Proposal for the classification of high-performance liquid chromatographic chiral stationary phases: How to choose the right column

Chiral chromatography method screening strategies: Past, present and future

Method screening is an integral part of chromatographic method development for the separation of racemates. Due to the highly complex retention mechanism of a chiral stationary-phase, it is often difficult, if not impossible, to device predefined method-development steps that can be successfully applied to a wide group of molecules. The standard approach is to evaluate or screen a series of stationary and mobile-phase combinations to increase the chances of detecting a suitable separation condition. Such a process is often the rate-limiting step for high-throughput analyses and purification workflows. To address the problem, several solutions and strategies have been proposed over the years for reduction of net method-screening time. Some of the strategies have been adopted in practice while others remained confined in the literature. The main objective of this review is to revisit, critically discuss and compile the solutions published over the last two decades. We expect that making the diverse set of solutions available in a single document will help assessing the adequacy of existing screening protocols in laboratories conducting chiral separation.

Enantiomeric Mixtures in Natural Product Chemistry: Separation and Absolute Configuration Assignment

Chiral natural product molecules are generally assumed to be biosynthesized in an enantiomerically pure or enriched fashion. Nevertheless, a significant amount of racemates or enantiomerically enriched mixtures has been reported from natural sources. This number is estimated to be even larger since the enantiomeric purity of secondary metabolites is rarely checked in the natural product isolation pipeline. This latter fact may have drastic effects on the evaluation of the biological activity of chiral natural products. A second bottleneck is the determination of their absolute configurations. Despite the widespread use of optical rotation and electronic circular dichroism, most of the stereochemical assignments are based on empirical correlations with similar compounds reported in the literature. As an alternative, the combination of vibrational circular dichroism and quantum chemical calculations has emerged as a powerful and reliable tool for both conformational and configurational analysis of natural products, even for those lacking UV-Vis chromophores. In this review, we aim to provide the reader with a critical overview of the occurrence of enantiomeric mixtures of secondary metabolites in nature as well the best practices for their detection, enantioselective separation using liquid chromatography, and determination of absolute configuration by means of vibrational circular dichroism and density functional theory calculations.

Enantioselective separation of all-E-astaxanthin and its determination in microbial sources

A method for the enantioselective separation of all-E-astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione), an important colorant in the feed industry, was developed. Different chiral stationary phases (CSPs) such as Pirkle phases (R,R Ulmo and l-leucine), modified polysaccharides and a beta-cyclodextrin have been investigated on their separation performance of astaxanthin enantiomers. Direct resolution was only achieved employing the Chiralcel OD-RH (cellulose-tris-3,5-dimethylphenyl-carbamate) under reversed phase conditions. The chiral separation of the enantiomeric forms of astaxanthin produced in microalgae and yeasts was reported. The yeast Xanthophyllomyces sp. produces astaxanthin predominantly in the R,R configuration, whereas in the green microalgae Scenedesmus sp. astaxanthin is built primarily in the S,S form. The separation method for the identification of astaxanthin enantiomers is of great interest since astaxanthin is used as functional food additive in human nutrition. Moreover the method may be used as a food chain indicator in farmed salmon.

Solvation of the N-(1-Phenylethyl)-N‘-[3-(triethoxysilyl)propyl]-urea Chiral Stationary Phase in Mixed Alcohol/Water Solvents

A theoretical and experimental study of alcohol/water and alcohol/alcohol solvent mixtures near a surface of N-(1-phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea (PEPU), a Pirkle-type chiral stationary phase, is presented. Molecular dynamics simulations are performed at room temperature for water/methanol, water/1-propanol, water/2-propanol, and methanol/1-propanol solvent mixtures confined between two PEPU surfaces. The interface was also prepared experimentally by attaching the PEPU molecules to atomic force microscopy tips and oxidized Si(111) substrates. Chemical force spectrometric measurements between such PEPU-terminated tips and samples were taken in the solvent mixtures, and the results are compared to the molecular dynamics study. We find that the extent of hydrogen bonding at the surface is the dominant contributor to the measured forces.

Chiral liquid chromatography contribution to the determination of the absolute configuration of enantiomers

The review covers examples in which chiral HPLC, as a source of pure enantiomers, has been combined with classical methods (X-ray, vibrational circular dichroism (VCD), enzymatic resolutions, nuclear magnetic resonance (NMR) techniques, optical rotation, circular dichroism (CD)) for the on- or off-line determination of absolute configuration of enantiomers. Furthermore, it is outlined that chiral HPLC, which associates enantioseparation process and classical purification process, opens new perspectives in the classical determination of absolute configuration by chemical correlation or chemical interconversion methods. The review also contains a discussion about the various approaches to predict the absolute configuration from the retention behavior of the enantiomers on chiral stationary phases (CSPs). Some examples illustrate the advantages and limitations of molecular modeling methods and the use of chiral recognition models. The assumptions underlying some of these methods are critically analyzed and some possible emerging new strategies are outlined.

Enantioselective high-performance liquid chromatography of therapeutically relevant aminoalcohols as their fluorescent 1-naphthyl isocyanate derivatives

A method for determining the enantiomers of 10 therapeutically relevant aminoalcohols using HPLC and precolumn derivatization was developed. Naphthyl isocyanate reacted with racemic aminoalcohols to form urea derivatives which were separated isocratically on a cellulose tris(3,5-dimethylphenylcarbamate) coated silica gel column, and detected fluorometrically in the lower ng mL(-1) range. The effluents can also be monitored at lower sensitivity, using an ultraviolet detector operated at 220 nm.

Atomistic modeling of enantioselection in chromatography

A review of atomistic molecular modeling studies related to chromatographic separations of enantiomers is presented. Only those types of calculations where direct interactions between a selector and a selectand are involved are described in this review; omitted are regression models. An emphasis is placed on comparing methods used for sampling potential energy surfaces implementing different methodologies like quantum and molecular mechanics for energy calculations, and molecular dynamics and Monte Carlo sampling strategies for simulations. Type I-V chiral stationary phases and additives for capillary electrophoresis and ion-pair chromatography are covered in this review.

Unexpected difference in enantioselective retention on cellulase (CHB I) silica stationary phase caused by exchange of potassium for sodium ion in the mobile phase

An increase in both retention and enantioselectivity for some beta-blocking agents was observed when exchanging potassium to sodium ion in the buffer used as mobile phase. A large effect of ionic strength on retention was observed, while the enantioselectivity was constant.

Enantiomeric separation of pirlindole by liquid chromatography using different types of chiral stationary phases

The enantioseparation of pirlindole by liquid chromatography (LC) was investigated using three different chiral stationary phases (CSPs) containing either cellulose tris-(3,5-dimethylphenylcarbamate) (Chiralcel OD-R), ovomucoid (OVM) or beta-cyclodextrin (beta-CD). The effects of the mobile phase pH on retention, enantioselectivity and resolution were studied. Methanol and acetonitrile were tested as organic modifiers while the influence of the addition to the mobile phase of sodium alkanesulfonates or sodium perchlorate was also investigated. Sodium perchlorate was only used on the Chiralcel OD-R column while sodium alkanesulfonates were tested as mobile phase additives on the three kinds of CSPs. The enantioseparation of pirlindole could be obtained on all CSPs tested, the best results with respect to chiral resolution being achieved on the Chiralcel OD-R and the OVM columns. The use of sodium octanesulfonate (NaOS) was found to improve the enantioseparation of pirlindole on the OVM column while enantioselectivity was considerably enhanced by addition of sodium perchlorate on the Chiralcel OD-R column.

Separation of the S(+) and R(-)-enantiomers of tiagabine.HCl and its two chiral precursors by chiral chromatography: application to chiral inversion studies

Chiral HPLC methods were developed and validated for tiagabine.HCl and its two chiral precursors to determine the chiral purity of the three compounds to ensure the quality of the final product which is used as a new antiepileptic drug. Tiagabine.HCl was derivatized with 1-napthalenemethylamine and was chromatographed on a Pirkle type phenyl glycine column with a mobile phase of 69:31, 0.1 M ammonium acetateacetonitrile (v/v). The two chiral precursors were chromatographed on a Chiralcel-OG column with a mobile phase of hexane, isopropanol etc. Each of the three HPLC methods have a selectivity factor (alpha) of 1-2 or higher. The validation of the methods was done by conducting standard addition and recovery studies of the S(+)-enantiomers in the samples. The %RSD of all three methods were < 5 with a limit of quantification of 0.05% (peak area) or lower. By using these methods, a study was conducted to investigate the effect of pH, temperature, and trace levels of transition metals such as Fe3+, Co2+, and Ni2+ on the conversion of R(-)-enantiomer to the S(+)-enantiomer of tiagabine.HCl and its two chiral precursors. The results of this study demonstrated that the two chiral precursors of tiagabine.HCl under reflux conditions are more sensitive to chiral inversion than tiagabine.HCl. Under reflux conditions, in the presence of trace metal ions and different pH, approximately 10, 11, and 1% of the R(-)-enantiomer was converted to the S(+)-enantiomer for ethyl nipecotate, ethylester of tiagabine, and tiagabine.HCl, respectively. However, at room temperature, tiagabine.HCl appears to be less chirally stable than its two chiral precursors. Approximately 0.4% R(-)-enantiomer of tiagabine.HCl was converted to the S(+)-enantiomer at room temerature and acidic conditions. Under similar conditions, the S(+)-enantiomer of ethyl nipecotate and ethylester of tiagabine.HCl was < 0.05%. The initial S(+)-enantiomer content for all three compounds was < 0.1%.

Separation and quantitation of the S-(+)-enantiomer in the bulk drug tiagabine x HCl by chiral high-performance-liquid chromatography using a Chiralcel-OD column

Tiagabine x HCl is being developed as an anti-convulsant/anti-epileptic agent for seizure disorders. The pharmacological activity of the R-(-)-enantiomer is higher than that of the S-(+)-enantiomer. Therefore, the drug is synthesized in the pure R-(-)-enantiomeric form. The enantiomers of tiagabine x HCl were separated on a modified cellulose stationary phase (Chiralcel-OD) with a mobile phase of hexane-isopropanol-ethanol (80:14:06, v/v/v). Approximately 5 ml of trifluoroacetic acid was added for each liter of the mobile phase mixture. The method is capable of separating the two enantiomers with a selectivity factor of 1.55 and a resolution factor of 3.4. The samples of tiagabine x HCl were monitored by a UV detector at 260 nm. The method was validated by conducting standard addition and recovery of the S-(+)-enantiomer in tiagabine x HCl. The R.S.D. of the method is 3.2%. The limit of quantification (LOQ) of the S-(+)-enantiomer present in tiagabine x HCl is about 0.03%.

Enantioselective bioanalysis of beta-blocking agents: focus on atenolol, betaxolol, carvedilol, metoprolol, pindolol, propranolol and sotalol

The recent developments in enantioselective HPLC-separation techniques are impressive and are driven by industrial and academic interests; thus there is for instance a high demand for developing stereoselective assays for chiral drugs in biological fluids. The beta-blocking agents, which possess an amino-propanol- or -ethanol side chain with at least one chiral centre, represent one of the most intensively investigated groups of more than 40 drugs introduced world wide. Seven of the most popular beta-blockers were chosen as representatives: atenolol; betaxolol; carvedilol; metoprolol; pindolol; propranolol; and sotalol, these span the whole range of lipophilicity to hydrophilicity (polarity). Enantioselective HPLC bioassays for these beta-blockers published so far, including techniques based on chiral derivatizing agents (CDAs), chiral stationary phases (CSPs) and chiral mobile phase additives (CMPAs) have been reviewed and documented in the light of general aspects together with pharmacokinetic and pharmacodynamic considerations.

The chiral chromatographic separation of β-adrenoceptor blocking drugs

Over 100 chromatographic procedures for the separation of beta-blocker enantiomers are reviewed including a large number for the analysis of biological samples. All the principal chiral chromatographic procedures have found use, namely Chiral Mobile Phase Additives (CMPA), Chiral Derivatization Agents (CDA) and Chiral Stationary Phases (CSP). Chiral Mobile Phase Additives are less frequently employed than the other two procedures and many of the earlier methods were based on the use of CDAs. However, the recent development of sophisticated custom-made CSPs has allowed the separation of native (underivatized) analytes and this approach appears to be gaining in popularity. The beta-blockers are an extensive group of drugs and stereoselective separations have been reported for 40 different structures.