HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review

Effective enantiomeric separations of racemic primary amines by the isopropyl carbamate-cyclofructan6 chiral stationary phase

A new chiral stationary phase (CSP) was developed by bonding isopropyl-carbamate functionalized cyclofructan6 (IP-CF6) to the silica gel. It was evaluated by injecting 119 racemic primary amine-containing compounds. This CSP showed pronounced enantioselectivity toward all types of primary amines, separating 93% of all tested compounds. Baseline separation was achieved even for some simple aliphatic racemic amines that contained no other functionality. The polar organic mode was shown to be the effective mobile phase owing to higher efficiency. This new chiral stationary phase showed great potential for preparative-scale separations. It is also interesting that the chiral selector, R-naphthylethyl-carbamate functionalized CF6 (RN-CF6), was found to provide complementary selectivity for the relatively few amine analytes that did not separate on IP-CF6. Thus between the two CSPs, 98% of attempted amine compounds were separated.

Chiral separations

The main goal of this review is to provide a brief overview of chiral separations to researchers who are versed in the area of analytical separations but unfamiliar with chiral separations. To researchers who are not familiar with this area, there is currently a bewildering array of commercially available chiral columns, chiral derivatizing reagents, and chiral selectors for approaches that span the range of analytical separation platforms (e.g., high-performance liquid chromatography, gas chromatography, supercritical-fluid chromatography, and capillary electrophoresis). This review begins with a brief discussion of chirality before examining the general strategies and commonalities among all of the chiral separation techniques. Rather than exhaustively listing all the chiral selectors and applications, this review highlights significant issues and differences between chiral and achiral separations, providing salient examples from specific classes of chiral selectors where appropriate.

Chiral recognition by enantioselective liquid chromatography: mechanisms and modern chiral stationary phases

An overview of the state-of-the-art in LC enantiomer separation is presented. This tutorial review is mainly focused on mechanisms of chiral recognition and enantiomer distinction of popular chiral selectors and corresponding chiral stationary phases including discussions of thermodynamics, additivity principle of binding increments, site-selective thermodynamics, extrathermodynamic approaches, methods employed for the investigation of dominating intermolecular interactions and complex structures such as spectroscopic methods (IR, NMR), X-ray diffraction and computational methods. Modern chiral stationary phases are discussed with particular focus on those that are commercially available and broadly used. It is attempted to provide the reader with vivid images of molecular recognition mechanisms of selected chiral selector-selectand pairs on basis of solid-state X-ray crystal structures and simulated computer models, respectively. Such snapshot images illustrated in this communication unfortunately cannot account for the molecular dynamics of the real world, but are supposed to be helpful for the understanding. The exploding number of papers about applications of various chiral stationary phases in numerous fields of enantiomer separations is not covered systematically.

[Separation of chiral pharmaceutical drugs by chromatographic and electrophoretic techniques]

SUMMARY

A large number of pharmaceutical drugs possess one or more centers of asymmetry giving rise to enantiomers whose pharmacological properties and toxicity are often different. At successive stages of drug discovery, the enantiomers of any chiral molecule must be isolated and analyzed and their enantiomeric purity determined. The electrophoretic and chromatographic techniques have become the most important tools to routinely determine the enantiomeric purity of pharmaceutical molecules. Liquid chromatography (LC) is the most widely used because of the large number of columns marketed, the variety of selectivities available and the ease at which analytical results can be scaled up to the preparative level. In particular, more than 80% of enantioseparations of pharmaceutical molecules are successful with polysaccharide-derivative stationary phases (cellulose, amylose) for multiple system solvents (normal phase, polar organic phase or reverse phase). Complementary selectivities can be achieved more rapidly with other types of stationary phase (glycopeptides, Pirkle, cyclodextrins) but their application is hindered by problems of stability (proteins) or transfer to the preparative scale (cyclodextrins). At the present time, glycopeptide phases offer very promising prospects for the separation of amino acids (and derivatives) and peptide enantiomers. In addition, because of its faster analysis and environmental benefits, supercritical chromatography (SFC) has given rise to renewed interest. Capillary electrophoresis (CE) is an orthogonal technique complementary to chromatographic methods. Its principle involves the formation of diastereoisomer complexes after addition of anionic (HS-beta-CD, HS-gamma-CD CM-beta-CD) or neutral (TM-beta -CD, HP-beta-CD, DM-beta-CD, HP-gamma-CD) cyclodextrins to the running buffer. Compared to LC, CE analyses are cheaper (no chiral column, no solvent, low consumption of chiral selector) and peak efficiencies are higher by one order of magnitude. Furthermore, the mechanism of separation in CE is much simpler to understand and predict. However, the low capacity of CD column prevents its use at the preparative scale and consequently hampers its development as an analytical technique. Today, the increasing number of new drug candidate molecules produced daily, and for which the determination of enantiomeric purity is required before further development, encourages the pharmaceutical industry to seek fast chiral analysis methods based on simple protocols. The speed of analysis is more important than resolution. Thus, screening strategies are implemented with HPLC, SFC and CE including the selection of a limited number of chiral selectors with strong powers of chiral recognition.

Enantiorecognition of profens by capillary electrophoresis using a novel chiral selector eremomycin

The evaluation of a macrocyclic glycopeptide antibiotic, eremomycin, as a chiral selector in capillary electrophoresis (CE) has been performed. The stability of eremomycin in solution and capillary electrolyte, as well as its optical and electrophoretic properties have been discussed. The effect of experimental parameters influencing the enantioseparation of several profens has been studied. Excellent enantioseparation of profens has been achieved and migration order has been validated. Comparison of enantioseparations of profens in CE by using eremomycin-mediated electrolytes and in HPLC with eremomycin immobilized on silica has revealed similar trends for both methods.

Comparison of performance of Chirobiotic T, T2 and TAG columns in the separation of beta2- and beta3-homoamino acids

The enantiomers of eight unusual beta(2)- and beta(3)-homoamino acids were separated on chiral stationary phases containing the macrocyclic glycopeptide antibiotic teicoplanin (Chirobiotic T or T2) or teicoplanin aglycone (Chirobiotic TAG) as chiral selectors. The effects of the organic modifier, the mobile phase composition, and temperature on the separations were investigated. Linear van't Hoff plots were observed in the studied temperature range, 280-318 K, and the changes in enthalpy, Delta(DeltaH(o)), entropy, Delta(DeltaS(o)), and free energy, Delta(DeltaG(o)) were calculated. The values of the thermodynamic parameters depended on the nature of the selectors, the structures of the analytes, and especially the positions of the substituents on the analytes. A comparison of the separation performances of the macrocyclic glycopeptide stationary phases revealed that the Chirobiotic TAG column exhibited much better selectivity for beta(2)-homoamino acids, while the separation of beta(3)-homoamino acid enantiomers was better on Chirobiotic T or T2. The elution sequence was determined in some cases and was observed to be R < S.

Retention mechanism of high-performance liquid chromatographic enantioseparation on macrocyclic glycopeptide-based chiral stationary phases

The development of methods for the separation of enantiomers has attracted great interest in the past 20 years, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of high-performance liquid chromatography (HPLC), thin-layer chromatography and electrophoresis. The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these macrocyclic glycopeptide antibiotics and, through their application, endeavors to demonstrate the mechanism of separation on macrocyclic glycopeptides. The sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Considerations on HILIC and polar organic solvent-based separations: use of cyclodextrin and macrocyclic glycopetide stationary phases

There is a natural tendency in science to prefer straightforward, logical classification systems. The use of mobile phase-stationary phase combinations that do not fit neatly into the standard "normal phase" or "reversed-phase" categories has been going on for over 50 years. The term "hydrophilic interaction chromatography" (HILIC) is sometimes being used as a general category for these "other" separations. In some cases, it may be appropriate and in others, not. Indeed the mechanistic constrains used to define the method seem to be varying with time. Given the name HILIC, it is assumed that water is not only present in the mobile phase, but also plays an essential role in the retention mechanism. However, there is residual water present in all organic solvents. Regardless, the number of reported separations in this alternative mode has increased tremendously in the last two decades. This is due to the advent of new stationary phases and an emphasis on polar, biologically important molecules. We discuss the relationships between HILIC and other chromatographic modes. We then examine two classes of stationary phases that have played a major role in these separations. These particular stationary phases can be used to provide appreciable mechanistic information as well.

High-performance liquid chromatographic separation of stereoisomers ofN-phthaloyl-protected amino acids and dipeptidomimetics

The stereoisomers of N-phthaloyl-protected amino acids and dipeptidomimetics were separated on macrocyclic glycopeptide and cellulose-based chiral stationary phases (CSPs) in the RP and polar-ionic modes. The effects of the organic modifier, the mobile phase composition, and the pH on the separations were investigated. Optimization of these separations was achieved through variation of the mobile-phase additive combinations. The elution sequence was determined for some of the samples. A practical application for the monitoring of the reaction conditions for N-phthaloylation of (S)-Phe was demonstrated.

Novel strong cation-exchange type chiral stationary phase for the enantiomer separation of chiral amines by high-performance liquid chromatography

The preparation of novel brush-type chiral cation-exchange materials based on de novo designed synthetic low molecular mass selectors (SOs) and their evaluation for enantioselective separation of chiral amines by HPLC are presented. The SO as the functional unit for enantioselectivity contains a beta-aminocyclohexanesulfonic acid moiety and is readily accessible via straightforward synthesis in both enantiomeric forms yielding chiral stationary phases (CSPs) with opposite configurations, CSPs 1 and 2, and reversed elution orders. For the evaluation of these novel CSPs by HPLC a sound set of chiral amines, mainly amino-alcohol type drug molecules, was selected. The chromatographic evaluations were carried out using polar organic mobile phase conditions. All of the analytes could be baseline separated, compared to common CSPs in parts with excellent peak efficiencies (up to 70000 theoretical plates per meter for the second eluted enantiomer). A number of experimental parameters have been varied to look at and prove the underlying ion-exchange process on CSPs 1 and 2, and to reveal suitable conditions for their operation. In this context, the influence of proton activity in the mobile phase and the effects of varying concentration and type of the counterion as well as type of co-ion and of bulk solvent components were thoroughly investigated.