Cyclodextrins and enantiomeric separations of drugs by liquid chromatography and capillary electrophoresis: basic principles and new developments

The impact of chirality on the analysis of alkaloids in plant

Most of the alkaloids are chiral compounds and are clinically administered as the racemic mixture, even though its enantiomers have been known to exert different pharmacological activity. The determination of the enantiomeric composition of alkaloid-containing plants is subject to severe attention from pharmacological and toxicological points of view. This review gives an overview of the chiral analysis of alkaloids that were used in theoretical studies and applications for plants in recent years.

Polysaccharides as separation media for the separation of proteins, peptides and stereoisomers of amino acids

Reliable separation of peptides, amino acids and proteins as accurate as possible with the maximum conformation and biological activity is crucial and essential for drug discovery. Polysaccharide, as one of the most abundant natural biopolymers with optical activity on earth, is easy to be functionalized due to lots of hydroxyl groups on glucose units. Over the last few decades, polysaccharide derivatives are gradually employed as effective separation media. The highly-ordered helical structure contributes to complex, diverse molecular recognition ability, allowing polysaccharide derivatives to selectively interact with different analytes. This article reviews the development, application and prospects of polysaccharides as separation media in the separation of proteins, peptides and amino acids in recent years. The chiral molecules mechanism, advantages, limitations, development status and challenges faced by polysaccharides as separation media in molecular recognition are summarized. Meanwhile, the direction of its continued development and future prospects are also discussed.

Noncovalent Complexes of Cyclodextrin with Small Organic Molecules: Applications and Insights into Host-Guest Interactions in the Gas Phase and Condensed Phase

Cyclodextrins (CDs) have drawn a lot of attention from the scientific communities as a model system for host–guest chemistry and also due to its variety of applications in the pharmaceutical, cosmetic, food, textile, separation science, and essential oil industries. The formation of the inclusion complexes enables these applications in the condensed phases, which have been confirmed by nuclear magnetic resonance (NMR) spectroscopy, X-ray crystallography, and other methodologies. The advent of soft ionization techniques that can transfer the solution-phase noncovalent complexes to the gas phase has allowed for extensive examination of these complexes and provides valuable insight into the principles governing the formation of gaseous noncovalent complexes. As for the CDs’ host–guest chemistry in the gas phase, there has been a controversial issue as to whether noncovalent complexes are inclusion conformers reflecting the solution-phase structure of the complex or not. In this review, the basic principles governing CD’s host–guest complex formation will be described. Applications and structures of CDs in the condensed phases will also be presented. More importantly, the experimental and theoretical evidence supporting the two opposing views for the CD–guest structures in the gas phase will be intensively reviewed. These include data obtained via mass spectrometry, ion mobility measurements, infrared multiphoton dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations.

Chiral Mobile-Phase Additives in HPLC Enantioseparations

High-performance liquid chromatography (HPLC) is one of the main separation techniques for chiral drugs. Among the chiral HPLC techniques available, the chiral mobile-phase additive (CMPA) technique is a valuable method for the direct enantioseparation of chiral chemical entities. In the CMPA method, the chiral selector is dissolved in the mobile phase while the stationary phase is achiral. Interaction with the analyte enantiomers results in the formation of transient diastereomeric complexes. These complexes differ in their formation constants and/or distribution between the (achiral) stationary phase and the mobile phase resulting in an enantioseparation. This chapter describes the HPLC separation applying CMPA methods by several most useful types of chiral selectors including chiral ligand exchangers, macrocyclic antibiotics, and cyclodextrins.

Recognition of Pharmacological Bi-Heterocyclic Compounds by Using Terahertz Time Domain Spectroscopy and Chemometrics

In this study, we presented the concept and implementation of a fully functional system for the recognition of bi-heterocyclic compounds. We have conducted research into the application of machine learning methods to correctly recognize compounds based on THz spectra, and we have described the process of selecting optimal parameters for the kernel support vector machine (KSVM) with an additional `unknown' class. The chemical compounds used in the study contain a target molecule, used in pharmacy to combat inflammatory states formed in living organisms. Ready-made medical products with similar properties are commonly referred to as non-steroidal anti-inflammatory drugs (NSAIDs) once authorised on the pharmaceutical market. It was crucial to clearly determine whether the tested sample is a chemical compound known to researchers or is a completely new structure which should be additionally tested using other spectrometric methods. Our approach allows us to achieve 100% accuracy of the classification of the tested chemical compounds in the time of several milliseconds counted for 30 samples of the test set. It fits perfectly into the concept of rapid recognition of bi-heterocyclic compounds without the need to analyse the percentage composition of compound components, assuming that the sample is classified in a known group. The method allows us to minimize testing costs and significant reduction of the time of analysis.

Chiral Stationary Phases for Liquid Chromatography: Recent Developments

The planning and development of new chiral stationary phases (CSPs) for liquid chromatography (LC) are considered as continuous and evolutionary issues since the introduction of the first CSP in 1938. The main objectives of the development strategies were to attempt the improvement of the chromatographic enantioresolution performance of the CSPs as well as enlarge their versatility and range of applications. Additionally, the transition to ultra-high-performance LC were underscored. The most recent strategies have comprised the introduction of new chiral selectors, the use of new materials as chromatographic supports or the reduction of its particle size, and the application of different synthetic approaches for preparation of CSPs. This review gathered the most recent developments associated to the different types of CSPs providing an overview of the relevant advances that are arising on LC.

Synthesis and applications of sulfopropyl ether γ-cyclodextrin polymer as chiral selector in capillary electrophoresis

A novel sulfopropyl ether γ-cyclodextrin polymer (SPE-γ-CDP) through polycondensating sulfated cyclodextrins (SCDs) was synthesized. This synthesis approach also has the potential of preparing other derived cyclodextrins (CDs) polymers. The polymerized SCDs took on both the properties of SCDs and certain characteristics of polymers, such as chiral selectivity and high viscosity. Synthesis parameters, including reactions sequence, sulfation, and polycondensation conditions were investigated systematically. The product was characterized by elemental analysis, infrared spectroscopy (IR), and indirect UV detections prior to use as background electrolytes additive. The separation conditions, including the concentration of SPE-γ-CDP, the concentration and pH of running buffer, separation voltage, as well as the additional organic solution were optimized during chiral separation of neutral, acidic, and basic enantiomers in capillary electrophoresis (CE). SPE-γ-CDP was proven to be an effective chiral resolving agent in CE with the advantages of simple synthesis process, low cost, similar ratio of charge-to-mass, low current, great reproducibility, and reusability. Graphical Abstract Synthesis and applications of sulfopropyl ether γ-cyclodextrin polymer.

Enantioseparation of Citalopram by RP-HPLC, Using Sulfobutyl Ether-β-Cyclodextrin as a Chiral Mobile Phase Additive

Enantiomeric separation of citalopram (CIT) was developed using a reversed phase HPLC (RP-HPLC) with sulfobutylether-β-cyclodextrin (SBE-β-CD) as a chiral mobile phase additive. The effects of the pH value of aqueous buffer, concentration of chiral additive, composition of mobile phase, and column temperature on the enantioseparation of CIT were investigated on the Hedera ODS-2 C₁₈ column (250 mm × 4.6 mm × 5.0 um). A satisfactory resolution was achieved at 25°C using a mobile phase consisting of a mixture of aqueous buffer (pH of 2.5, 5 mM sodium dihydrogen phosphate, and 12 mM SBE-β-CD), methanol, and acetonitrile with a volumetric ratio of 21 : 3 : 1 and flow rate of 1.0 mL/min. This analytical method was evaluated by examining the precision (lower than 3.0%), linearity (regression coefficients close to 1), limit of detection (0.070 µg/mL for (R)-CIT and 0.076 µg/mL for (S)-CIT), and limit of quantitation (0.235 µg/mL for (R)-CIT and 0.254 µg/mL for (S)-CIT).

Chiral mobile phase additives in HPLC enantioseparations

In recent years, high performance liquid chromatography (HPLC) has become one of the main separation techniques for chiral drugs. Among the chiral HPLC techniques available, the chiral mobile phase additive (CMPA) technique is a valuable method for the direct enantioseparation of chiral chemical entities. In the CMPA method, the chiral selector is dissolved in the mobile phase while the stationary phase is achiral. Interaction with the analyte enantiomers results in the formation of transient diastereomeric complexes. These complexes differ in their formation constants and/or distribution between the (achiral) stationary phase and the mobile phase resulting in an enantioseparation. This chapter describes the HPLC separation applying CMPA methods by several most useful types of chiral selectors including chiral ligand exchangers, macrocyclic antibiotics, and cyclodextrins.

Cationic cyclodextrins chemically-bonded chiral stationary phases for high-performance liquid chromatography

Two covalently bonded cationic β-CD chiral stationary phases (CSPs) prepared by graft polymerization of 6(A)-(3-vinylimidazolium)-6-deoxyperphenylcarbamate-β-cyclodextrin chloride or 6(A)-(N,N-allylmethylammonium)-6-deoxyperphenylcarbamoyl-β-cyclodextrin chloride onto silica gel were successfully applied in high-performance liquid chromatography (HPLC). Their enantioseparation capability was examined with 12 racemic pharmaceuticals and 6 carboxylic acids. The results indicated that imidazolium-containing β-CD CSP afforded more favorable enantioseparations than that containing ammonium moiety under normal-phase HPLC. The cationic moiety on β-CD CSPs could form strong hydrogen bonding with analytes in normal-phase liquid chromatography (NPLC) to enhance the analytes' retention and enantioseparations. In reversed-phase liquid chromatography (RPLC), the analytes exhibited their maximum retention when the pH of mobile phase was close to their pK(a) value. Inclusion complexation with CD cavity and columbic/ionic interactions with cationic substituent on the CD rim would afford accentuated retention and enantioseparations of the analytes.

Enhanced chiral recognition by cyclodextrin dimers

In this article we investigate the effect of multivalency in chiral recognition. To this end, we measured the host-guest interaction of a β-cyclodextrin dimer with divalent chiral guests. We report the synthesis of carbohydrate-based water soluble chiral guests functionalized with two borneol, menthol, or isopinocampheol units in either (+) or (−) configuration. We determined the interaction of these divalent guests with a β-cyclodextrin dimer using isothermal titration calorimetry. It was found that—in spite of a highly unfavorable conformation—the cyclodextrin dimer binds to guest dimers with an increased enantioselectivity, which clearly reflects the effect of multivalency.

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.

Optical isomer separation of flavanones and flavanone glycosides by nano-liquid chromatography using a phenyl-carbamate-propyl-beta-cyclodextrin chiral stationary phase

In this paper a phenyl-carbamate-propyl-beta-cyclodextrin stationary phase was employed for the enantioseparation of several flavonoids, including flavanones and methoxyflavanones by using nano-liquid chromatography (nano-LC). The same stationary phase was also used for the diastereoisomeric separation of two flavanone glycosides. The compounds: flavanone, 2'-hydroxyflavanone, 4'-hydroxyflavanone, 6-hydroxyflavanone, 7-hydroxyflavanone, 4'-methoxyflavanone, 6-methoxyflavanone, 7-methoxyflavanone, hesperetin, hesperidin, naringenin and naringin were studied using reversed, polar organic and normal elution modes. The effect of the nature and composition of the mobile phase (organic modifier type, buffer and water content in the reversed phase mode) on the enantioresolution (R(s)), retention factor (k) and enantioselectivity (alpha) were investigated. Baseline resolution of all studied flavonoids, with the exception of 2'-hydroxyflavanone and naringin, was achieved in reversed phase mode using a mixture of MeOH/H(2)O at different ratios as mobile phase. Good results, in terms of peak efficiency and short analysis time, were obtained adding 1% triethylammonium acetate pH 4.5 buffer to MeOH/H(2)O mixture. The separation of the studied compounds was also performed in polar organic mode. By using 100% of MeOH as mobile phase, the resolution was achieved for the studied analytes, except for 7-hydroxyflavanone, 2'-hydroxyflavanone, naringenin, hesperidin and naringin. Normal mode was tested employing a mixture of EtOH/hexane/TFA as mobile phase achieving the enantiomeric and diastereomeric separation of only hesperetin and hesperidin, respectively. The use of nano-LC technique for the resolution of flavanones optical isomers allowed to achieve good resolutions in shorter analysis time compared to the results reported in literature with conventional HPLC.

[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.

Separation efficiencies in hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) is important for the separation of highly polar substances including biologically active compounds, such as pharmaceutical drugs, neurotransmitters, nucleosides, nucleotides, amino acids, peptides, proteins, oligosaccharides, carbohydrates, etc. In the HILIC mode separation, aqueous organic solvents are used as mobile phases on more polar stationary phases that consist of bare silica, and silica phases modified with amino, amide, zwitterionic functional group, polyols including saccharides and other polar groups. This review discusses the column efficiency of HILIC materials in relation to solute and stationary phase structures, as well as comparisons between particle-packed and monolithic columns. In addition, a literature review consisting of 2006-2007 data is included, as a follow up to the excellent review by Hemström and Irgum.