An instance of temperature-dependent elution order of enantiomers from a chiral brush-type HPLC 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.

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.

Solvent-induced enantioselectivity reversal in a chiral metal organic framework

Solvent-induced enantioselectivity reversal is a rarely reported phenomenon in porous homochiral materials. Similar behavior has been studied in chiral high performance liquid chromatography, where minor modifications to the mobile phase can induce elution order reversal of two enantiomers on a chiral stationary phase column. We report the first instance of solvent-induced enantioselectivity reversal in a homochiral metal organic framework. Further, we highlight the complex enantioselectivity behavior of homochiral metal organic frameworks toward racemic mixtures in the presence of solvents through racemate-solvent enantioselectivity and loading experiments as well as enantiopure-solvent loading experiments. We hypothesize that this interesting selectivity reversal behavior is likely to be observed in other competitive adsorption, nonchiral selective processes involving a solvent.

How mobile phase composition and column temperature affect enantiomer elution order of liquid crystals on amylose tris(3-chloro-5-methylphenylcarbamate) as chiral selector

A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod-like liquid crystalline materials. The analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed-phase modes using amylose tris(3-chloro-5-methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature-induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.

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.

Design and synthesis of a stereodynamic catalyst with reversal of selectivity by enantioselective self-inhibition

Chirality plays a pivotal role in an uncountable number of biological processes, and nature has developed intriguing mechanisms to maintain this state of enantiopurity. The strive for a deeper understanding of the different elements that constitute such self-sustaining systems on a molecular level has sparked great interest in the studies of autoinductive and amplifying enantioselective reactions. The design of these reactions remains highly challenging; however, the development of generally applicable principles promises to have a considerable impact on research of catalyst design and other adjacent fields in the future. Here, we report the realization of an autoinductive, enantioselective self-inhibiting hydrogenation reaction. Development of a stereodynamic catalyst with chiral sensing abilities allowed for a chiral reaction product to interact with the catalyst and change its selectivity in order to suppress its formation, which caused a reversal of selectivity over time.

Chromatographic and thermodynamic comparison of amylose tris(3-chloro-5-methylphenylcarbamate) coated or covalently immobilized on silica in high-performance liquid chromatographic separation of the enantiomers of select chiral weak acids

The separation of enantiomers of some chiral weak acids was studied in HPLC with chiral HPLC columns prepared by coating or covalent immobilization of the same chiral selector, namely amylose tris(3-chloro-5-methylphenylcarbamate) onto silica. After screening some representatives of arylpropionic acid derivatives, coumarins and barbiturates in hydrocarbon-alcohol type mobile phases, we studied the temperature dependence of separation parameters for ketoprofen and naproxen. Instances of reversal of the enantiomer elution order were observed function of column temperature, nature of polar modifier and its content in the mobile phase, as well as between the coated and covalently immobilized versions of the columns made with more-or-less the same chiral selector. Thermodynamic parameters such as Gibb's free energy, the standard molar entropy and the standard molar enthalpy of analyte transfer from the mobile to the stationary phase were calculated in some cases in order to explain the differences observed in the enantiomer separation ability and pattern of coated and covalently immobilized columns.

Temperature and eluent composition effects on enantiomer separation of carvedilol by high-performance liquid chromatography on immobilized amylose-based chiral stationary phases

Carvedilol is a chiral drug with potent antihypertensive and antianginal activities. Although it is clinically used as a racemic mixture, its enantiomers show different pharmacokinetic and pharmacodynamic profiles. Here, the direct chiral separation of racemic drug by high performance liquid chromatography using two immobilized-type amylose-based chiral stationary phases is presented. Some chromatographic parameters, such as retention and selectivity, were determined under multimodal eluent conditions and different temperatures. A temperature-dependent inversion of the elution order of enantiomers was observed in the operative temperature range of chiral chromatographic support. Finally, an effective direct enantioselective method was successfully applied to the separation of the enantiomers of carvedilol on a semipreparative scale.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.

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.

Liquid chromatographic resolution of secondary amino alcohols on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

Two types of secondary amino alcohols were successfully resolved on a liquid chromatographic chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. The secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center were resolved much better than those containing a hydroxyl group attached directly to the stereogenic center. In addition, the dependence of the separation factor (alpha) on column temperature was exactly opposite for the two different types of secondary amino alcohols. From the thermodynamic calculations based on Van't Hoff plots, we conclude that the enantioselectivity for the resolution of the secondary amino alcohols containing a secondary amino group connected directly to the stereogenic center is enthalpically controlled while that for the resolution of the secondary amino alcohols containing a hydroxyl group attached directly to the stereogenic center is entropically controlled.

Resolution of beta-blockers on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid: unusual temperature effect

A chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid has been successfully employed in the liquid chromatographic resolution of eleven beta-blockers containing a secondary amino functional group. As the result of an effort to find out the optimal mobile phase condition, the mixture of trifluoroacetic acid-triethylamine-ethanol-acetonitrile with the ratio of 0.1/0.5/20/80 (v/v/v/v) was concluded to be the best mobile phase condition, the separation (alpha) and the resolution factors (Rs) for the resolution of 11 beta-blockers being in the range of 1.13-1.85 and 1.36-5.79, respectively. Surprisingly, in contrast to the resolution of other racemic compounds containing a primary amino functional group, the separation factors (alpha) for the resolution of beta-blockers were observed to improve as the column temperature increased and these unusual chromatographic behaviors were rationalized as the entropically controlled enantioselectivity.

Characterization of a Thermally Induced Irreversible Conformational Transition of Amylose Tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase in Enantioseparation of Dihydropyrimidinone Acid by Quasi-Equilibrated Liquid Chromatography and Solid-State NMR

A thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenylcarbamate) (i.e., Chiralpak AD) chiral stationary phase (CSP) in the enantioseparation of dihydropyrimidinone (DHP) acid racemate was studied for the first time by quasi-equilibrated liquid chromatography with cyclic van't Hoff and step temperature programs and solid-state ((13)C CPMAS and (19)F MAS) NMR using ethanol and trifluoroacetic acid (TFA)-modified n-hexane as the mobile phase. The conformational transition was controlled by a single kinetically driven process, as evidenced by the chromatographic studies. Solid-state NMR was used to study the effect of the temperature on the conformational change of the solvated phase (with or without the DHP acid enantiomers and TFA) and provided some viable structural information about the CSP and the enantiomers.

Enantioseparation of various amino acid derivatives on a quinine based chiral anion-exchange selector at variable temperature conditions. Influence of structural parameters of the analytes on the apparent retention and enantioseparation characteristics

The influence of temperature on the performance of an enantioselective anion-exchange type chiral selector (SO) was systematically investigated. The resolution of the enantiomers of 23 N-acylated amino acids (selectands, SAs) on a covalently immobilized quinine tert.-butylcarbamate chiral stationary phase (CSP) was studied under linear chromatographic conditions over a temperature range of 0-85 degrees C with hydro-organic buffers (pHa 6.0) as mobile phases. The apparent enantioseparation factors increased considerably at low column temperatures, indicating that enthalpic contributions are the dominating thermodynamic driving force for chiral recognition for all investigated SAs. Retention factors gave non-linear van't Hoff plots, while the corresponding apparent enantioseparation factors showed linear van't Hoff behavior. Correlations between magnitude and sign of the relative thermodynamic parameters of enantioselective adsorption (deltadeltaG, deltadeltaH and deltadeltaS) and specific structural features of the analytes, i.e., steric and electronic nature of the various side chains and the N-acyl groups, are discussed with the aim to rationalize their possible contributions to the overall chiral recognition.

Unusual effect of column temperature on chromatographic enantioseparation of dihydropyrimidinone acid and methyl ester on amylose chiral stationary phase

This paper reports an unusual effect of column temperature on the separation of the enantiomers of dihydropyrimidinone (DHP) acid and its methyl ester on a derivatized amylose stationary phase by normal-phase liquid chromatography. The separation of the DHP acid enantiomers was investigated using both carbamate-derivatized amylose and cellulose stationary phases (Chiralpak AD and Chiralcel OD) with an ethanol-n-hexane (EtOH-n-Hex) mobile phase. On the amylose phase, the van 't Hoff plot of the retention factor of the S-(+)-DHP acid was observed to be non-linear while that of R-(-)-DHP acid was linear. Likewise, the van 't Hoff plot for DHP acid enantioselectivity was non-linear with a transition occurring at approximately 30 degrees C. Furthermore, the van 't Hoff plot for the DHP acid enantioselectivity factor for data taken when heating the column from 5 to 50 degrees C was not superimposable with the same plot prepared with data from the cooling process from 50 to 5 degrees C. This observation suggested that the stationary phase was undergoing a thermally induced irreversible conformational change that altered the separation mechanism between the heating and cooling cycles. Similar phenomena were observed for the separation of the enantiomers of the DHP ester probe compound. The conformational change of the AD phase was shown to depend on the polar component of the mobile phase. When 2-propanol (2-PrOH) was used as the modifier instead of EtOH, the van 't Hoff plots for DHP acid were linear and thermally reversible, suggesting that no such irreversible conformational change occurs with this modifier. Conversely, when the AD phase was pre-conditioned with a more polar methanol (MeOH) or water containing mobile phase, thermal irreversibility of DHP acid enantioselectivity was once again observed. Interestingly, when the stationary phase was changed to its cellulose analogue, the Chiralcel OD, all van 't Hoff plots for the retention and selectivity of DHP acid were thermally reversible for both EtOH-n-Hex and 2-PrOH-n-Hex mobile phases.

Effects of temperature on retention of chiral compounds on a ristocetin A chiral stationary phase

The isocratic retention of enantiomers of chiral analytes, i.e. tryptophan, 1,2,3,4-tetrahydroisoquinoline and gamma-butyrolac tone analogs, was studied on a ristocetin A chiral stationary phase at different temperatures and with different mobile phase compositions, using the reversed-phase, polar-organic and normal-phase modes. By variation of the both mobile phase composition and the temperature, baseline separations could be achieved for these enantiomers. The retention factors and selectivity factors for the enantiomers of all investigated compounds decreased with increasing temperature. The natural logarithms of the retention factors (ln k) of the investigated compounds depended linearly on the inverse of temperature (1/T). van't Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (deltaH(o)), the apparent change in entropy (deltaS(o)) and the apparent change in Gibbs free energy (deltaG(o) ) for the transfer of analyte from the mobile to the stationary phase. The thermodynamic parameters (deltaH(o), deltaS(o) and deltaG(o)) were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system.

Reversal of elution order during the chiral separation in high performance liquid chromatography

The elution order of the enantiomers is one of the most important topics in the field of chiral separations. The reversal of enantiomeric elution order, although rare, could be observed during the investigations of chiral discrimination in high performance liquid chromatography (HPLC). Scrutinizing these phenomena must be helpful in determining the optical purities because it is favorable to elute the minor enantiomeric impurity before the major component. In this mini-review, several examples of such unusual behavior will be described from the point of mechanistic rationale.

Unusual effects of separation conditions on chiral separations

Unusual effects in liquid chromatographic separations of enantiomers on chiral stationary phases are reviewed with emphasis on polysaccharide phases. On protein phases and Pirkle phases reversal of the elution order between enantiomers due to variation of temperature and mobile phase composition has been reported. Most of the nonanticipated observations have dealt with the widely used polysaccharide phases. Reversed retention order and other stereoselective effects have been observed by variation of temperature, organic modifier and water content in nonpolar organic mobile phases.

Separation of drugs by packed-column supercritical fluid chromatography

Packed-column supercritical fluid chromatography (pSFC) has been expected to analyze various kinds of compounds. Many researchers have expected a new chromatographic technique that overcomes the limitations of other techniques, HPLC and GC. In pharmaceutical development, chromatography plays an important role in the evaluation of safety and efficacy of a new compound. This article provides an overview of the separation of drugs by pSFC. The effects of the chromatographic parameters were studied for the separation of steroids. In chiral separation, the successful results were shown and compared with HPLC.

Temperature-induced inversion of elution order in the enantioseparation of sotalol on a cellobiohydrolase I-based stationary phase

The effect of temperature on the resolution of (RS)-sotalol by immobilized cellobiohydrolase I (CBH I) was studied between 5 and 40 degrees C and Van 't Hoff plots of ln k versus 1/T were acquired at different pH values of the aqueous mobile phase and in the presence of varying organic cosolvents. The elution order of the enantiomers reverses in the range between 17 and 28 degrees C. Beyond this range, enantioseparations with comparatively high resolution factors are achieved either by decreasing or by increasing the temperature. The composition of the mobile phase influences the "crossover" temperature as well as the character of the global adsorption process of the (R)-(-)-enantiomer. Under certain conditions, (R)-(-)-sotalol exhibits an unusual endothermic adsorption behavior. Its retention time increases with increasing temperature. At room temperature (23 degrees C) the enantiomeric elution order can also be regulated by the solvent additive.

Intermolecular (1)H-(1)H Two-Dimensional Nuclear Overhauser Enhancements in the Characterization of a Rationally Designed Chiral Recognition System

Chiral stationary phases (CSPs) for liquid chromatography derived from N-(acyl)proline-3,5-dimethylanilides separate the enantiomers of N-(3,5-dinitrobenzoyl)-alpha-amino esters and amides with high levels of selectivity. These CSPs have been used to assemble a large body of chromatographic data which indirectly supports the validity of the mechanistic rationale originally used in the design of these CSPs. We herein report (1)H and (13)C chemical shift data obtained when the (S)-enantiomer of chiral solvating agent (CSA) 3, a soluble analogue of the selector used in CSP (S)-1, acts on each of the enantiomers of the dimethylamide of N-(3,5-dinitrobenzoyl)leucine, 2. The changes in chemical shift in the mixture of (S)-2 and (S)-3 support the existence of those interactions thought to be essential to chiral recognition in this system. In addition, significant intermolecular NOESY enhancements are observed in this mixture. These NOE data are consistent with the structure expected for the more stable diastereomeric adsorbate formed between (S)-2 and the (S)-proline-derived CSP 1. No intermolecular NOEs are observed for corresponding mixtures of the chiral solvating agent (S)-3 and (R)-2, the enantiomer least retained on (S)-CSP 1.