Strong versus weak chiral cation exchangers: Comparative evaluation for enantiomer separation of chiral bases by non-aqueous CEC

Chiral recognition without π-π-interactions: Highly efficient chiral strong cation exchangers lacking an aromatic unit in the molecular structure

Current state-of-the-art chiral stationary phases (CSPs) enable chiral resolution of almost any racemic mixture of choice. The exceptions represent ionizable and ionized substances that fail at any attempts to resolve on commercially available CSPs. These compounds, however, can be efficiently separated on chiral ion exchangers. Commercially available Cinchona alkaloids-based chiral weak ion-exchangers are typically used for chiral resolution of organic acids, while zwitterion ion-exchangers are efficient in the resolution of acids, bases, and zwitterions. The latter possess in their structure a cation exchange unit, which alone can serve as a cornerstone of chiral strong cation exchangers facilitating chiral separation of various basic racemic mixtures. Although chiral strong cation exchangers (cSCX) are efficient CSPs, their structural variations have not been thoroughly studied so far. It was assumed that the mechanism of chiral recognition of basic compounds by cSCX is based predominantly on π-π-interactions, hydrogen bonding and steric interactions (CSP I). To verify this assumption, we aimed in our study on the design and synthesis of cSCX first lacking lateral polar substituents on the aromatic unit in the selector's structure (CSP II), and second, to replace the aromatic unit by a cyclohexane ring (CSP III and IV), thereby to omit completely the π-π-interactions. We hypothesized that this structural change should lead to a partial or complete loss of enantiorecognition power of the selectors. Surprisingly, the non-aromatic cSCXs have shown chiral recognition capability comparable to that of previously described chiral cation exchange-type CSPs: from 16 analytes screened, 11 analytes were baseline resolved and 5 partially resolved on CSP I, while non-aromatic CSP III resolved 10 analytes baseline and 6 partially. We discuss the structural motifs of the known cSCX and the novel non-aromatic selectors in a relationship with their chromatographic performance using a set of basic analytes. Moreover, we present a theory of an effective chiral recognition mechanism by two novel non-aromatic cSCXs based on the chromatographic results and quantum mechanical calculations.

Design and synthesis of naphthalene-based chiral strong cation exchangers and their application for chiral separation of basic drugs

In continuation of our efforts to synthesize a highly dedicated strong cation exchanger, we introduce four chiral stationary phases based on a laterally substituted naphthalene core featuring chiral 2-aminocyclohexansulfonic acid as the chiral cation-exchange site. The selectors were modified with two different terminal units, which enabled immobilization to the silica support by thiol-ene radical reaction or azide-yne click chemistry. The chromatographic parameters of these chiral stationary phases were determined using a set of chiral amines, mainly from the family of β-blocker pharmaceuticals. The chiral stationary phases immobilized by means of click chemistry were found to be superior to those possessing the sulfide linker to the silica support. The chromatographic results and visualization of density functional theory-calculated conformations of the selectors hint at a combination of a steric and electronic effect of the triazole ring in the course of chiral resolution of the target analytes.

Electrostatic attraction-repulsion model with Cinchona alkaloid-based zwitterionic chiral stationary phases exemplified for zwitterionic analytes

In the present paper, we demonstrated that Cinchona alkaloid cyclohexyl sulfonic acid-based zwitterionic chiral selectors (SOs) and the respective chiral stationary phases (CSPs) can be successfully employed for the enantioseparation of underivatized thus zwitterionic amino acids (the selectands, SAs) even in the absence of ionic additives in the eluent, generally used as displacer counter-ions in ion exchange chromatography. Therefore, we provided evidence that cooperative "intramolecular and intermolecular counter-ion effects" of the zwitterionic SO moiety and the zwitterionic SAs can be sufficient to modulate alone the retention characteristics without a loss of stereoselectivity. Four fully constrained β-amino acids were used as target compounds for this study. The analyses were carried out with either neat methanol, acetonitrile, water or their binary hydro-organic mixtures. A U-shaped retention profile was observed both with methanol- and acetonitrile-based eluents. Except a few cases, enantioselectivity experienced a remarkable amelioration at the "balanced region" of a buffer free hydroorganic mobile phase composition. At "the bottom" of the U-shaped curve, high α- and resolution values could be reached with most of the screened mobile phases. An electrostatically driven "attraction-repulsion model" was postulated to explain the very favourable characteristic of the two studied zwitterion-type CSPs for the retention and enantiomer separation of zwitterionic analytes.

Early history of the recognition of molecular biochirality

This opening chapter recalls the history of the discoveries that led to the appreciation of the nature and importance of molecular chirality in biology, as well as the development of stereochemistry as an interdisciplinary field connecting chemistry and biology. The discoveries described cover roughly the period of ca. 1840-1940, although certain relevant events of earlier or later times are also addressed. A large number of chiral substances occur in nature in unichiral (i.e., single-enantiomer) form, and for centuries many such substances were used in crude extracts for relief from diseases. For the science of biochirality, the first milestone was the discovery of molecular chirality by Louis Pasteur in 1848. Thereafter, fundamental advances were made, beginning in 1857 with Pasteur's discovery of biological enantioselectivity, in the metabolism of (±)-tartaric acid. With the advances in organic chemistry during the second half of the nineteenth century, the structures of many organic molecules were elucidated and new chiral compounds synthesized, and by the turn of the twentieth century studies of stereoselectivity in the biological activity or enzymatic transformations of natural or synthetic substances were proliferating, and chiroselectivity was often found. Among the names associated with important discoveries in biochirality appear Pasteur, Piutti, Fischer, Cushny, Easson and Stedman, and others. The findings soon prompted attempts to explain the phenomenon of enantioselectivity in biological action, beginning with Pasteur's proposal to account for enantioselectivity in the metabolism of tartaric acid. In 1894 Fischer announced his "lock-and-key" metaphor to explain enantioselectivity in enzyme-substrate interactions and in 1933 Easson and Stedman advanced the first chemical-structure-based model, the three-point-attachment paradigm, to rationalize enantioselectivity at adrenergic receptors. This model has been generalized as the simplest basis for enantioselectivity in biological activity. Today molecular chirality is widely recognized as an important modulator of the effects of chiral substances in a variety of branches of biology and medicine.

Advances in the enantioseparation of β-blocker drugs by capillary electromigration techniques

β-Blocker drugs or β-adrenergic blocking agents are an important class of drugs, prescribed with great frequency. They are used for various diseases, particularly for the treatment of cardiac arrhythmias, cardioprotection after myocardial infarction (heart attack), and hypertension. Almost all β-blocker drugs possess one or more stereogenic centers; however; only some of them are administered as single enantiomers. Since both enantiomers can differ in their pharmacological and toxicological properties, enantioselective analytical methods are required not only for pharmacodynamic and pharmacokinetic studies but also for quality control of pharmaceutical preparations with the determination of enantiomeric purity. In addition to the chromatographic tools, in recent years, capillary electromigration techniques (CE, CEC, and MEKC) have been widely used for enantioselective purposes employing a variety of chiral selectors, e.g. CDs, polysaccharides, macrocyclic antibiotics, proteins, chiral ion-paring agents, etc. The high separation efficiency, rapid analysi,s and low consumption of reagents of electromigration methods make them a very attractive alternative to the conventional chromatographic methods. In this review, the development and applications of electrodriven methods for the enantioseparation of β-blocker drugs are reported. The papers concerning this topic, published from January 2000 until December 2010, are summarised here. Particular attention is given to the coupling of chiral CE and CEC methods to MS, as this detector provides high sensitivity and selectivity.

Transient isotachophoresis with field-amplified sample injection for on-line preconcentration and enantioseparation of some β-agonists

Transient isotachophoresis with field-amplified sample injection (FASI), using β-CD as the chiral selector and tetrabutylammonium hydroxide (TBAOH) as the additive, was applied for on-line preconcentration and enantioseparation of three β-agonists, namely, cimaterol, clenbuterol and terbutaline. The experimental conditions for both simultaneous enantioseparation and on-line preconcentration methods have been investigated in detail. Under the optimum conditions, the detection limits (defined as S/N = 3) of this method were found to be 1 ng mL-1 for all three pairs of β-agonists enantiomers. Compared with conventional electrokinetic injection, the enhancement factors were greatly improved to be 250-fold. Finally, the proposed method has been applied for the analysis of human urine samples.

Sulfated and sulfonated polysaccharide as chiral stationary phases for capillary electrochromatography and capillary electrochromatography-mass spectrometry

The applications of polysaccharide phenyl carbamate derivatives as chiral stationary phases (CSPs) for capillary electrochromatography (CEC) are often hindered by longer retention times, especially using a normal-phase (NP) eluent due to very low electroosmotic flow (EOF). Therefore, in this study, we propose an approach for the aforementioned problems by introducing two new types of negatively charged sulfate and sulfonated groups for polysaccharide CSPs. These CSPs were utilized to pack CEC columns for enantioseparation with a NP eluent. Compared to conventional cellulose tris(3,5-dimethylphenyl carbamate) or CDMPC CSPs, the sulfated CDMPC CSP (sulfur content 4.25%, w/w) shortened the analysis time up to 50% but with a significant loss of enantiomeric resolution (approximately 60%). On the other hand, the sulfonated CDMPC CSP (sulfur content 1.76%, w/w) not only provided fast throughput but also maintained excellent resolving power. In addition, its synthesis is much more straightforward than the sulfated one. Furthermore, we studied several stationary phase parameters (CSP loading and silica gel pore size) and mobile phase parameters (including type of mobile phase and its composition) to evaluate the throughput and enantioselectivity. Using the optimized conditions, a chiral pool containing 66 analytes was screened to evaluate the enantioselectivity under three different mobile phase modes (i.e., NP, polar organic phase (POP) and reversed-phase (RP) eluents). Among these mobile phase modes, the RP mode showed the highest success rate, whereas some degree of complementary enantioselectivity was observed with NP and POP. Finally, the feasibility of applying this CSP for CEC-MS enantioseparation using internal tapered column was evaluated with NP, POP and RP eluents. In particular, the NP-CEC-MS provided significantly enhanced sensitivity when methanol was replaced with isopropanol in the sheath liquid. Using aminoglutethimide as model chiral analyte, all three modes of CEC-MS demonstrated excellent durability as well as excellent reproducibility of retention time and enantioselectivity.

Statistical optimization of the silylation reaction of a mercaptosilane with silanol groups on the surface of silica gel

Thiol-modified silica is often used as an intermediate product for further synthesis of modified stationary phases for chromatography or purification processes. Different conditions were used to synthesize such thiol-modified particles, but systematic optimizations remained scarce. In this study the reaction conditions for the synthesis of mercaptopropyl-modified silica were optimized. The general synthetic method consists in slurrying the silica gel in toluene before adding 3-mercaptopropyldimethoxymethylsilane together with a tertiary amine as catalyst (here dimethylaminopyridine). Reaction time and temperature were optimized using a full factorial design of experiment (DoE) from 3 to 25h with temperature varying between 45 and 105 degrees C. The surface coverage of the silica with mercaptopropyl-groups was analyzed by two different ways (elemental analysis and chemical surface reaction with 2,2'-dipyridyl disulfide followed by HPLC-UV analysis of stoichiometrically liberated pyridyl-2-thione). We obtained a three-dimensional (3D) plot of the surface coverage as a function of reaction time and temperature. The arch-shaped hyperplane allowed us to determine an optimum with regard to time and temperature, which yields to the highest surface coverage possible. We also verified that the increase of the surface coverage does not lead to a decrease of the stability of the surface modification by subjecting the gels to treatment with high temperature and acidic conditions. The stability was monitored by different chromatographic methods. Moreover, (29)Si cross-polarization-magic angle spinning (CP-MAS) NMR spectra of materials prepared by different conditions allowed to confirm that the Si species on the surface were essentially the same, while there was only a minute difference in signal intensities for the individual Si species for materials obtained by distinct temperatures.

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.

Monolithic silica-based capillary column with strong chiral cation-exchange type surface modification for enantioselective non-aqueous capillary electrochromatography

A silica-based monolithic stationary phase prepared by the sol-gel process in a 100 microm I.D. fused-silica (FS) capillary has been modified chemically with 3-mercaptopropyl trimethoxysilane followed by immobilization of a strong cation-exchange (SCX) type chiral selector, (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutane phosphonic acid, by radical addition reaction onto the reactive sulfhydryl surface. After a fine-tuning of the mobile phase composition, the enantioselective capillary column was evaluated for the separation of various chiral basic drugs by enantioselective non-aqueous capillary electrochromatography (CEC), in comparison to capillary column analogs packed with 3.5 microm silica particles having attached the same selector. The performance of the monolithic silica column was further compared to corresponding polymethacrylate-based organic polymer monoliths. The study indicated that strong counter-ions such as 2-aminobutanol or N,N,N',N'-tetramethylethylenediamine are needed, although they reduce the electroosmotic flow velocity and separation factors in comparison to less efficient counter-ions, in order to allow the elution of the oppositely charged solutes in the ion-exchange retention mode within reasonable run time and as sharp zones. In contrast, weak counter-ions such as N,N-diisopropylethylamine (Huenig base) provided stronger electroosmotic flow and much better separation factors, but relatively poor peak efficiencies. Overall, with the chemically functionalized monolithic silica column the high quality separations of packed column analogs could be approximated, with regards to both separation factors and peak performances. On the other hand, the monolithic capillary column certainly outperformed the packed column in terms of system robustness under capillary electrochromatography conditions and showed excellent column longevity. The enantioselective strong cation-exchange-type monolithic silica column performed also well in comparison to the organic polymer monolith.

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.

Polymethacrylate-type monoliths functionalized with chiral amino phosphonic acid-derived strong cation exchange moieties for enantioselective nonaqueous capillary electrochromatography and investigation of the chemical composition of the monolithic polymer

In situ prepared monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) (poly(GMA-co-EDMA)) capillary columns were activated to reactive thiol-monoliths and subsequently functionalized with (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutanephosphonic acid as chiral selector by radical addition to afford enantioselective strong cation exchanger (SCX) capillary columns (100 microm inner diameter (ID)). These monolithic capillaries were devised for the enantioseparation of chiral bases by nonaqueous and aqueous capillary electrochromatography (CEC) and the results obtained for mefloquine and its tert-butylcarbamate as test compounds were compared to those obtained with particulate silica-based analogs (packed columns). Despite abolishment of nonspecific ionic interactions between the cationic solutes and residual silanols that may diminish separation factors of the silica-based chiral SCX particles, the poly(GMA-co-EDMA)-supported SCX monolith did not, as expected, show better enantioselectivities, which was assumed to be due to detrimental nonspecific interactions between the analytes and the lipophilic polymer backbone. In order to minimize these unfavorable contributions, less lipophilic monoliths were developed by copolymerization of different amounts of the hydrophilic monomer 2-hydroxyethyl methacrylate (HEMA) with GMA and EDMA, leading to GMA-co-HEMA-co-EDMA-terpolymeric monoliths. By this increase of the hydrophilicity of the monolithic support the enantioselectivity of the resultant SCX stationary phase could be enhanced and reached values comparable to the packed silica-based enantioselective SCX capillaries. Additionally, the mobile phase composition and other variables were examined and it could be shown that the separation factors are considerably affected by diverse parameters such as acetonitrile-methanol ratio and type and concentration of the counterion. Mefloquine enantiomers could be separated with alpha-values up to 1.56 and a maximum plate count of ca. 60,000 m(-1) could be achieved.

Capillary electrochromatographic chiral separations with potential for pharmaceutical analysis

The use of capillary electrochromatography as a chiral separation technique for pharmaceutical applications is reviewed. Publications of the past 10 years that provide a potential practical application in pharmaceutical analysis are considered. Method development or validation, separation strategies, and potential routine analysis by the methods/applications cited are the main subjects on which we focused our attention. The indirect chiral separation method was only used once in CEC mode. In the direct chiral separations, the use of chiral stationary phases was obviously preferred over the use of chiral mobile phases with non-chiral stationary phases. Amongst the chiral stationary phases, those based on macrocyclic antibiotics and polysaccharide selectors were the most frequently used. Monolithic stationary phases also have several applications, but not so extended as those with packed capillary electrochromatography. The considered papers not only describe the applicability of the technique for relatively large sets of chiral analytes, they also showed that various types of stationary phases can be produced in-house in a simple manner. However, to survive as a mature separation technique, considerable time and effort are still needed to solve some disadvantages currently characterizing capillary electrochromatography.

Chiral separations by capillary electromigration techniques in nonaqueous media

A review on the advantages, peculiarities, and the potential of enantioselective capillary electrochromatography (CEC) in nonaqueous media is presented. Some fundamentals on CEC with particular focus on enantioselective CEC are discussed. The strategies, concepts, preferentially utilized chiral selectors and column technologies that have been utilized to succeed in highly efficient enantiomer separations by nonaqueous CEC are described thoroughly.