generally accepted that the selective binding of enantiomers of the chiral analyte to a chiral selector is necessary for enantioseparations in CE, whereas the role of mobility differences between the temporary diastereomeric associates formed between the enantiomers and the chiral selector has been commonly neglected. One of the authors of this study suggested in 1997 that the mobility difference between the diastereomeric associates of two enantiomers with the chiral selector may be solely responsible for a separation of enantiomers in CE and enantioselective selector-selectand binding may be not necessarily required. Several indirect confirmations of this hypothesis have been described in the literature within the last few years but a dedicated study proving this concept has not been published yet. The present data obtained for the two chiral antimycotic drugs ketoconazole and terconazole by CE and NMR spectroscopy unequivocally support this concept.

Study on complex formation of biologically active pyridine derivatives with cyclodextrins by capillary electrophoresis

The capillary electrophoretic separation of the pyridine derivatives pyridoxine, pyridoxal, nicotinamide, nicotinic acid and isonicotinic acid in phosphate buffer using cyclodextrins as buffer additives was studied at pH 2.0 and 3.5. Superior separation was achieved at pH 2.0. Addition of alpha- and beta-cyclodextrin and the respective 2-hydroxypropyl derivatives as well as carboxymethyl-alpha-cyclodextrin to the running buffer did not significantly improve the resolution of the compounds. The interactions of alpha- and beta-cyclodextrin as well as their hydroxypropyl derivatives with the pyridine derivatives were investigated by capillary electrophoresis at pH 2.0. No complex formation was observed between the cyclodextrins and pyridoxine, pyridoxal and nicotinamide. alpha-Cyclodextrin and 2-hydroxypropyl-alpha-cyclodextrin form weak 1:1 complexes with nicotinic and isonicotinic acids in aqueous media at 298.15K, while beta-cyclodextrin and its hydroxypropyl derivative did not form complexes. The apparent stability constants (K) of the complexes calculated from the electrophoretic mobility data ranged between 3 and 33 kg/mol. The negative values of enthalpy and entropy of complex formation obtained from the graphical plot of the van't Hoff equation indicate an important role of van der Waals and electrostatic interactions in the binding of nicotinic acid with alpha-cyclodextrin.

Complexation of tauro- and glyco-conjugated bile salts with three neutral β-CDs studied by ACE

Complexation of the bile salts (BS) taurocholate, tauro-beta-muricholate, taurodeoxycholate, taurochenodeoxycholate, glycocholate, glycodeoxycholate, and glycochenodeoxycholate common in rat, dog, and man with natural beta-CD and the chemically modified beta-CDs 2-hydroxypropyl-beta-CD and 2-O-methyl-beta-CD was studied using mobility shift ACE. The CDs were selected due to their frequent use in preformulation and drug formulation as oral excipients for the solubilization of drug substances with low aqueous solubility. ACE was demonstrated to be a feasible and efficient technique for investigation of the interactions between BS and beta-CDs. All the investigated BS possessed affinity for the three CDs with stability constants ranging from 2x10(3) to 4x10(5) M(-) (1). The requirements and assumptions related to the use of ACE for estimating high affinity stability constants were discussed. The extent and pattern of hydroxylation significantly influenced the affinity of the glyco- and tauro-conjugated BS toward the beta-CDs (chenodeoxycholates >> deoxycholates > cholates) whereas the nature of the beta-CD derivatization and BS conjugation played a minor role only. The results indicate that displacement of drug substances from beta-CD inclusion complexes is likely to occur in the small intestine where BS are present potentially influencing drug bioavailability.

Aptamer-oligonucleotide binding studied by capillary electrophoresis: cation effect and separation efficiency

Novel features of DNA structure, recognition and discrimination have been recently elucidated through the solution structural characterization of DNA aptamers that bind cofactors, amino acids and peptides with high affinity and specificity. Multidimensional nuclear magnetic resonance methodologies have been successfully applied to solve the solution structures. In this work, it was demonstrated that capillary electrophoresis was a powerful tool allowing the fundamental study of the binding mechanism between a DNA aptamer and three ligands, adenosine and adenylate compounds, i.e., adenosine diphosphate (ADP) and adenosine triphosphate (ATP). In order to gain further insight into this binding, thermodynamic measurements under different values of parameters (such as salt nature and its concentration (x) in the run buffer) were carried out. The results showed that dehydration at the binding interface, van der Waals interactions, H-bonding and adjustment of the aptamer recognition surface were implied in the aptamer-ligand association. As well, it was demonstrated that the addition in the medium of the sodium monovalent cation Na(+) or the nickel divalent cation Ni(2+) decreased the complex formation. Separation efficiency and peak shape can also be improved by Mg(2+) divalent cation, which increased the mass transfer kinetics during the ligand-aptamer binding process. A significant separation for the worst separated pair of peaks on the electropherogram ((ADP, ATP) peak pair) was thus achieved.

Effects of nonequilibrium on velocity and plate height in reactive capillary electrophoresis

Models for velocity and plate height for reactive CE are developed under the formalism of generalized nonequilibrium theory, as described by Giddings. The resultant equations are consistent with chromatographic theory and validated with an independent stochastic simulation. Moreover, unlike prior methods for CE, this model allows calculation of thermodynamic equilibrium constants and kinetic rate constants from a single, undistorted peak. The theoretical development shows that velocity is directly dependent on the equilibrium constant and is independent of the rate constant. On the other hand, plate height varies little with equilibrium constant and is inversely proportional to rate constant. The ability to evaluate equilibrium constants from velocity and rate constants from plate height is most greatly influenced by electric field strength and mobility difference. The accuracy in calculated equilibrium constants is limited by mobility difference; however, the accuracy in rate constants is limited by plate height and equilibrium constant.

Chiral separation in capillary electrophoresis using dual neutral cyclodextrins: Theoretical models of electrophoretic mobility difference and separation selectivity

Simple equations and theoretical models, related to enantioselectivity (kappa) and C, have been developed for prediction of electrophoretic mobility difference (Deltamu) and separation selectivity (alpha) for enantiomers in CE using dual CDs, where alpha and kappa are defined as the ratio of mu and the ratio of binding constant (K) for enantiomers to each CD, respectively, C the CD concentration, and the average K for enantiomers and each CD. Experiments were carried out using dual CDs as beta-CD and dimethyl-beta-cyclodextrin (DM-beta-CD) and test analytes as five pairs of amphetamine drug enantiomers. A change in observed Deltamu and alpha of enantiomers in dual CDs was found to be in excellent agreement with the theoretical models. For example, in comparison with single CD1, dual CDs can enhance Deltamu and alpha up to the maximum value when enantiomers migrate with the same order in CD1 and CD2, and have the value of rho > 1.0, where rho is the enantioselectivity ratio for CD2 to CD1, while worse Deltamu and alpha are obtained for enantiomers with rho < 1.0.

Comparison of the separation of aziridine isomers applyingheptakis(2,3-di-O-methyl-6-sulfato)β-CD andheptakis(2,3-di-O-acetyl-6-sulfato)β-CD in aqueous and nonaqueous systems

Aziridines are attracting interest as protease inhibitors, which might be used, e.g., for treatment of parasitic diseases. Within the framework of greater projects dealing with the search of new selective protease inhibitors, a huge number of aziridines with two stereogenic centers will be synthesized. Thus, a fast and reliable screening method for the evaluation of the isomeric composition is needed. Robust baseline separations were obtained using heptakis(2,3-di-O-acetyl-6-sulfato)beta-CD (HDAS) in acidic methanol and sulfated beta-CD in acidic phosphate buffer. With HDAS the resolutions were higher and migration times shorter. Thus, the method will be used as a screening method for further isomeric mixtures of aziridines.

Application of cyclodextrin-mediated capillary electrophoresis to determine the apparent binding constants and thermodynamic parameters of the alkylnaphthalene derivatives

The separation and migration behavior of seven positional and structural neutral alkylnaphthalene derivatives in cyclodextrin-mediated capillary electrophoresis were systematically investigated. The effective separation conditions were to use 10 mM phosphate buffer with negatively charged carboxymethyl-beta-cyclodextrin (CM-beta-CD) at pH 6.0. The guest-host interactions with 1:1 or both 1:1 and 1:2 binding stoichiometries for various derivatives were evaluated by comparing their apparent binding constants. The results reveal that the substituent group(s) attached to the naphthalene ring significantly affected the inclusion stoichiometric behaviors. Alkylnaphthalene derivatives with the substituent(s) at the 1-position(s), such as 1-ethylnaphthalene, 1,4-dimethylnaphthalene, may undergo complexation with one and two CM-beta-CD molecules. The binding constants of these derivatives were consistent with the data obtained by a spectrophotometric method. The thermodynamic parameters were also calculated in order to improve our understanding of the interaction between the neutral alkylnaphthalene derivatives and CM-beta-CD at various temperatures. The positive entropy (deltaS degrees) values of the alkylnaphthalenes with the substituent(s) at the 2-position(s) indicate that the inclusion of the guest molecule into the cavity of CM-beta-CD is favored at all temperatures.

Determination of equilibrium constants from chromatographic and electrophoretic measurements

Chemical interactions, such as acid-base, complex-forming, ion association and other equilibria, are widely exploited to improve the separation efficiency in liquid chromatography as well as in electrophoresis. On the other hand, these techniques can be advantageously used to study the chemical equilibria affecting the separations. If the equilibium is sufficiently fast in comparison with the separation process, then the retention characteristics in chromatography (retention factors) or the migration characteristics in electrophoresis (effective mobilities) may be expressed as functions of the composition of mobile phase or background electrolyte (BGE), respectively. Using a proper experimental arrangement, the dependencies of retention (migration) characteristics on the mobile phase (background electrolyte) composition can be measured and utilized to calculate the equilibrium constants for equlibria taking place in the mobile phase (background electrolyte). Although principles of these measurements have been known for a long time, only more recent studies utilizing HPLC and capillary electrophoretic techniques are reviewed in this paper.

Comparative performances of selected chiral HPLC, SFC, and CE systems with a chemically diverse sample set

Pharmaceutical companies have a continuous need to resolve new racemates. Analysis may be required in aqueous and nonaqueous media, or in the presence of several different sets of potentially interfering compounds. There is often a preparative requirement. For these reasons analysts may require a number of different separation systems capable of resolving a given pair of enantiomers. We wished to improve upon existing approaches that address this situation and undertook a program of work to screen over 100 racemates, selected for their chemical diversity, on over 100 different chiral HPLC, SFC, and CE systems. Here we report results of this comparison and illustrate the use of rapid gradient screening as a valuable tool for chiral method development.

Indirect capillary electrophoresis with 8-anilino-1-naphthalenesulfonic acid as a fluorescence probe for determining the apparent stability constant of an inclusion complex formed between a cyclodextrin and a solute

An indirect capillary electrophoresis (CE) method was developed based on two competitive chemical equilibria for determining the stability constant of an inclusion complex formed between a cyclodextrin and a solute. 8-Anilino-1-naphthalenesulfonic acid was employed as a fluorescence probe. A linear relationship between mobility difference and concentration of uncomplexed ligand was theoretically established and experimentally verified. The principle of the method was explained using an example of determining stability constant of an inclusion complex formed between a ligand of hydroxypropyl-beta-cyclodextrin and a solute of amantadine. The stability constant was determined to be approximately 2 x 10(2) M(-1). It was calculated without knowledge of the mobility of the complex measured at saturating ligand concentrations. This indirect method can be applied to solutes and ligands lacking signal response on the selected detector in the CE. In addition, the indirect method is valid for both charged and neutral solutes and ligands.

Mechanistic study of enantiomeric recognition with native gamma-cyclodextrin by capillary electrophoresis, reversed-phase liquid chromatography, nuclear magnetic resonance spectroscopy, electrospray mass spectrometry and circular dichroism techniques

The possible mechanisms for the chiral recognition of 2(S)-(3,5-bis-trifluoromethyl-phenyl)-2-[3(S)-(4-fluorophenyl)-4-(1H-[1,2,4]triazol-3-ylmethyl)-morpholin-2(R)-yloxy]-ethanol (compound A) and its enantiomer with native gamma-cyclodextrin (gamma-CD) were investigated using capillary electrophoresis (CE), reversed-phase liquid chromatography (RPLC), proton (1H), fluorine (19F) and carbon (13C) nuclear magnetic resonance spectroscopy (NMR), electrospray mass spectrometry (ESI-MS) and circular dichroism (CD). All experiments provided clear evidence of the formation of diastereomeric complexes between the enantiomers and gamma-CD. Proton, fluorine and carbon NMR spectra suggested that both aromatic rings, with mono-fluoro and bis-tri-fluoro functional groups, on the guest molecule were partially included into the cavity of the gamma-CD. ESI-MS spectra indicated that the diastereomeric complexes have a 1:1 stoichiometric ratio. The binding constants of the diastereomeric complexes obtained by CE, RPLC and CD were compared. The effects of the gamma-CD concentration, organic modifiers and temperature on the CE-chiral separation were also investigated.

Chiral separation of 2,3-allenoic acid by capillary zone electrophoresis using cyclodextrin derivatives

In this paper, five of six samples of 2,3-allenoic acid enantiomers were separated by capillary zone electrophoresis (CZE) using hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) as chiral selectors. Using HP-beta-CD for chiral separation, three of the six enantiomers were separated. Five experimental conditions including HP-beta-CD concentration, pH, buffer concentration, temperature, and running voltage were investigated for their influence on separation and migration using enantiomers of 2-methyl-4-phenyl-2,3-butadienoic acid (A) and 2-(n-propyl)-4-phenyl-2,3-butadienoic acid (B) as samples. Good separation results were observed when [HP-beta-CD] = 3-12 mmol/L and pH = 7-9 for samples A and B. The temperature range of 15-25 degrees C can be selected for convenience. According to the chiral separation results, HP-beta-CD and HP-gamma-CD should be valuable selectors to separate 2,3-allenoic acids and HP-gamma-CD was suggested to separate the 2,3-allenoic acid samples with a group at 4-position bulkier than phenyl.

Chiral separation of methoxamine and lobeline in capillary zone electrophoresis using ethylbenzene-deactivated fused-silica capillary columns and cyclodextrins as buffer additives

The complete chiral separation of methoxamine and lobeline was achieved by capillary zone electrophoresis on an ethylbenzene-deactivated fused-silica capillary column and with cyclodextrins (CDs) as buffer additives. Among the CDs investigated in this study, i.e. alpha-CD, beta-CD, dimethyl-beta-CD, hydroxypropyl-beta-CD and gamma-CD, all the three beta-type CDs showed chiral recognition on the two drugs investigated. Under the investigated conditions, the baseline chiral separation of methoxamine can be achieved with 90 mM Tris-H3PO4 (pH 2.5) containing 11.5 mM of the three beta-type CDs, with dimethyl-beta-CD giving the best resolution, whereas the baseline chiral separation of lobeline can be realized by using 90 mM Tris-H3PO4 buffer (pH 2.5) containing 5.8 mM dimethyl-beta-CD or 29.5 mM hydroxypropyl-beta-CD.

Quantitative approach for the screening of cyclodextrins by nuclear magnetic resonance spectroscopy in support of chiral separations in liquid chromatography and capillary electrophoresis enantioseparation of norgestrel with alpha-, beta- and gamma-cyclodextrins

A quantitative NMR approach is proposed for the screening of cyclodextrins with regard to their enantioselectivity as chiral mobile phase additives in column reversed-phase chromatography and capillary electrophoresis. Similarities and differences between the mechanism of enantiomeric peak-separation in NMR and HPLC and CE are interpreted. The affinity of d-norgestrel to bind to (alpha-, beta-, gammay-) cyclodextrins in aqueous solution was quantified and compared by determining the association constants from chemical shift data. The association constant of l-norgestrel was estimated from titration of the racemate. Differences between the apparent association constants of the enantiomerically pure drug and the racemate are discussed from the point of view of enantiomeric competition for the cyclodextrin. The apparent association constants and chiral selectivities determined by 'H NMR for dl-norgestrell/gamma-CD system at various water-methanol ratios are correlated with the corresponding chromatographic results found in the literature. The pitfalls of previously proposed screening methods based on comparison of chemical shift differences with separation parameters are discussed.

Chiral recognition of functionalized cyclodextrins in capillary electrophoresis

The chiral recognition of hydroxypropylated, dimethylated, and sulfated cyclodextrins was evaluated by utilizing them as chiral additives in capillary electrophoresis. Although each selector yielded enantiomeric separations of most of the target analytes, differences were observed in the electrophoretic results for the different derivatized cyclodextrins and for additives having varying degrees of substitution. The results for the sulfated cyclodextrins also highlighted the importance of knowing the degree of substitution as well as the location of the substituents when comparing chiral selectors.

New triazine spectroscopic reagent for the separation of DL-amino acids by micellar electrokinetic chromatography

An approach to the chiral separation of racemic mixtures of amino acids by means of micellar electrokinetic chromatography after derivatization with a new triazine spectroscopic reagent, 3-(4,6-dichloro-1,3,5-triazinylamino)-7-dimethylamino-2-methylphenazine (DTDP), has been evaluated. It was found that the derivatives of the aliphatic amino acids such as serine, valine and arginine, could produce a strong UV absorption at 282 nm, whose apparent molar absorptivities are of 10(-4) M(-1) cm(-1), and thus the concentration of the amino acids down to 3 x 10(-7) M can still give a detectable signal (S/N = 3). Beta-Cyclodextrin (beta-CD) added to the buffer system was used as a chiral selector, and separation conditions were optimized. The presence of an organic modifier (2-propanol) was also a prerequisite for the chiral separation. The best results for the chiral separation of DTDP-amino acids were achieved in a mixed sodium dodecylsulfate-beta-CD-borate-2-propanol medium at pH 9.0. Compared to some of the commonly used derivatization methods, the present one offers a relatively stable derivative and strong UV absorption for the spectroscopically inert amino acids, thus enabling amino acids to be separated and detected by CE even with a simpler UV detector.

Accurately describing weak analyte-additive interactions by capillary electrophoresis

When modeling analyte-additive interactions in capillary electrophoresis (CE), it is necessary to correct for all changes in the apparent electrophoretic mobility of an analyte that are not due to specific binding. Current models based on dynamic complexation have corrected for bulk viscosity changes in the background electrolyte (BGE) when additives are used, while assuming negligible changes in the dielectric constant and other physicochemical properties of the solution. In this report, a study of weak interactions between deoxyribonucleotides and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) revealed significant nonideality in binding isotherms. Changes in the dielectric properties of the solution due to the addition of high concentrations of HP-beta-CD to the BGE was observed to alter the electrophoretic mobility of analytes. A relative dielectric correction factor was required to normalize analyte mobilities to a reference state of zero additive concentration. The use of both a relative dielectric factor and a viscosity correction factor was found to increase the accuracy of the model, reflected by a higher degree of correlation between predicted and measured analyte mobilities. This type of correction is particularly relevant when studying weak analyte binding interactions or when using high concentrations of additive in the BGE. This work is vital for accurate determination of weak binding constants and mobility values, as well as providing a deeper understanding of the fundamental parameters influencing a separation in CE.

Selected fundamental aspects of chiral electromigration techniques and their application to pharmaceutical and biomedical analysis

While capillary electrophoresis has been established as a major enantioseparation technique within the last decade, the potential of capillary electrochromatography is still studied extensively. This review summarizes recent applications of electromigration techniques with regard to the enantioseparation of chiral drugs. The first part discusses the general aspects of migration models and the enantiomer migration order. The application of capillary electrophoresis to chiral pharmaceutical analysis considers recent literature on: (1) chiral resolutions of non-racemic mixtures of enantiomers for the development of assays and the determination of the stereochemical purity of the drugs, (2) chiral separations of compounds in pharmaceutical formulations and products, and (3) enantioseparations of drugs in biological samples. A shorter section devoted to chiral electrochromatography discusses some fundamental aspects as well as the application to the chiral analysis of drugs including bioanalysis.

Nonlinear analysis of dynamic binding in affinity capillary electrophoresis demonstrated for inclusion complexes of beta-cyclodextrin

The stability of the molecular host-guest inclusion complexes of beta-cyclodextrin with benzoate and four different hydroxybenzoates is investigated. For the measurement of the binding constants an experimental method is devised that is based on affinity capillary electrophoresis (ACE) with indirect UV absorbance detection. We derive an explicit equation for effective mobilities in ACE experiments without violation of rigorous mass balance. This equation is employed in the nonlinear least-squares analyses of the experimental data yielding binding constants of 48+/-2 M(-1) for benzoate, 299+/-38 M(-1) for 2-hydroxybenzoate, 37+/-1 M(-1) for 3-hydroxybenzoate, 228+/-9 M(-1) for 4-hydroxybenzoate, and 895+/-110 M(-1) in the case of 2,4-dihydroxybenzoate.

In vivo monitoring of amine neurotransmitters using microdialysis with on-line capillary electrophoresis

Microdialysis sampling was coupled via a flow-gated interface on-line to capillary electrophoresis with laser-induced fluorescence (LIF) detection for in vivo monitoring of neuroactive amino acids and amines. In the instrument, analytes are derivatized precolumn with o-phthaldehyde and beta-mercaptoethanol to form fluorescent isoindole products. The instrument was improved over previous designs by incorporating a sheath-flow cuvette for reduced background in LIF detection which improved sensitivity by 15-fold. The methodology was improved by utilizing a voltage ramped injection which allowed generation of 500000 theoretical plates with 20 s separations. Resolution of the isoindole derivatives was further improved by addition of hydroxypropyl-modified beta-cyclodextrin to the electrophoresis buffer. The new instrumentation and methods allow resolution and detection of glutamate, gamma-aminobutyric acid, glycine, aspartate, serine, taurine, glutamine and dopamine (if levels are elevated) collected from in vivo sampling probes every 20 s. The technique is suited to continuous monitoring for dynamic measurements of these compounds in vivo.