Enantioseparation of derivatized amino acids by capillary isoelectric focusing using cyclodextrin complexation

Separation and fractionation of glutamic acid and histidine via origami isoelectric focusing

We demonstrated the fractionation of two amino acids, glutamic acid and histidine, separated via isoelectric focusing (IEF) on filter paper folded and stacked in an origami fashion. Channels for electrophoresis were fabricated as circular zones acquired via wax printing onto the filter paper. An ampholyte solution with amphiphilic samples was deposited on all the circle zones, which was followed by folding to form the electrophoresis channels. IEF was achieved by applying an electrical potential between the anodic and cathodic chambers filled with phosphoric acid and sodium hydroxide solutions, respectively. A pH gradient was formed using either a wide-range ampholyte with a pH of 3 to 10 or a narrow-range version with a pH of 5 to 8, which was confirmed by adding pH indicators to each layer. The origami IEF was used to separate the amino acids, glutamic acid and histidine, by mixing with the ampholytes, which were deposited on the layers. The components in each layer were extracted with water and measured by high-performance liquid chromatography using pre-column derivatization with dansyl chloride. The results indicated that the focus for glutamic acid and that for histidine were at different layers, according to their isoelectric points. The origami isoelectric focusing achieved the fractionation of amino acids in less than 3 min using voltage as low as 30 V.

Some thoughts about enantioseparations in capillary electrophoresis

In this overview the goal of the authors was to analyze from the historical perspective the reasons of success and failure of chiral capillary electrophoresis. In addition, the current trends are analyzed, unique advantages of capillary electrophoresis are highlighted and some future directions are discussed.

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.

Application of capillary electrophoresis in determination of acid dissociation constant values

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

Enantioseparations by using capillary electrophoretic techniques. The story of 20 and a few more years

This paper provides the author's insight on the past, present and future of performing enantioseparations using capillary electrophoretic (CE) techniques. These techniques are discussed from the historical point of view, as well as based on their potential as the separation techniques of today and the future. The overview covers mechanistic as well as practical aspects of CE techniques.

Analysis of pD-Dependent complexation ofN-benzyloxycarbonylaminophosphonic acids by α-cyclodextrin. Enantiodifferentiation of phosphonic acid pKa values

The influence of aqueous solution pD on stereoselective complexation of N-benzyloxycarbonylaminophosphonic acids with alpha-cyclodextrin was investigated by means of nuclear magnetic resonance spectroscopy. The highest enantiodiscrimination was achieved at pD close to the pKa of less acidic hydroxyl group of the phosphonic moiety of analytes (6.5-7.5). This effect results from the stereoselective differentiation of pKa (up to 0.28 pD unit) upon complexation with applied chemical shift reagent. Moreover, analysis of 2D-ROESY spectra proved that the host-guest inclusion mode is strongly influenced by pD.

Determination of amino acids in saliva using capillary electrophoresis with fluorimetric detection

In the present study a sensitive method for the quantification of main free amino acids in saliva using capillary electrophoresis with laser induced fluorescence detection was developed. As background electrolyte 20 mM borate buffer pH 9.5 was used. Amino acids were separated after derivatization with fluorescein isothiocyanate (FITC) and the conditions for derivatization were optimized. The main amino acids occurring in saliva (Pro, Ser, Gly and Glu) were separated in less than 7 min. The parameters of validation such as linearity of response, precision and detection limits were determined. The detection limits were obtained in the range from 0.1 to 2.4 nM. The developed method was employed for determination of amino acids in real saliva samples.

Capillary electrophoresis direct enantioseparation of aromatic amino acids based on mixed chelate-inclusion complexation of aminoethylamino-beta-cyclodextrin

6(A)-(2-Aminoethylamino)-6(A)-deoxy-beta-cyclodextrin (CDen) was synthesized and formed a binary complex with Cu(II) which was shown to be an effective chiral selector for separation of underivatized amino acid enantiomers in capillary electrophoresis (CE). Moreover, the chiral resolution was greatly enhanced by the presence of polyethyl glycol (PEG) and tert-butyl alcohol in the running buffer. The optimum experimental conditions were 20 mmol/L CDen, 20 mmol/L CuSO(4).5H(2)O, 5.0 mg/mL PEG20000 and 1.0% v/v tert-butyl alcohol, pH 5.80. With the proposed method, the four selected aromatic chiral amino acid pairs were separated in less than 15 min.

Preparative-scale isoelectric trapping enantiomer separations

The new Gradiflow BF200 IET unit, developed for isoelectric trapping protein separations has been modified and used to carry out preparative-scale enantiomer separations. Hydroxypropyl beta-cyclodextrin was used as the chiral resolving agent to induce an isoelectric point difference between the enantiomers. Three isoelectric membranes with isoelectric points below, in between and above the isoelectric points of the complexed enantiomers were used to trap the separated enantiomers in the anodic and cathodic separation compartments of the Gradiflow BF200 IET apparatus, respectively. The production rates were about 15 times higher than those previously obtained with another isoelectric trapping device and about 30% higher than those obtained in a continuous free-flow electrophoretic device operated in the isoelectric focusing mode. The remarkable separation speed observed in the modified Gradiflow BF200 IET unit is attributed to the favorable interplay of the short electrophoretic transfer distance, the high electric field strength and the large effective surface areas of the isoelectric membranes.

Effect of feed zone width on product purity in preparative-scale, continuous free-flow isoelectric focusing separation of enantiomers

The effects of the increased width of the sample feed stream upon the purity of the collected fractions were examined in the continuous free-flow isoelectric focusing separation of the enantiomers of dansyl-tryptophan. Compared to the reference separation obtained with a narrow feed stream introduced through the central sample feed port of the continuous free-flow isoelectric focusing separation unit, the final pH gradient, the position of the enantiomer band centroids and the values of the cumulative product recoveries and cumulative product purities remained essentially identical as the width of the feed band of the racemic sample dissolved in the carrier ampholyte was increased up to the full width of the separation chamber suggesting that the current, limiting practice of narrow, central feed bands can be safely abandoned and dilute feedstock solutions can be utilized in preparative-scale isoelectric focusing enantiomer separations.

Use of full-column imaging capillary isoelectric focusing for the rapid determination of the operating conditions in the preparative-scale continuous free-flow isoelectric focusing separation of enantiomers

A rapid, simple method is proposed here for the identification of the experimental conditions that lead to satisfactory preparative-scale isoelectric focusing enantiomer separations in continuous free-flow electrophoretic units. The method first calls for the use of a commercially available, full-column imaging capillary electrophoretic system to find the background electrolyte composition that generates the largest pI difference between the bands of the enantiomers. The method then calls for the finding of the minimum residence time that permits full development of the pH gradient across the separation chamber of the continuous free-flow electrophoretic unit by measuring the pH in the sample-free carrier electrolyte fractions collected during these runs. Finally, the quality of the predicted preparative-scale separation is verified by analyzing the enantiomer-containing collected fractions by capillary electrophoresis using a 14-sulfated, single-isomer cyclodextrin as resolving agent. The pI difference values and production rate values observed in this work agree well with the literature values that were obtained by much more time-consuming methods.

Recent developments in chiral capillary electrophoresis and applications of this technique to pharmaceutical and biomedical analysis

This paper provides an overview of the current status of chiral capillary electrophoresis (CE). The emphasis is placed on the application of CE in chiral separation of various racemic compounds. During the last two years about 280 papers, several review articles, and two entire issues, edited by S. Fanali (Electrophoresis 1999, 20, 2577-2798, and H. Nishi and S. Terabe (J. Chromatogr. A 2000, 879, 1-471.) have been devoted to chiral CE. Enantiomeric separations of various compounds, e.g., pharmaceuticals, drug candidates, drugs and related metabolites in biological fluids, amino acids, di- and tri peptides, pesticides and fungicides, have been performed using different chiral selectors. Native and derivatized cyclodextrins continue to be the most widely used chiral selectors. Other chiral selectors such as natural and synthetic chiral micelles, crown ethers, chiral ligands, proteins, oligo- and polysaccharides, and macrocyclic antibiotics have also been applied to chiral CE separations.

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.

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.