Comparative Enantiomer-Resolving Ability of Coated and Covalently Immobilized Versions of Two Polysaccharide-Based Chiral Selectors in High-Performance Liquid Chromatography

The separation of enantiomers of over 175 randomly selected chiral acidic, basic, and neutral compounds was studied on 4 polysaccharide-based chiral columns made by coating or covalent attachment of cellulose 3,5-dichlorophenylcarbamate or amylose 3,5-dimethylphenylcarbamate on the surface of silica. Triscarbamate derivatives of cellulose or amylose were used for the preparation of coated-type columns, while in the case of covalently immobilized chiral stationary phases, the respective polysaccharides were not completely carbamoylated but only close to triscarbamates. It was found that this minimal difference in the chemical composition of the polysaccharide derivatives resulted in significantly different enantiomer-resolving ability for certain groups of chiral compounds while only marginally different for other chiral analytes. This potential difference between coated- and covalently immobilized versions of the “same” chiral selector must be considered in method development with these columns, as well as in method transfer between them.     

Separation of terbutaline enantiomers in capillary electrophoresis with neutral cyclodextrin-type chiral selectors and investigation of the structure of selector-selectand complexes using nuclear magnetic resonance spectroscopy

The major goal of this study was to determine the affinity pattern of the terbutaline (TB) enantiomers toward α-, β-, γ-, and heptakis(2,3-di-O-acetyl)-β-cyclodextrins and using NMR spectroscopy for the understanding of the fine mechanisms of interaction between the cyclodextrins (CD) and TB enantiomers. It was shown once again that CE in combination with NMR spectroscopy represents a sensitive tool to study the affinity patterns and structure of CD complexes with chiral guests. Opposite affinity patterns of TB enantiomers toward native α- and β-CDs were associated with significant differences between the structure of the related complexes in solution. In particular, the complex between TB enantiomers and α-CD was of the external type, whereas an inclusion complex was formed between TB enantiomers and β-CD. One of the possible structures of the complex between TB and heptakis(2,3-di-O-acetyl)-β-CD (HDA-β-CD) was quite similar to that of TB and β-CD, although the chiral recognition pattern and enantioselectivity of TB complexation with these two CDs were very different.     

The Effect of Enantiomer Elution Order on the Determination of Minor Enantiomeric Impurity in Ketoprofen and Enantiomeric Purity Evaluation of Commercially Available Dexketoprofen Formulations

In a recent study, opposite enantiomer elution order was observed for ketoprofen enantiomers on two amylose-phenylcarbamate-based chiral columns with the same chemical composition of the chiral selector but in one case with coated while in the other with an immobilized chiral selector. In the present study, the influence of this uncommon effect on method validation parameters for the determination of minor enantiomeric impurity in dexketoprofen was studied. The validated methods with two alternative elution orders for enantiomers were applied for the evaluation of enantiomeric impurity in six marketed dexketoprofen formulations from various vendors. In most of these formulations except one the content of enantiomeric impurity exceeded 0.1% (w/w).     

Enantioseparation of glutethimide and its 5-OH-metabolite in capillary electrophoresis and study of selector-selectand interactions using one-dimensional rotating frame nuclear Overhauser and exchange spectroscopy

Enantioseparation of glutethimide (GT) and its 5-hydroxy metabolite (5-OH-GT) has been studied with several charged cyclodextrin (CD) derivatives. The emphasis was made on the enantiomer migration order of GT and simultaneous enantioseparation of GT and 5-OH-GT. The possible structural differences of GT complexes with three different single isomer charged CD derivatives were studied using one-dimensional rotating frame nuclear Overhauser and exchange spectroscopy (1-D ROESY).     

Native and substituted cyclodextrins as chiral selectors for capillary electrophoresis enantioseparations: Structures,features, application,and molecular modeling

CDs are cyclic oligosaccharides consisting of α-d -glucopyranosyl units linked through 1,4-linkages, which are obtained from enzymatic degradation of starch. The coexistence of hydrophilic and hydrophobic regions in the same structure makes these macrocycles extremely versatile as complexing host with application in food, cosmetics, environmental, agriculture, textile, pharmaceutical, and chemical industries. Due to their inherent chirality, CDs have been also successfully used as chiral selectors in enantioseparation science, in particular, for CE enantioseparations. In the last decades, multidisciplinary approaches based on CE, NMR spectroscopy, X-ray crystallography, microcalorimetry, and molecular modeling have shed light on some aspects of recognition mechanisms underlying enantiodiscrimination. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a useful tool to model at molecular level the dynamics of diastereomeric associate formation to sample low-energy conformations, the binding energies between the enantiomer and the CD, and to profile noncovalent interactions contributing to the stability of CD/enantiomer association. On this basis, the aim of this review is to provide the reader with a critical overview on the applications of CDs in CE. In particular, the contemporary theory of the electrophoretic technique and the main structural features of CDs are described, with a specific focus on techniques, methods, and approaches to model CE enantioseparations promoted by native and substituted CDs. A systematic compilation of all published literature has not been attempted.     

High‐performance liquid chromatographic separations of stereoisomers of chiral basic agrochemicals with polysaccharide‐based chiral columns and polar organic mobile phases

The separation of the stereoisomers of 23 chiral basic agrochemicals was studied on six different polysaccharide‐based chiral columns in high‐performance liquid chromatography with various polar organic mobile phases. Along with the successful separation of analyte stereoisomers, emphasis was placed on the effect of the chiral selector and mobile phase composition on the elution order of stereoisomers. The interesting phenomenon of reversal of enantiomer/stereoisomer elution order function of the polysaccharide backbone (cellulose or amylose), type of derivative (carbamate or benzoate), nature, and position of the substituent(s) in the phenylcarbamate moiety (methyl or chloro) and the nature of the mobile phase was observed. For several of the analytes containing two chiral centers all four stereoisomers were resolved with at least one chiral selector/mobile phase combination.     

Updating a chiral separation strategy for non-acidic drugs with capillary electrochromatography applicable for both chlorinated and non-chlorinated polysaccharide selectors

A generic strategy for the chiral separation of non-acidic pharmaceuticals was updated to complete an approach defined earlier. The selected chiral stationary phases are all polysaccharide selectors, chlorinated, and non-chlorinated, namely Lux(?) Amylose 2, Chiralcel(?) OD-RH, Lux(?) Cellulose 4, and Chiralpak(?) AD-RH. In this study, the screening step of a strategy defined earlier was updated and the optimization steps were re-evaluated for the applied chiral stationary phases. These screening and optimization conditions were studied by analyzing 20 pharmaceuticals at different organic modifier contents, temperatures, or applied voltages. The proposed chiral separation strategy was then evaluated with a test set of 19 non-acidic drugs. Seventeen compounds (89.5%) of the latter set could be resolved of which eight (42%) were baseline separated. The strategy thus proved to be applicable on compounds different from those used for its development.     

Separation of enantiomers of ephedrine by capillary electrophoresis using cyclodextrins as chiral selectors: comparative CE, NMR and high resolution MS studies

The enantiomer migration order (EMO) of ephedrine was investigated in the presence of various CDs in CE. The molecular mechanisms of chiral recognition were followed for the ephedrine complexes with native α- and β-CD and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-CD (HDAS-β-CD) by CE, NMR spectroscopy and high-resolution MS. Minor structural differences were observed between the complexes of ephedrine with α- and β-CD although the migration order of enantiomers was opposite when these two CDs were applied as chiral selectors in CE. The EMO was also opposite between β-CD and HDAS-β-CD. Significant structural differences were observed between ephedrine complexes with the native CDs and HDAS-β-CD. The latter CD was advantageous as chiral CE selector not only due to its opposite electrophoretic mobility compared with that of the cationic chiral analyte, but also primarily due to its enhanced chiral recognition ability towards the enantiomers of ephedrine.     

Enantiomeric separation of FMOC-amino acids by nano-LC and CEC using a new chiral stationary phase, cellulose tris(3-chloro-4-methylphenylcarbamate)

A novel polysaccharide-based chiral stationary phase (CSP), cellulose tris(3-chloro-4-methylphenylcarbamate), also known as Sepapak-2 or Lux Cellulose-2, has been evaluated for the enantiomeric separation of FMOC derivatives of amino acids. After mobile-phase optimization in nano liquid chromatography (nano-LC) the column enabled the enantiomeric separation of 19 out of 23 amino acids tested, indicating the high chiral recognition power of this new CSP. Subsequently, a comparison of the driving force employed (pressure or voltage) was carried out comparing nano-LC and CEC under the same conditions. Better peak efficiencies and resolution were observed by using CEC experiments, which enabled the chiral discrimination of 20 out of 23 amino acids tested. Finally, in order to show the potential of this new CSP, the determination of the content and the enantiomeric purity of the non-protein amino acid citrulline in food supplements was performed. For that purpose, the method was optimized, evaluated and applied to different commercial samples.