Enantiomeric resolution of primary amines by capillary electrophoresis and high-performance liquid chromatography using chiral crown ethers

Past, present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.

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.

Computational modeling of capillary electrophoretic behavior of primary amines using dual system of 18-crown-6 and β-cyclodextrin

Using capillary electrophoresis (CE) three chiral primary amine compounds 1-aminoindan (AI), 1-(1-naphthyl)ethylamine (NEA) and 1,2,3,4-tetrahydro-1-naphthylamine (THAN), exhibited only partial or no separation when β-cyclodextrin (βCD) was used as chiral selector. The use of 18-crown-6 (18C6) as a second additive with βCD resulted in an enhanced separation. A molecular modeling study, using molecular mechanics and the semiempirical PM6 calculations, was used to help explaining the mechanism of the enantiodifferentiation and to predict the separation process. Optimization of the structures of the complexes by the PM6 method indicate that the poor separation obtained in the presence of the βCD chiral selector alone is due to the small binding energy differences (ΔΔE) of 4.7, 1.1 and 1.2 kcal mol(-1) for AI, NEA and THAN, respectively. In the presence of 18C6 it was suggested that a sandwich compound between 18C6, amine and βCD is formed. Theoretical calculations show that a significant increase in the binding energy is obtained for the sandwich compounds indicating strong hydrophobic and van der Waals interactions that show enhanced enantiodifferentiation.

Development of new HPLC chiral stationary phases based on native and derivatized cyclofructans

An unusual class of chiral selectors, cyclofructans, is introduced for the first time as bonded chiral stationary phases. Compared to native cyclofructans (CFs), which have rather limited capabilities as chiral selectors, aliphatic- and aromatic-functionalized CF6s possess unique and very different enantiomeric selectivities. Indeed, they are shown to separate a very broad range of racemic compounds. In particular, aliphatic-derivatized CF6s with a low substitution degree baseline separate all tested chiral primary amines. It appears that partial derivatization on the CF6 molecule disrupts the molecular internal hydrogen bonding, thereby making the core of the molecule more accessible. In contrast, highly aromatic-functionalized CF6 stationary phases lose most of the enantioselective capabilities toward primary amines, however they gain broad selectivity for most other types of analytes. This class of stationary phases also demonstrates high "loadability" and therefore has great potential for preparative separations. The variations in enantiomeric selectivity often can be correlated with distinct structural features of the selector. The separations occur predominantly in the presence of organic solvents.

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.

Effect of the residual silanol group protection on the liquid chromatographic resolution of racemic primary amino compounds on a chiral stationary phase based on optically active (3,3′-diphenyl-1,1′-binaphthyl)-20-crown-6

A liquid chromatographic chiral stationary phase (CSP) based on (3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6, which has been utilized in the resolution of alpha-amino acids, amines and amino alcohols, was treated with excess of n-octyltriethoxysilane to prepare a new improved CSP. The residual silanol groups of the original CSP were protected by n-octyl groups in the new CSP. The chiral recognition ability of the new CSP was superior to that of the original CSP in the resolution of alpha-amino acids, amines and amino alcohols. Retention factors (k1) for the resolution of alpha-amino acids were lower on the new CSP than on the original CSP while those for the resolution of amines and amino alcohols were higher on the new CSP than on the original CSP. The improved chiral recognition ability of the new CSP and the retention behaviors of the two enantiomers on the new CSP have been rationalized to stem from the removal of the non-enantioselective interactions between the analytes and the residual silanol groups of the original CSP and the improved lipophilicity of the CSP.

Newly synthesized tetraoxa-diaza crown ether derivatives versus commercialized crown ethers in the separation of positional isomers with capillary electrophoresis

Three new tetraoxa-diaza derivatives of 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (R-1, R-2 and R-3) and three commercially available crown ethers, 18-crown-6 (18C6), (+)-18-crown-6-tetracarboxilic acid (18C6H4) and 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane, were investigated to separate the positional isomers of aminophenol, aminobenzoic acid and aminocresol. The running electrolyte, in which the crown ethers were dissolved, was a 50 mM Tris solution adjusted to pH 2.0 with hydrochloric acid. Using 50 mM H3PO4 buffer, whose pH was adjusted to 2.0 with Tris, or only hydrochloric acid solution with the same pH, did not allow good separations for the tested components. The effect of the crown ether concentration on the separation of the 11 positional isomers was studied in the concentration range of 10-50 mM. The best separations were achieved using the 18C6 and the 18C6H4 crown ethers: 9 isomers out of 11 could be separated within one run. The m- and p-aminophenol isomers could not be separated under the investigated experimental conditions. The newly synthesized tetraoxa-diaza crown ether derivatives were only found suitable for the separation of aminobenzoic acid positional isomers. The macrocyclic ring of the tetraoxa-diaza crown ethers was not able to form a stable inclusion complex with the tested positional isomers. Consequently, the aminophenol and aminocresol isomers were not separated, the isomers migrated with the same or very similar velocities.

Enantiomeric separation by capillary electrophoresis using a crown ether as chiral selector

This paper reviews chiral separations of primary amines by capillary electrophoresis and crown ether as chiral selector. Two possible mechanisms of chiral recognition by host-guest complexation are discussed: (i) The substituents of the crown ether act as barriers for the guest compounds, and (ii) lateral electrostatic interactions between host and guest occur. Experimental conditions affecting the separation are discussed in detail. A literature overview of practical applications is presented as well. More than 80 different primary amines were analyzed, whereupon the majority could be resolved using a screening method. It is shown that a synergistic effect on the resolution of chiral amines is observed when the chiral crown ether and cyclodextrins are simultaneously used in the same buffer system. This approach opens interesting perspectives for further method optimization.

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.

Advances in capillary electrophoresis

This review summarizes the advancement in operational modes and selected applications of the title technique over the past five years. Regarding operational modes particular emphasis is put upon increasing selectivity and resolution, hyphenation of capillary electrophoresis with techniques based on other than electromigration principles, the so-called chip technology and new ways of detection. In applications selected examples of chiral separation and separation of biopolymers (proteins, nucleic acids) are emphasized. It is demonstrated that capillary electrophoresis represents a complementary technique to high-performance column chromatography and in a number of cases it offers better separations than standard chromatographic procedures.

Mechanism of capillary electrophoresis enantioseparations using a combination of an achiral crown ether plus cyclodextrins

The addition of an achiral crown ether (18-crown-6) to a cyclodextrin-based separation can significantly affect the capillary electrophoresis (CE) enantioresolution of organic racemates that contain a primary amine functional group. In most cases an enhancement of the enantioseparation was observed. However, there are also cases where the addition of 18-crown-6 was detrimental to a cyclodextrin-based CE enantioseparation. The effect of concentration of the two complexing additives as well as the effect of pH and added potassium ion were examined. A specific three-body complex involving simultaneous, dual inclusion complex formation can be used to explain both the enhanced and diminished enantioselectivities observed when 18-crown-6 is added to the run buffer.

Chiral separation of primary amino compounds using a non-chiral crown ether with beta-cyclodextrin by capillary electrophoresis

A non-chiral crown ether (18-crown-6) along with beta-cyclodextrin (beta-CD) was used to achieve enantioselective separations of primary amino compounds in capillary electrophoresis. In this new method, the amino group of these compounds is protonated in a low pH separation buffer and forms a selective host-guest complex with the crown ether (amino compound+18-crown-6). The hydrophobic portion of the host-guest complex is then incorporated into the cavity of the beta-cyclodextrin. The amino compound is sandwiched between the crown ether and the cyclodextrin (18-crown-6+amino compound+beta-CD) and thus determines or enhances the enantioselective recognition. It is postulated that the formation of this sandwich results in a more selective chiral interaction between the molecule and beta-cyclodextrin. The chiral recognition is dependent upon the formation of this sandwich complex. This method has been used to achieve enantioselectivity of primary amino compounds with a wide variety of substitutions.

HPCE determination of R(+) and S(-) mepivacaine in human serum using a derivatized cyclodextrin and ultraviolet detection

A high performance capillary electrophoresis assay for the quantitative determination of R(+) and S(-) mepivacaine in human serum is reported using heptakis (2,6-di-O-methyl) beta-cyclodextrin as the chiral selector for the separation of the enantiomers. The background electrolyte was a 100 mM phosphate buffer (pH 2.5) containing 20 mM heptakis (2,6-di-O-methyl) beta-cyclodextrin and 30 nM hexadecyltrimethylammonium bromide (HTAB). A 72 cm uncoated fused silica capillary was used for the analysis. HTAB was used as the buffer additive to decrease the adsorption of endogenous substances onto the silica wall. To separate the analytes of interest from the endogenous serum substances, a liquid-liquid extraction procedure was used. The extraction recoveries were greater than 70% for both R(+) and S(-) mepivacaine. The detection limits were around 150 ng ml-1 using 1 ml of serum and the limits of quantitation were 200 ng ml-1. The calibration curves were linear over a range of 200-2000 ng ml-1 with R(-) prilocaine as internal standard (IS) and coefficients of determination were greater than 0.999 (n=3). Precision and accuracy of the method were 4.1-7.2 and 2.6-5.9%, respectively, for R(+) mepivacaine and 4.0-7.4 and 3.2-7.4% for respectively, for S(-) mepivacaine. The HPCE method was compared to an existing HPLC method in terms of sensitivity and selectivity for the routine analysis of the drugs.