HPLC separation of dihydropyridine derivatives enantiomers with emphasis on elution order using polysaccharide-based chiral columns

Separation of isotopologues of amphetamine with various degree of deuteration on achiral and polysaccharide-based chiral columns in high-performance liquid chromatography

Hydrogen/deuterium (H/D) isotope effects are not unusual in chromatography and such phenomena have been observed in both gas- and liquid-phase separations. Despite the numerous reports on this topic, the understanding of mechanisms and the underlying noncovalent interactions at play remains rather challenging. In our recent study, we reported baseline separation of isotopologoues of some amphetamine (AMP) derivatives on achiral and polysaccharide-based chiral columns, as well as some correlations between the degree of separation of enantiomers and isotopologues on (the same) polysaccharide-based chiral column(s). Following our previous findings on isotope effects in high-performance liquid chromatography, we report herein a comparative study on the isotope effects observed with AMP and methamphetamine (MET). The impact of some pivotal factors such as the number of deuterium atoms part of AMP isotopologues, the structure of its isotopomers, the chemical structure of the achiral and chiral stationary phases used in this study, and the use of methanol- vs acetonitrile-containing mobile phases on the isotope effects was examined and discussed. Quantitative correlations between the observed isotope effects and the enantioselectivity of the chiral columns used are also shortly discussed. Furthermore, considering the chromatographic results as benchmark experimental data, we attempted to elucidate the molecular bases of the observed phenomena using quantum mechanics calculations.

Separation of undeuterated and partially deuterated enantioisotopologues of some amphetamine derivatives on achiral and polysaccharide-based chiral columns in high-performance liquid chromatography

The different behavior of enantiomers of chiral compounds in non-isotropic environments (among them in living organism) is well known. On the other hand, the importance of a kinetic isotope effect in the biomedical field has become evident during past few decades. Thus, separation of both, enantiomers and isotopologues is now critical. Only very few published studies have attempted the simultaneous separation of enantioisotopologues. In this article we report baseline separation of partially deuterated isotopologues of a few amphetamine derivatives in high-performance liquid chromatography (HPLC) using achiral columns. In addition, the simultaneous separations of enantiomers and isotopologues (i.e. enantioisotopologues) were attempted on polysaccharide-based chiral columns. For several compounds the isotope effect was tunable and could be switched from a "normal" to "inverse" by making changes to the mobile-phase composition. A stronger isotope effect was observed in acetonitrile-containing mobile phases compared to methanol-containing ones with both chiral and achiral columns. In a separation system where both "normal" and "inverse" isotope effects were observed the "normal" isotope effect was favored in polar organic solvents while increasing content of the aqueous component in the reversed-phase (RP) mobile phase favored an "inverse" isotope effect. This observation indicates that polar, hydrogen bonding-type noncovalent interactions are involved in the "normal" isotope effect, while apolar hydrophobic-type interactions are mostly responsible for the "inverse" isotope effect.

Simultaneous Determination of Enantiomeric Purity and Organic Impurities of Dexketoprofen Using Reversed-Phase Liquid Chromatography-Enhancing Enantioselectivity through Hysteretic Behavior and Temperature-Dependent Enantiomer Elution Order Reversal on Polysaccharide Chiral Stationary Phases

A reversed-phase high-performance liquid chromatographic (HPLC) method was developed for the simultaneous determination of the potential impurities of dexketoprofen, including the distomer R-ketoprofen. After screening the separation capability of four polysaccharide columns (Lux Amylose-1, Lux Amylose-2, Lux Cellulose-1 and Lux Cellulose-2) in polar organic and in reversed-phase modes, appropriate enantioseparation was observed only on the Lux Amylose-2 column in an acidified acetonitrile/water mixture. A detailed investigation of the mobile phase composition and temperature for enantio- and chemoselectivity showed many unexpected observations. It was observed that both the resolution and the enantiomer elution order can be fine-tuned by varying the temperature and mobile phase composition. Moreover, hysteresis of the retention times and enantioselectivity was also observed in reversed-phase mode using methanol/water mixtures on amylose-type columns. This could indicate that the three-dimensional structure of the amylose column can change by transitioning from a polar organic to a reversed-phase mode, which affects the enantioseparation process. Temperature-dependent enantiomer elution order and rare enthalpic/entropic controlled enantioseparation in the operative temperature range were also observed in reversed-phase mode. To find the best methodological conditions for the determination of dexketoprofen impurities, a full factorial optimization design was performed. Using the optimized parameters (Lux Amylose-2 column with water/acetonitrile/acetic acid 50/50/0.1 (v/v/v) at a 1 mL/min flow rate at 20 °C), baseline separations were achieved between all compounds within 15 min. Our newly developed HPLC method was validated according to the current guidelines, and its application was tested on commercially available pharmaceutical formulations. According to the authors' knowledge, this is the first study to report hysteretic behavior on polysaccharide columns in reversed-phase mode.

Effect of water and protic solvents on polysaccharide-based column efficiency

In the present study, polysaccharide-based columns were used to evaluate the efficiency of columns in response to the introduction of water and protic solvents (methanol and ethanol) into the mobile phase, replacing acetonitrile. While increasing water content frequently enhances enantiomer resolution, the inclusion of water, particularly when combined with methanol and ethanol in the mobile phase, has an adverse impact on mass transfer, thus influencing the column plate height. These effects are more pronounced with ethanol, and in many cases, van Deemter plots exhibit the absence of a minimum point optimal in the explored range. Consequently, acetonitrile and its water mixtures are the preferred choices to mitigate these effects for situations in which the chiral column is operated at a relatively high flow rate (> 1 mL/min in a 4.6 mm column).

The Applicability of Chromatographic Retention Modeling on Chiral Stationary Phases in Reverse-Phase Mode: A Case Study for Ezetimibe and Its Impurities

Mechanistic modeling is useful for predicting and modulating selectivity even in early chromatographic method development. This approach is also in accordance with current analytical quality using design principles and is highly welcomed by the authorities. The aim of this study was to investigate the separation behavior of two different types of chiral stationary phases (CSPs) for the separation of ezetimibe and its related substances using the mechanistic retention modeling approach offered by the Drylab software (version 4.5) package. Based on the obtained results, both CSPs presented with chemoselectivity towards the impurities of ezetimibe. The cyclodextrin-based CSP displayed a higher separation capacity and was able to separate seven related substances from the active pharmaceutical ingredient, while the cellulose-based column enabled the baseline resolution of six impurities from ezetimibe. Generally, the accuracy of predicted retention times was lower for the polysaccharide CSP, which could indicate the presence of additional secondary interactions between the analytes and the CSP. It was also demonstrated that the combination of mechanistic modeling and an experimental design approach can be applied to method development on CSPs in reverse-phase mode. The applicability of the methods was tested on spiked artificial placebo samples, while intraday and long-term (2 years) method repeatability was also challenged through comparing the obtained retention times and resolution values. The results indicated the excellent robustness of the selected setpoints. Overall, our findings indicate that the chiral columns could offer orthogonal selectivity to traditional reverse-phase columns for the separation of structurally similar compounds.

Simultaneous determination of enantiomeric and organic impurities of vildagliptin on a cellulose tris(3-chloro-4-methylphenylcarbamate) column under revered-phase conditions

A new, reversed-phase HPLC (RP-HPLC) method was developed for the simultaneous determination of the dipeptidyl-peptidase-IV-inhibitor antidiabetic drug vildagliptin (VIL) enantiomeric impurity and four other achiral related impurities. An initial screening was performed on five polysaccharide-type chiral stationary phases (Lux Amylose-1, Lux Amylose-2, Lux-Cellulose-1, Lux-Cellulose-2, Lux-Cellulose-3) in polar organic mode with methanol, ethanol, 2-propanol, or acetonitrile containing 0,1% diethylamine as mobile phase to identify the best conditions for the separation of VIL enantiomers. Lux-Cellulose-2 column was found to provide the best chiral resolution for VIL enantiomers. Further experiments were conducted using different aqueous-organic mobile phases to achieve the simultaneous chiral-achiral separation of the selected compounds. Experimental design-based optimization was performed by using a face-centered central composite design. The optimal separation conditions (Lux Cellulose-2 stationary phase, 45 °C, mobile phase consisting of methanol/water/diethylamine 80:20:0.2 (v/v/v), and 0.45 mL/min flow rate) provided baseline separation for all 6 compounds. The optimized method was validated according to the ICH guideline and proved to be reliable, specific, linear, precise, and accurate for the determination of at least 0.1% for all impurities in VIL samples. The validated method was applied for determinations from a commercially available drug formulation and proved to be suitable for routine quality control of both enantiomeric and organic impurities of VIL.

Effect of mobile-phase modifiers on the enantioselective retention behavior of methyl mandelate with an amylose 3,5-dimethylphenylcarbamate chiral stationary phase under reversed-phase conditions

In this study, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, acetone, and tert-butanol were used as organic modifiers in reversed-phase mode chiral liquid-chromatography to systematically investigate the effects of mobile phase components on the enantioselective retention behavior of methyl mandelate with immobilized amylose 3,5-dimethylphenylcarbamate-based sorbent called Chiralpak IA. A two-site enantioselective model was used to obtain information on the recognition mechanisms by observing the dependence of the enantioselectivity and retention factor difference on the modifier content. Similar enantioselective retention behaviors were observed for all modifiers, and characteristic modifier concentration points (P_L , P_M , and P_H ) were identified. At modifier concentrations up to P_M , the weakened hydrophobic environment resulted in polymer structural relaxation, which changed the recognition mechanisms. By contrast, at concentrations beyond P_H , considerably different enantioselectivity behaviors were observed, indicating that the existence of dipole-dipole interaction, which was stronger at higher modifier concentrations, contributed to the retention mechanisms. The concentrations at which these characteristic points occurred were dependent on the carbon number of the modifier molecule. Modifiers with more carbon numbers facilitated the transition in the enantioselective behaviors. These results demonstrated that the proposed method can provide a physically consistent quantitative description of enantioselective retention behavior in reversed-phase mode.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Our research cooperation with Professor Yoshio Okamoto

This article summarizes our cooperation with the research group of Prof. Yoshio Okamoto at Nagoya University during the period of time between 1992 and 2005. Although the text deals entirely with enantioseparations in high-performance liquid chromatography, capillary electrophoresis, and capillary electrochromatography, this is not a detailed review in any of these areas. The text highlights selected aspects of these techniques, which have been the subject of our joint research and in part their reflection in follow-up research by our and other research groups. Together with more systematically studied topics, aspects such as ultrafast separation of enantiomers, uncommonly high separation factor of enantiomers and other related issues are also addressed.

Comparative Chiral Separation of Thalidomide Class of Drugs Using Polysaccharide-Type Stationary Phases with Emphasis on Elution Order and Hysteresis in Polar Organic Mode

The enantioseparation of four phthalimide derivatives (thalidomide, pomalidomide, lenalidomide and apremilast) was investigated on five different polysaccharide-type stationary phases (Chiralpak AD, Chiralpak AS, Lux Amylose-2, Chiralcel OD and Chiralcel OJ-H) using neat methanol (MeOH), ethanol (EtOH), 1-propanol (PROP), 2-propanol (IPA) and acetonitrile (ACN) as polar organic mobile phases and also in combination. Along with the separation capacity of the applied systems, our study also focuses on the elution sequences, the effect of mobile phase mixtures and the hysteresis of retention and selectivity. Although on several cases extremely high resolutions (R_s > 10) were observed for certain compounds, among the tested conditions only Chiralcel OJ-H column with MeOH was successful for baseline-separation of all investigated drugs. Chiral selector- and mobile-phase-dependent reversals of elution order were observed. Reversal of elution order and hysteresis of retention and enantioselectivity were further investigated using different eluent mixtures on Chiralpak AD, Chiralcel OD and Lux Amylose-2 column. In an IPA/MeOH mixture, enantiomer elution-order reversal was observed depending on the eluent composition. Furthermore, in eluent mixtures, enantioselectivity depends on the direction from which the composition of the eluent is approached, regardless of the eluent pair used on amylose-based columns. Using a mixture of polar alcohols not only the selectivities but the enantiomer elution order can also be fine-tuned on Chiralpak AD column, which opens up the possibility of a new type of chiral screening strategy.

Chiral separation of oxazolidinone analogues by liquid chromatography on polysaccharide stationary phases using polar organic mode

The enantioseparation of four oxazolidinone and one biosimilar thiazolidine derivatives was performed on seven different polysaccharide-type chiral stationary phases (Lux Amylose-1, Lux i-Amylose-1, Lux Amylose-2, Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-3, Lux Cellulose-4) differing in backbone (cellulose or amylose), substituent or the immobilization technologies (coated or immobilized). Polar organic mode was employed using neat methanol (MeOH), ethanol (EtOH), 2-propanol (IPA) and acetonitrile (ACN) either alone or in combinations as mobile phases. Amylose-based columns with ACN provided the highest enantioselectivities for the studied compounds. The replacement of an oxygen with a sulfur atom in the backbone of the studied analytes significantly alters the enantiomer recognition mechanism. Chiral selector-, mobile-phase-, and interestingly immobilization-dependent enantiomer elution order reversal was also observed. Reversal of elution order and hysteresis of retention and enantioselectivity was further investigated using different mixtures of IPA:MeOH and ACN:MeOH on amylose-type chiral stationary phases. Hysteresis of retention and enantioselectivity was observed on all investigated amylose-type columns and binary eluent mixtures, which can be further utilized for fine-tuning chiral separation performance of the studied columns.

Chiral HPLC separation of enantiomeric blebbistatin derivatives and racemization analysis in vertebrate tissues

Simple and consistent chiral HPLC methods for the efficient separation of enantiomeric blebbistatin derivatives, namely parent compound blebbistatin and derivatives 4-nitroblebbistatin, 4-aminoblebbistatin, 4-dimethylaminoblebbistatin, and 4-t-butylblebbistatin were developed using cellulose tris(3,5-dimethylphenylcarbamate) as a stationary phase (Lux cellulose-1 column). Blebbistatin, 4-aminoblebbistatin, and 4-dimethylaminoblebbistatin racemates were well-separated in normal-phase HPLC conditions while 4-nitroblebbistatin and 4-t-butylblebbistatin were effectively separated in both normal- and reversed-phase HPLC conditions. Furthermore, the order of elution of enantiopure compounds was found to be independent of mobile phase compositions and conditions used, and solely depends on the interaction between the enantiomer and the chiral stationary phase. We found that despite the chiral center being present far from the D-ring in the blebbistatin structure, the D-ring substitutions prominently affect the chiral separation. Ex vivo racemization studies of the most popular blebbistatin derivative (S)-(-)-4-aminoblebbistatin in rat blood and brain tissues revealed that the compound does not convert into the inactive enantiomer. This confirms that (S)-(-)-4-aminoblebbistatin is a useful tool compound in cellular and molecular biology studies without the risks of racemization and degradation effects.

A comparison of chiral resolution of antifungal agents on different polysaccharide chiral columns under various mobile phase modes: Application in the biological samples

The current article describes the chiral separation of tioconazole, miconazole, isoconazole, sertaconazole and terconazole, with Lux i-Cellulose 5 and Lux i-Amylose-1 chiral columns under organic polar, normal and reversed mobile phases modes. The mobile phase flow rate was 1 mL/min with 230 nm detection at 25 ± 1 °C temperature. The polar organic mobile phases offered certain advantages for separation such as short analysis time, order of elution, high plate numbers and favorable signal to noise ratio. The values of k, α and Rs were ranged from 0.6 to 7.87, 1.10 to 1.62 and 0.37 to 5.72 in polar organic, 0.15 to 43.86, 1.02 to 2.01 and 0.36 to 8.03 in normal, and 0.34 to 15.99, 1.03 to 1.40 and 0.59 to 4.18 in reversed phases modes, respectively. The reported methods were applied in urine samples and the results were satisfactory. The reported methods were applied to the analysis of urine samples.

Unexpected effects of mobile phase solvents and additives on retention and resolution of N-acyl-D,L-leucine applying Cinchonane-based chiral ion exchangers

Chiral ion exchangers based on quinine (QN) and quinidine (QD), namely Chiralpak QN-AX and QD-AX as anionic and ZWIX(+) and ZWIX(-) as zwitterionic ion exchanger chiral stationary phases (CSPs) have been investigated with respect to their retention and chiral resolution characteristics. For the evaluation of the effects of the composition of the polar organic bulk solvents of the mobile phase (MP) and those of the organic acid and base additives acting as displacers necessary for a liquid chromatographic ion-exchange process, racemic N-(3,5-dinitrobenzoyl)leucine and other related analytes were applied. The main aim was to evaluate the impact of the MP variations on the observed, and thus the apparent enantioselectivity (α_app), and the retention factor. Significant differences were found using either polar protic methanol (MeOH) or polar non-protic acetonitrile (MeCN) solvents in combination with the acid and base additives as counter- and co-ions. It became clear, that the charged sites of both the chiral selectors of the CSPs and the analytes get specifically solvated, accompanied by the adsorption of all MP components on the CSP, thereby building a stagnant "stationary phase layer" with a composition different from the bulk MP. Via a systematic change of the MP composition, trends of resulting α_app and retention factors have been identified and discussed. In a detailed set of experiments, the effect of the concentration of the acid component in the MP containing MeOH or MeCN was specifically investigated, with the acid considered to be a displacer in anion-exchange type chromatographic systems. Surprisingly, all four chiral columns retained and resolved the tested N-acyl-Leu analytes with α_app values up to 21 within a retention factor window of 0.03 and 10 with pure MeOH as eluent. However, using pure MeCN as eluent, an almost infinite-long retention of the acidic analyte was noticed in all cases. We suggest that the rather different thickness of the solvation shells generated by MeOH or MeCN around the charged/chargeable sites of the chiral selector determines eventually the strength of the electrostatic selector-selectand interactions. As a control experiment we included the non-chiral N-acylglycine derivatives as analyte in all cases to support the interpretations with respect to the contribution of the enantioselective and non-enantioselective retention factor increments as a part of the observed α_app.

Immobilization of Chondroitin Sulfate A onto Monolithic Epoxy Silica Column as a New Chiral Stationary Phase for High-Performance Liquid Chromatographic Enantioseparation

Chondroitin sulfate A was covalently immobilized onto a monolithic silica epoxy column involving a Schiff base formation in the presence of ethylenediamine as a spacer and evaluated in terms of its selectivity in enantioseparation. The obtained column was utilized as a chiral stationary phase in enantioseparation of amlodipine and verapamil using a mobile phase consisting of 50 mM phosphate buffer pH 3.5 and UV detection. Sample dilution by organic solvents (preferably 25% v/v acetonitrile-aqueous solution) was applied to achieve baseline enantioresolution (Rs > 3.0) of the individual drug models within 7 min, an excellent linearity (R² = 0.999) and an interday repeatability of 1.1% to 1.8% RSD. The performance of the immobilized column for quantification of racemate in commercial tablets showed a recovery of 86–98% from tablet matrices. Computational modeling by molecular docking was employed to investigate the feasible complexes between enantiomers and the chiral selector.

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).

A study of tetrapeptide enantiomeric separation on crown ether based chiral stationary phases

The chiral separations of small peptides is an important challenge in the biological and medical sciences, because different stereoisomers of chiral drugs can often possess different pharmacological, pharmacokinetic, and/or toxicological activities. Commercially available crown ether chiral stationary phases based on S-(3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6 (CROWNPAK CR-I (+)) and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (ChiroSil RCA (+)) have been successfully used for separating enantiomers of various racemic compounds containing primary amino groups. In this investigation, enantioresolution of more complex model analyte - tetrapeptide Tyr-Arg-Phe-Lys-NH₂, has been reported on crown ether chiral stationary phases. Organic and acidic modifier content in aqueous mobile phase was tested. All Tyr-Arg-Phe-Lys-NH₂ stereoisomers showed U-shaped retention plots, based on ACN content in mobile phase. Increased retention of tetrapeptide stereoisomers was observed at low (<35%) and at high (>70%) acetonitrile content in the mobile phase, indicating that different separation mechanisms are most likely involved. As a result, baseline separation of all eight tetrapeptide enantiomer pairs was achieved under isocratic elution mode on both chiral columns.

Effect of Various Parameters and Mechanism of Reversal Order of Elution in Chiral HPLC

Background:

Chiral separation involves many phenomena in which the elution order of the enantiomers has its unique position. The phenomenon of elution order of the enantiomers has also been used in the determination of optical purity which is favorable to elute the major component after minor enantiomeric impurity but the main problem is that, this phenomenon is rare.

Results:

This review rumors the reversal order of elution of many chiral molecules in HPLC. Besides, this review pronounces the effects of pH, derivatisation of drugs, the composition of the mobile phase, and temperature on the reversal order of elution of chiral drugs. The efforts are also made to discuss the possible future perspectives of reversal order of elution.

Conclusion:

Various parameters such as pH, mobile phase composition, temperature, and chemical structure of the analytes play a role in the phenomena of the reversal order of elution of many chiral molecules which are discussed in the article.

Liquid chromatography separation of α- and γ-linolenic acid positional isomers with a stationary phase based on covalently immobilized cellulose tris(3,5-dichlorophenylcarbamate)

α-Linolenic acid (ALA) and its most important positional isomer γ-linolenic acid (GLA), are essential fatty acids (vitamin F). Therefore, ALA- and GLA-rich edible oils hold great potential in human and animal nutrition, as well as in nutraceutics and cosmetics. Quality control and nutritional validation of oil products is thus of increasing importance. In the present study, the cellulose tris(3,5-dichlorophenylcarbamate)-based chiral stationary phase was successfully used for separation of ALA and GLA, a major challenge in the liquid chromatography of these isomers. The chromatographic conditions were firstly optimized on a HPLC system with UV detection, and the use of a reversed-phase eluent system made up of aqueous 10 mM ammonium acetate/acetonitrile (40/60, v/v; _w^spH6.0) with a 25 °C column temperature resulted optimal for the simultaneous discrimination of the two isomers at a 0.5 mL/min flow rate (α = 1.10; R_S = 1.21). The method was then optimized for LC-MS/MS implementation. The proposed innovative separation method holds a great potential for the quantification of ALA and GLA in food and biological matrices, thus opening the way to further investigations involving the two positional isomers.

Comparative study on enantiomer resolving ability of amylose tris(3-chloro-5-methylphenylcarbamate) covalently immobilized onto silica in nano-liquid chromatography and capillary electrochromatography

In the present study separation of enantiomers of some chiral neutral and weakly acidic analytes was investigated on the chiral stationary phase (CSP) made by covalent immobilization of amylose tris(3-chloro-5-methylphenylcarbamate) onto silica in nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) in acetonitrile and aqueous acetonitrile. Few comparisons were made also between the enantioseparations in nano-LC and high-performance liquid chromatography (HPLC) with the chiral column of 4.6 × 250 mm dimension. Slightly better separation of enantiomers was observed in HPLC mode compared to nano-LC mode. It was shown that in the capillary columns packed with the CSP containing about 20% (w/w) of a covalently immobilized neutral chiral selector, amylose tris(3-chloro-5-methylphenylcarbamate), sufficient electroosmotic flow has been generated and enantioseparations with reasonable analysis time were performed also in CEC mode. It was shown once again that CEC offers a clear advantage over nano-LC from the viewpoint of plate numbers and peak resolution.

Direct Enantiomeric Resolution of Seventeen Racemic 1,4-Dihydropyridine-Based Hexahydroquinoline Derivatives by HPLC

1,4-Dihydropyridine (DHP) scaffold holds an outstanding position with its versatile pharmacological properties among all heterocyclic compounds. Although most of the commercially available DHPs are marketed as a racemic mixture, the chiral center at C-4 can lead to even opposite pharmacological activities between the enantiomers. In the present study, enantioseparation of seventeen DHP structural analogues, consisting of either pharmacologically active or newly synthesized derivatives, (M2-4, MD5, HM2, HM10, CE5, N11, N10, N7, M11, MC6-8, MC13, MD23, and 42IIP) by high-performance liquid chromatography was investigated using immobilized polysaccharide-based chiral stationary phase, Chiralpak IC column. Due to the solvent versatility of the covalently immobilized chiral stationary phase in enantiomer separation, multiple elution modes including standard normal phase, nonstandard mobile phase, and reversed phase were used to expand the possibility to find the optimum enantioselective conditions for the tested analytes. Under appropriate separation conditions, complete enantiomeric separation was obtained for nearly all compounds except MC6-8 and MC13 which contained two chiral centers. Additionally, the effects of the polar modifier, the additive, and column temperature on the chiral recognition were evaluated. The thermodynamic parameters calculated according to the linear van’t Hoff equation indicated that the chiral separations in this study were enthalpy-driven or entropy-driven. Some parameters of method validation such as linearity, limit of quantitation, and repeatability were also measured for all studied compounds to prove the reliability of the method.

New Brush-Type Chiral Stationary Phases for Enantioseparation of Pharmaceutical Drugs

The importance of chirality in drug development is unquestionable, with chiral liquid chromatography (LC) being the most adequate technique for its analysis. Among the various types of chiral stationary phases (CSPs) for LC, brush-type CSPs provide the base for interaction analysis of CSPs and enantiomers, which provide valuable results that can be applied to interaction studies of other CSP types. In order to analyze the influence of aromatic interactions in chiral recognition, we designed a set of ten new brush-type CSPs based on (S)-N-(1-aryl-propyl)-3,5-dinitrobenzamides which differ in the aromatic unit directly linked to the chiral center. Thirty diverse racemates, including several nonsteroidal anti-inflammatory drugs and 3-hydroxybenzodiazepine drugs, were used to evaluate the prepared CSPs. Chromatographic analysis showed that the three new CSPs separate enantiomers of a wide range of compounds and their chromatographic behavior is comparable to the most versatile brush-type CSP—Whelk-O1. The critical role of the nonbonding interactions in positioning of the analyte (naproxen) in the cleft of CSP-6, as well as the analysis of interactions that make enantioseparation possible, were elucidated using computational methods. Furthermore, the influence of acetic acid as a mobile phase additive, on this enantiorecognition process was corroborated by calculations.

Hybrid Organic-Inorganic Materials Containing a Nanocellulose Derivative as Chiral Selector

Hybrid organic-inorganic materials (HOIM), with high mechanical stability, large surface area, tailored pore size, controlled morphology, and organic loading have shown superior chiral separation performance. In this chapter, the preparation of hybrid organic-inorganic materials of core-shell silica microspheres by a layer-by-layer self-assembly method is described. The enantioseparation performance by high-performance liquid chromatography is illustrated by various types of chiral compounds under normal- and reversed-phase elution conditions. The chiral selector of nanocrystalline cellulose derivative hybrid organic-inorganic materials showed good performance in the separation of enantiomers.

HPLC Enantioseparations with Polysaccharide-Based Chiral Stationary Phases in HILIC Conditions

In contrast to achiral hydrophilic interaction liquid chromatography (HILIC), which is a popular and largely applied technique to analyze polar compounds such as pharmaceuticals, metabolites, proteins, peptides, amino acids, oligonucleotides, and carbohydrates, the introduction of the HILIC concept in enantioselective chromatography has been relatively recent and scarcely debated. In this chapter, the HILIC enantioseparations carried out on polysaccharide-based chiral stationary phases are grouped and discussed. Another objective of this chapter is to provide a comprehensive overview and insight into the experimental conditions needed to operate under HILIC mode. Finally, to stimulate and facilitate the application of this chromatographic technique, a detailed experimental protocol of a chiral resolution on a chlorinated cellulose-based chiral stationary phase under HILIC conditions is described.

A chromatographic study on the retention behavior of the amylose tris(3-chloro-5-methylphenylcarbamate) chiral stationary phase under aqueous conditions

In this study, the retention properties of the immobilized polysaccharide-derived Chiralpak IG-3 chiral stationary phase under aqueous-organic conditions were investigated. A systematic evaluation of the retention factors of the enantiomers of the chiral sulfoxide oxfendazole, endowed with anthelmintic activity and selected as test compound, was carried out changing progressively the water content in hydro-organic eluents containing methanol, ethanol or acetonitrile. From the results obtained with acetonitrile/water mobile phases and the associated retention plots, clear U-shape retention dependencies, indicative of the interplay of both hydrophilic interaction liquid chromatography and reversed-phase modes, were highlighted. A U-turn point of retention mechanism was recorded in correspondence of the acetonitrile/water 100:40 v/v mobile phase. Retention was significantly affected by small percentages of trifluoroacetic acid or diethylamine additives incorporated in the mobile phase. It is worth emphasizing that the basic additive was more effective in reducing retention in the reversed-phase region, while the action of acid additive was more pronounced in the hydrophilic interaction liquid chromatography region. Finally, either in the transition from hydrophilic interaction liquid chromatography to reversed-phase conditions or after additive addition, the enantioselectivity did not vary significantly.

The Composition of the Mobile Phase Affects the Dynamic Chiral Recognition of Drug Molecules by the Chiral Stationary Phase

More than half of all pharmaceuticals are chiral compounds. Although the enantiomers of chiral compounds have the same chemical structure, they can exhibit marked differences in physiological activity; therefore, it is important to remove the undesirable enantiomer. Chromatographic separation of chiral enantiomers is one of the best available methods to get enantio-pure substances, but the optimization of the experimental conditions can be very time-consuming. One of the most widely used chiral stationary phases, amylose tris(3,5-dimethylphenyl carbamate) (ADMPC), has been extensively investigated using both experimental and computational methods; however, the dynamic nature of the interaction between enantiomers and ADMPC, as well as the solvent effects on the ADMPC-enantiomer interaction, are currently absent from models of the chiral recognition mechanism. Here we use QM/MM and molecular dynamics (MD) simulations to model the enantiomers of flavanone on ADMPC in either methanol or heptane/2-propanol (IPA) (90/10) to elucidate the chiral recognition mechanism from a new dynamic perspective. In atomistic MD simulations, the 12-mer model of ADMPC is found to hold the 4/3 left-handed helical structure in both methanol and heptane/IPA (90/10); however, the ADMPC polymer is found to have a more extended average structure in heptane/IPA (90/10) than in methanol. This results from the differences in the distribution of solvent molecules close to the backbone of ADMPC leads to changes in the distribution of the (φ, ψ) dihedral angles of the glycoside bond (between adjacent monomers) that define the structure of the polymer. Our simulations have shown that the lifetime of hydrogen bonds formed between ADMPC and flavanone enantiomers in the MD simulations are able to reproduce the elution order observed in experiments for both the methanol and the heptane/IPA solvent systems. Furthermore, the ratios of hydrogen-bonding-lifetime-related properties also capture the solvent effects, in that heptane/IPA (90/10) is found to make the separation between the two enantiomers of flavanone less effective than methanol, which agrees with the experimental separation factors of 0.9 versus 0.4 for R/S, respectively.

Enantiomers of triclabendazole sulfoxide: Analytical and semipreparative HPLC separation, absolute configuration assignment, and transformation into sodium salt

Direct HPLC separation of the enantiomers of triclabendazole sulfoxide (TCBZ-SO), which is the main metabolite of the anthelmintic drug triclabendazole, was carried out using the polysaccharide-based Chiralpak AS-H and Chiralpak IF-3 chiral stationary phases (CSPs). The chromatographic behaviour of both CSPs was evaluated and compared using normal-phase and reversed-phase eluents at different column temperatures. The eluent mixture of n-hexane-2-propanol-trifluoroacetic acid 70:30:0.1 (v/v/v) and a column temperature of 40°C were identified as the best operational conditions to carry out semipreparative enantioseparations on a 1-cm I.D. AS-H column. Under these conditions, 12.5mg of racemic sample were resolved in a single chromatographic run within 15min. Comparison of calculated and experimental chiroptical properties provided the absolute configuration assignment at the sulfur atom. The salification of the isolated enantiomers of TCBZ-SO by reaction with sodium hydroxide solution produced water-soluble Na salts which are potentially useful in the development of new anthelmintic enantiomerically pure formulations.

Effect of pore-size optimization on the performance of polysaccharide-based superficially porous chiral stationary phases for the separation of enantiomers in high-performance liquid chromatography

Our earlier studies on the preparation of chiral stationary phases (CSP) based on superficially porous (or core-shell) silica (SPS) particles for the separation of enantiomers in HPLC have provided proof to the advantages of such sorbents. In particular, higher enantioselectivity was observed with the columns packed with superficially porous CSP compared to the columns packed with fully-porous (FP) silica-based CSPs at comparable content of chiral selector (polysaccharide derivative) in CSP. Also, less dependence of plate height on mobile phase flow rate and higher plate numbers and resolution calculated per unit time (i.e. speed of separation) were observed with SPS-based CSPs. Thirty years of CSP development have demonstrated that wide-pore silica has to be used as a support for large molecular weight chiral selectors such as the ones based on polysaccharides. In this study the effect of pore size of the core-shell silica support and of other experimental factors on column performance is demonstrated. Reduced plate heights in the range 1.4-1.5 were obtained, as well as highly effective baseline separations of enantiomers were observed with analysis times of less than 15s.

High-performance liquid chromatography enantioseparation of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases under multimodal elution

An investigation on the high-performance liquid chromatography enantioseparation of 12 polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases is described. The overall study was directed toward the generation of efficient separations in order to obtain pure atropisomers that will serve as ligands for building homochiral metal organic frameworks. Four coated columns--namely, Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-4, and Lux Amylose-2--and two immobilized columns--namely, Chiralpak IC and IA--were used under normal, polar organic, and reversed-phase elution modes. Moreover, Chiralcel OJ was considered under normal-phase and polar organic conditions. The effect of the chiral selector and mobile phase composition on the enantioseparation, the enantiomer elution order and the beneficial effect of nonstandard solvents were studied. The effect of water in the mobile phase on the enantioselectivity and retention was investigated and retention profiles typical of hydrophilic interaction liquid chromatography were observed. Interesting phenomena of solvent-induced enantiomer elution order reversal occurred under normal-phase mode. All the considered 4,4'-bipyridines were enantioseparated at the multimilligram level.