Histamine-modified beta-cyclodextrins for the enantiomeric separation of dansyl-amino acids in capillary electrophoresis

Chiral selectors in CE: recent developments and applications

This review article provides an overview of the recent advances in enantioanalysis by use of electrophoretic techniques. Due to the big number of publications in the subject mentioned above, this article is focused on chiral method developments and applications published from 2008 until 2011, and it demonstrates chiral selectors used in CE. Numerous chiral selectors have been used over the years, and these include the cyclic and the linear oligo- and polysaccharides, the branched polysaccharides, the polymeric and monomeric surfactants, the macrocyclic and other antibiotics, and the crown ethers. Different dual-selector systems are also presented in this article, and the results are compared with those obtained by use of a single chiral selector. Finally, several pharmaceutical and biomedical applications based on chiral recognition are summarized.

Chiral separation and determination of excitatory amino acids in brain samples by CE-LIF using dual cyclodextrin system

Chiral capillary electrophoresis method has been developed to separate aspartate and glutamate enantiomers to investigate the putative neuromodulator function of D-Asp in the central nervous system. To achieve appropriate detection sensitivity fluorescent derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole and laser-induced fluorescence detection was applied. Although, simultaneous baseline separation of the two enantiomer pairs could be achieved by using 3 mM 6-monodeoxy-6-mono(3-hydroxy)propylamino-β-cyclodextrin (HPA-β-CD), further improvement of the chemical selectivity was required because of the high excess of L-enantiomers in real samples to be analyzed. The system selectivity was fine-tuned by combination of 8 mM heptakis(2,6-di-O-methyl)-β-cyclodextrin and 5 mM HPA-β-CD in order to increase the resolution between aspartate and glutamate enantiomers. The method was validated for biological application. The limits of detection for D-Asp and D-Glu were 17 and 9 nM, respectively, while the limit of quantification for both analytes was 50 nM. This is the lowest quantification limit reported so far for NBD-tagged D-Asp and D-Glu obtained by validated capillary electrophoresis laser-induced fluorescence method. The applicability of the method was demonstrated by analyzing brain samples of 1-day-old chickens. In all the studied brain areas, the D-enantiomer contributed 1-2 % of the total aspartate content, corresponding to 17-45 nmol/g wet tissue.

Selectively functionalized cyclodextrins and their metal complexes

Cyclodextrins (CDs) are cyclic oligomers of alpha-1,4-linked D-glucopyranose. Due to their unique structure, marked by a chiral and hydrophobic cavity, CDs have been extensively used as chiral selectors and drug delivery systems. The functionalization both improve the CD applications and widen their use in many other fields, such as molecular recognition and enzyme mimicking. Moreover, the functionalization highly increases the metal binding properties of the CDs. This critical review is a report of recent applications concerning the CD derivatives and their metal complexes. The metal ion assists the host-guest interaction often increasing the properties of CDs to act as chiral receptors. Furthermore, it can act as a catalytic center in the mimicking of metalloenzymes based on functionalized CDs (164 references).

Chiral separation of dansyl amino acids in capillary electrophoresis using mono-(3-methyl-imidazolium)-beta-cyclodextrin chloride as selector

Enantioseparations of fourteen dansyl amino acids were achieved by using a positively-charged single-isomer beta-cyclodextrin, mono-(3-methyl-imidazolium)-beta-cyclodextrin chloride, as a chiral selector. Separation parameters such as buffer pH, selector concentration, separation temperature, and organic modifier were investigated for the enantioseparation in order to achieve the maximum possible resolution. Chiral separation of dansyl amino acids was found to be highly dependent on pH since the degree of protonation of these amino acids can alter the strength of electrostatic interaction and/or inclusion complexation between each enantiomer and chiral selector. In general, the chiral resolution of dansyl amino acids was enhanced at higher pH, which indicates that the carboxylate group on the analytes may interact with the imidazolium group of cationic cyclodextrin. For most analytes, a distinct maximum in enantioresolution was obtained at pH 8.0. Moreover, the chiral separation can be further improved by careful tuning of the separation parameters such as higher selector concentration (e.g. 10 mM), lower temperature, and addition of methanol. Enantioseparation of a standard mixture of these dansyl amino acids was further achieved in a single run within 30 min.

Synthesis of mono-6-deoxy-6-N,N,N′,N′,N′-pentamethylethylenediammonio-cyclomaltoheptaose, a single-isomer, monosubstituted, permanently dicationicβ-CD and its use for enantiomer separations by CE

The dichloride salt of mono-6-deoxy-6-N,N,N',N',N'-pentamethylethylenediammonio-cyclomaltoheptaose (PEMEDA-BCD), the first single-isomer, monosubstituted, permanently dicationic beta-CD has been synthesized, analytically characterized, and used for the capillary electrophoretic separation of the enantiomers of a group of analytes in acidic and basic BGEs. When the concentration of PEMEDA-BCD was changed in the BGEs, the resulting effective mobilities of the analytes and the respective separation selectivities followed the predictions of the ionic strength-corrected charged resolving agent migration model. Good separation selectivities and favorable normalized EOF mobilities allowed for the rapid, efficient separation of the enantiomers of anionic, weak acid and nonionic analytes in the low- and/or high-pH BGEs.

Effect of alkylimidazolium substituents on enantioseparation ability of single-isomer alkylimidazolium-β-cyclodextrin derivatives in capillary electrophoresis

A family of 6-mono(3-alkylimidazolium)-beta-cyclodextrins with one primary hydroxyl group replaced by an alkylimidazolium cation has been developed. The effect of alkyl substitutents on the enantioresolution ability of these single-isomer cyclodextrins towards dansyl amino acids has been studied by capillary electrophoresis. Systematical investigations on the effect of buffer pH and selector concentration on the enatioseparation show that chiral selectors with a shorter alkyl chain (R=C(n)H(2n+1), n</=4) presented more powerful chiral recognition ability. These newly introduced single-isomer beta-cyclodextrin derivatives proved to be effective chiral selectors for most selected dansyl amino acids at low buffer pH (e.g. pH 5.0) with selector concentration no less than 3mM. The apparent complex stability constants between alkylimidazolium beta-CDs and dansyl amino acids were also theoretically determined by using the mobility difference model proposed by Wren and Rowe. The side alkyl chains from both dansyl amino acids and alkylimidazolium beta-CDs displayed significant effect on the apparent complex stability constants. Both the optimum selector concentrations calculated according to the model, however, were much lower than the experimental values giving the maximum chiral resolution of enantiomers.

Synthesis and application of single-isomer 6-mono(alkylimidazolium)-β-cyclodextrins as chiral selectors in chiral capillary electrophoresis

Novel single isomers of positively charged beta-CDs were prepared via nucleophilic substitution of 6-monotosyl-beta-CD with alkylimidazoles to afford 6-mono(alkylimidazolium)-beta-CD tosylates and then 6-mono(alkylimidazolium)-beta-CD chlorides by anion exchange. The chiral resolution abilities of these cationic CDs were studied by CE using dansyl (Dns)-amino acids as model analytes. From the experimental results, it was found that both resolution and selectivity of these cationic CDs were dependent on the following parameters: concentration of chiral selectors, pH of the running buffer, counteranions of the CDs, side chain length of the n-alkyl-imidazolium cation, temperature of the capillary column, and organic modifier used. The concentration of chiral selectors required for enantioseparation varied from 3 to 30 mM. The BGE pH played an important role in the resolution of Dns-amino acids. For acidic BGEs, chiral resolution increased with pH (4.0-6.0) and reached a local maximum at pH 6.0. However, better resolutions were obtained with basic phosphate buffer at pH 9.6. Methanol was found to be an effective organic modifier for the resolution of Dns-amino acids by CE.

Enantioseparation of acidic enantiomers in capillary electrophoresis using a novel single-isomer of positively charged β-cyclodextrin: Mono-6A-N- pentylammonium-6A-deoxy-β-cyclodextrin chloride

The new single-isomer of positively charged beta-cyclodextrin, mono-6A-N-pentylammonium-6A-deoxy-beta-cyclodextrin chloride (PeAM-beta-CD), was employed for the first time for the enantioseparation of anionic and ampholytic analytes by capillary electrophoresis (CE). The synthesis and characterization of PeAM-beta-CD were reported. The effect of background electrolyte (BGE) pH and selector concentration on the enantioseparation was investigated. Good separation was obtained at low BGE pH (ca. 5.0-6.0). The effective mobilities of all analytes were found to decrease with increasing CD concentration. PeAM-beta-CD proved to be an effective chiral selector for most studied anionic analytes.

A family of single-isomer positively charged cyclodextrins as chiral selectors for capillary electrophoresis: Mono-6A-butylammonium-6A-deoxy-β-cyclodextrin tosylate

A novel single-isomer positively charged beta-cyclodextrin (beta-CD), mono-6(A)-butylammonium-6(A)-deoxy-beta-cyclodextrin tosylate (BuAM-beta-CD), has been synthesized, characterized, and used for the enantioseparations of alpha-hydroxy acids, carboxylic acids, and ampholytic analytes by capillary electrophoresis in acidic aqueous background electrolytes. The effective mobilities of all studied analytes decreased with increasing concentration of CD. Satisfactory resolutions were obtained for alpha-hydroxy acids over a wide range of chiral selector concentration. The optimum CD concentration was lower than 5 mM for the carboxylic acids, while higher than 20 mM for alpha-hydroxy acids. Inclusion complexation in combination with ion pair interaction seemed to account for the chiral discrimination process. The hydrogen bonding may provide secondary contribution for the chiral resolution of alpha-hydroxy acids. In addition, BuAM-beta-CD was further proved to be an effective chiral selector for anionic analytes by the baseline enantioseparation of a six-acid mixture within 20 min.

Enantioseparation of dansyl amino acids by a novel permanently positively charged single-isomer cyclodextrin: Mono-6-N-allylammonium-6-deoxy-β-cyclodextrin chloride by capillary electrophoresis

A permanently positively charged single-isomer β-CD, mono-6-N-allylammonium-6-deoxy-β-CD chloride, ALAM-β-CD, has been synthesized and successfully used as chiral selector in capillary electrophoresis for enantioseparation of amino acids and dansyl amino acids. The effects of pH, CD's concentration and capillary length on enantioseparation were studied in order to investigate its potential as chiral selector for acidic racemates. By increasing buffer's pH and CD's concentration, fairly robust separations were achieved. Good separations were also obtained with a shorter capillary. In addition, baseline enantiomeric separations of a mixture of six pairs of dansyl dl-amino acids were achieved under different pH conditions.

The 6-derivative of β-cyclodextrin with succinic acid: a new chiral selector for CD-EKC

6-O-Succinil-beta-cyclodextrin (CDsuc6) was synthesized with very good yield by one pot synthesis and characterized by NMR spectroscopy and ESI-MS. It was used as a chiral selector in capillary electrophoresis to resolve catecholamine racemates, namely norepinephrine, epinephrine, terbutaline and norphenilephrine. The CE experiments at pH 5.6 show very promising selector ability by 6-O-succinil-beta-cyclodextrin for the chiral recognition of all the catecholamines tested, while at pH 9.2, only racemic terbutaline was successfully separated.

Capillary electrophoresis of highly sulfated flavanoids and flavonoids

Flavanoids and flavonoids are natural products present in our diet and known to possess multiple biological activities. Sulfated species of these natural products represent highly charged water-soluble organic molecules that possess unique biochemical properties. We describe here the first studies on capillary electrophoresis of these highly charged molecules. Fully sulfated flavanoids and flavonoids can be electrophoresed and resolved under reverse polarity at pH 3.5 using 5-10 kV in less than 20 min. In contrast, at high pH under normal polarity these species can be electrophoresed only if a pressurized capillary is employed. (+/-)-Catechin sulfate, a racemic sulfated flavanoid, was resolved into its enantiomers using 15% beta-cyclodextrin, a chiral selector, but not with alpha- or gamma-cyclodextrins. Yet, the high charge density of these molecules challenges the resolving capability of capillary electrophoresis as diastereomers (-)-epicatechin sulfate and (+)-catechin sulfate do not resolve, even in the presence of cyclodextrins or chiral positively charged amino acids. Overall, capillary electrophoresis of highly sulfated flavanoids and flavonoids is expected to be useful in rapid structure analysis of sulfated flavonoids, either synthetic or natural.

Enantiomeric separation of hydroxy acids and carboxylic acids by diamino-beta-cyclodextrins (AB,AC,AD) in capillary electrophoresis

Selectively modified 6,6'-dideoxy-6,6'-L-diamino-beta-cyclodextrins (AB, AC, AD) were successfully used as chiral selectors for the enantiomeric separation of hydroxy acids and carboxylic acids (in particular, phenoxyalkanoic acid herbicides) in capillary electrophoresis (CE). Chiral separations were obtained at a low selector concentration (1 mM) with good enantioselectivity and resolution factors. Separations were optimized as a function of pH. The different position of the charged groups on the upper rim greatly influenced the separation, accounting for electrostatic interactions between the protonated amino groups of the cyclodextrins (CDs) and the carboxylate of the selectands. The best enantiomeric separation of hydroxy acids was obtained with the AC regioisomer, whereas carboxylic acids were well resolved only by the AB regioisomer. A recognition model is proposed, based on two-dimensional nuclear magnetic resonance (2-D NMR) experiments, in which the orientation of the guest in the inclusion complex is determined by the electrostatic interactions between the selectand and the CD upper rim.

Enantioseparations in capillary electromigration techniques: recent developments and future trends

This review summarizes the current status of enantioseparations using capillary electromigration techniques and gives the authors insights on the selected fundamental aspects and future trends in this field. The most recent developments in the field of chiral separations using capillary electrophoresis (CE) and capillary electrochromatography (CEC) are summarized. The status of chiral electromigration techniques is evaluated tacking into account the most recent developments in related techniques such as chiral HPLC, GC and SFC.

Recent progress in chiral separation principles in capillary electrophoresis

This review summarizes recent developments in the field of chiral separations by electromigration techniques including capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), isotachophoresis (ITP), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC). This overview focuses on the development of new chiral selectors and the introduction of new techniques rather than applications of already established selectors and methods. The mechanisms of the different chiral separation principles are discussed.

Recent innovations in the use of charged cyclodextrins in capillary electrophoresis for chiral separations in pharmaceutical analysis

A review is presented on the use of charged cyclodextrins (CDs) as chiral selectors in capillary electrophoresis (CE) for the separation of analytes in pharmaceutical analysis. An overview is given of theoretical models that have been developed for a better prediction of the enantiomeric resolution and for a better understanding of the separation mechanism. Several types of charged CDs have been used in chiral capillary electrophoretic separation (anionic, cationic, and amphoteric CDs). Especially the anionic CDs seem to be valuable due to the fact that many pharmaceutically interesting compounds can easily be protonated (e.g., amine groups). For that reason several anionic CDs are now commercially available. Cationic and amphoteric CDs are less common in chiral analysis and only a few are commercially available. Attention is paid to the most common synthesis routes and the characterization of the CDs used in chiral capillary electrophoretic separations. The degree of substitution in the synthesized CDs may vary from one manufacturer to another or even from batch to batch, which may have a detrimental effect on the reproducibility and ruggedness of the separation system. In Sections 4, 5, and 6 the applications of anionic, cationic, and amphoteric CDs for the chiral separation in CE are described. Many interesting examples are shown and the influence of important parameters on the enantioselectivity is discussed.

Enantioselective determination by capillary electrophoresis with cyclodextrins as chiral selectors

This review surveys the separation of enantiomers by capillary electrophoresis using cyclodextrins as chiral selector. Cyclodextrins or their derivatives have been widely employed for the direct chiral resolution of a wide number of enantiomers, mainly of pharmaceutical interest, selected examples are reported in the tables. For method optimisation, several parameters influencing the enantioresolution, e.g., cyclodextrin type and concentration, buffer pH and composition, presence of organic solvents or complexing additives in the buffer were considered and discussed. Finally, selected applications to real samples such as pharmaceutical formulations, biological and medical samples are also discussed.

Chiral separation of amino acids and peptides by capillary electrophoresis

Chiral separation of amino acids and peptides by capillary electrophoresis (CE) is reviewed regarding the separation principles of different approaches, advantages and limitations, chiral recognition mechanisms and applications. The direct approach details various chiral selectors with an emphasis on cyclodextrins and their derivatives, antibiotics and chiral surfactants as the chiral selectors. The indirect approach deals with various chiral reagents applied for diastereomer formation and types of separation media such as micelles and polymeric pseudo-stationary phases. Many derivatization reagents used for high sensitivity detection of amino acids and peptides are also discussed and their characteristics are summarized in tables. A large number of relevant examples is presented illustrating the current status of enantiomeric and diastereomeric separation of amino acids and peptides. Strategies to enhance the selectivity and optimize separation parameters by the application of experimental designs are described. The reversal of enantiomeric elution order and the effects of organic modifiers on the selectivity are illustrated in both direct and indirect methods. Some applications of chiral amino acid and peptide analysis, in particular, regarding the determination of trace enantiomeric impurities, are given. This review selects more than 200 articles published between 1988 and 1999.

Recent advancements in amino acid analysis using capillary electrophoresis

Recent advances in the analysis of amino acids using capillary electrophoresis are addressed. This area of research continues to receive increased attention as is evident from the 62 references reviewed. This review discusses current detection strategies including UV absorbance, laser-induced fluorescence, electrochemical, and others. Separation methodologies for both derivatized and underivatized amino acids are reviewed. Both direct and indirect enantiomeric resolution of amino acids are addressed. Applications utilizing capillary electrophoresis for the analysis of amino acids are discussed. This review covers literature published in 1997 and 1998.

Histamine-modified cationic beta-cyclodextrins as chiral selectors for the enantiomeric separation of hydroxy acids and carboxylic acids by capillary electrophoresis

The enantiomeric separation of alpha-hydroxy acids and carboxylic acids was successfully performed by using 6-deoxy-6-N-histamino-beta-cyclodextrin (CD-hm), a monosubstituted positively charged beta-cyclodextrin (beta-CD) bearing a histamine moiety linked to the C6 of a glucose unit in the upper CD rim via the amino group. Good results were obtained at a low selector concentration (1 mM). The number of positive charges on the upper rim may be modulated as a function of pH, because of the different pKa of the amino and the imidazolyl groups, and was found to affect both the enantioselectivity and resolution factors. With the analogous 6-deoxy-[4-(2-aminoethyl)imidazolyl]-beta-cyclodextrin (CD-mh) bearing the histamine moiety linked to the C6 via the imidazolyl group, very poor results were obtained, showing that the proximity of the positive charge to the cavity plays an important role in the enantiomeric recognition. The complexation mode was studied by electrospray ionization-mass spectrometry (ESI-MS) and two-dimensional nuclear magnetic resonance (2-D NMR) ROESY experiments: the recognition model is consistent with an inclusion complexation of the aromatic ring of the analyte within the CD cavity coupled to electrostatic interactions between the carboxylate and the protonated amino group of the cyclodextrin.

Enantiomeric separations of dansyl amino acids using the macrocyclic antibiotic A35512B as a chiral selector in capillary electrophoresis

The macrocyclic antibiotic A35512B was examined as a chiral selector for capillary electrophoresis (CE) using thirteen racemic dansyl amino acids as test analytes. The chiral selectivity of A35512B was evaluated as a function of the run buffer pH, antibiotic concentration, and organic modifier composition. After optimizing these parameters, the macrocylic antibiotic A35512B provided high resolutions of all the enantiomers for the thirteen dansyl amino acids tested in this study.

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.

Determination of aspartic acid enantiomers in bio-samples by capillary electrophoresis

Enantiomeric separation and detection of D,L-aspartic acid (Asp) derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) by capillary electrophoresis (CE) using modified cyclodextrins as chiral selectors was studied. Heptakis(2,3, 6-tri-O-methyl)-beta-cyclodextrin(TM-beta-CD) was most effective for enantiomeric separation of NBD-D,L-Asp with optimum conditions of 30 mM TM-beta-CD in 50 mM phosphate buffer (pH 4.0) and the limit of detection (LOD) attained was 100 nM for each enantiomer. The method proposed in the present study was convenient for both D- and L-Asp determination since the other amino compounds migrated differently and D-Asp in bio-samples such as rat pineal gland and foods was determined with a simple sample pretreatment and a short analysis run time.

Separation of chiral compounds by capillary electrophoresis

This review presents the different chiral selectors used in capillary electrophoresis (CE) for the separation of enantiomers. The use of charged cyclodextrins, crown ethers, polysaccharides, proteins, natural and synthetic micelles, macrocyclic antibiotics and ergot alkaloids is discussed in detail. Neutral native and derivatized cyclodextrins are not treated because several review articles have already been published on this topic. Recent developments like the application of two chiral selectors in the same background electrolyte are highlighted.

Cationic beta-cyclodextrin derivative for chiral separations

A novel hepta-substituted beta-cyclodextrin bearing the methoxyethylamine group linked to the upper cyclodextrin rim was successfully used as a chiral selector for enantiomeric separation of non-steroidal anti-inflammatory drugs (NSAIDs) and phenoxypropionic acid herbicides (PPAHs). Separation parameters such as pH and concentration were found to have major influences on enantiomeric resolution of the NSAIDs and PPAHs. Results indicate that heptakis(6-methoxyethylamine-6-deoxy)-beta-cyclodextrin [beta-CD-OMe (VII)] performs exceptionally well for the enantiomeric resolution of NSAIDs: indoprofen and fenoprofen (Rs = 11 and 14, respectively). In addition, baseline enantiomeric separation of a mixture of six pairs of PPAHs was achieved in under 30 min. Compared to other cationic beta-cyclodextrins reported in the literature, the beta-CD-OMe (VII) showed improved selectivity for both classes of the aforementioned anionic racemates.