Comparative enantioseparation of chiral 4,4’-bipyridine derivatives on coated and immobilized amylose-based chiral stationary phases

Update on chiral recognition mechanisms in separation science

The stereospecific analysis of chiral molecules is an important issue in many scientific fields. In separation sciences, this is achieved via the formation of transient diastereomeric complexes between a chiral selector and the selectand enantiomers driven by molecular interactions including electrostatic, ion-dipole, dipole-dipole, van der Waals or π-π interactions as well as hydrogen or halogen bonds depending on the nature of selector and selectand. Nuclear magnetic resonance spectroscopy and molecular modeling methods are currently the most frequently applied techniques to understand the selector-selectand interactions at a molecular level and to draw conclusions on the chiral separation mechanism. The present short review summarizes some of the recent achievements for the understanding of the chiral recognition of the most important chiral selectors combining separation techniques with molecular modeling and/or spectroscopic techniques dating between 2020 and early 2024. The selectors include polysaccharide derivatives, cyclodextrins, macrocyclic glycopeptides, proteins, donor-acceptor type selectors, ion-exchangers, crown ethers, and molecular micelles. The application of chiral ionic liquids and chiral deep eutectic solvents, as well as further selectors, are also briefly addressed. A compilation of all published literature on chiral selectors has not been attempted.

Chiral high-performance liquid chromatography analysis of mono-, di-, and triacylglycerols with amylose- and cellulose-phenylcarbamate-based stationary phases

The ever-increasing technological advancement in the (ultra)high-performance liquid chromatography tandem (high-resolution) mass spectrometry platforms have largely contributed to steeply intensify the interest towards lipidomics research. However, mass spectrometers alone are unable to distinguish between enantiomers. This obstacle is especially evident in the case of glycerolipids analysis due the prochiral nature of glycerol. Until a couple of decades ago, the stereoselective analysis of triacylglycerols (TAGs) was performed on the end products generated either from their enzymatic or chemical hydrolysis, namely on mono- or diacyl-sn-glycerols (MAGs and DAGs, respectively). These were then mostly analyzed with Pirkle-type chiral stationary phases (CSPs) after dedicated multi-step derivatization procedures. One of the most significant drawbacks of these traditional methods for enantioselective TAGs analysis (actually of the produced MAGs and DAGs, often investigated as target species per se) was the difficulty to totally abolish the migration of fatty acyls between glycerol positions. This made difficult to control and keep unaltered the stereochemistry of the original molecules. Over the last two decades, it has been widely demonstrated that the enantioselective analysis of intact TAGs as well as of non-derivatized MAGs and DAGs can be efficiently obtained using polysaccharide-based CSPs incorporating either amylose- or cellulose-phenylcarbamate derivatives chiral selectors. In this paper, the enantioselective methods developed with these CSPs for the enantioselective direct LC analysis of MAGs, DAGs and TAGs embedding different types of fatty acid residues are comprehensively reviewed.

Unravelling dispersion forces in liquid-phase enantioseparation. Part I: Impact of ferrocenyl versus phenyl groups

BACKGROUND

Highly ordered chiral secondary structures as well as multiple (tunable) recognition sites are the keys to success of polysaccharide carbamate-based chiral selectors in enantioseparation science. Hydrogen bonds (HBs), dipole-dipole, and π-π interactions are classically considered the most frequent noncovalent interactions underlying enantioselective recognition with these chiral selectors. Very recently, halogen, chalcogen and π-hole bonds were also identified as interactions working in polysaccharide carbamate-based selectors to promote enantiomer distinction. On the contrary, the function of dispersion interactions in this field was not explored so far.

RESULTS

The enantioseparation of chiral ferrocenes featuring chiral axis or chiral plane as stereogenic elements was performed by comparing five polysaccharide carbamate-based chiral columns, with the aim to identify enantioseparation outcomes that could be reasonably determined by dispersion forces, making available a reliable experimental data set for future theoretical studies to confirm the heuristic hypothesis. The effects of mobile phase polarity and temperature on the enantioseparation were considered, and potential recognition sites on analytes and selectors were evaluated by electrostatic potential (V) analysis and molecular dynamics (MD). In this first part, the enantioseparation of 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine bearing two ferrocenylethynyl units linked to an axially chiral core was performed and compared to that of the analyte featuring the same structural motif with two phenyl groups in place of the ferrocenyl moieties. The results of this study showed the superiority of the ferrocenyl compared to the phenyl group, as a structural element favouring enantiodifferentiation.

SIGNIFICANCE AND NOVELTY

Even if dispersion (London) forces have been envisaged acting in liquid-phase enantioseparations, focused studies to explore possible contributions of dispersion forces with polysaccharide carbamate-based selectors are practically missing. This study allowed us to collect experimental information that support the involvement of dispersion forces as contributors to liquid-phase enantioseparation, paving the way to a new picture in this field.

Recent advances in chiral selectors immobilization and chiral mobile phase additives in liquid chromatographic enantio-separations: A review

For decades now, the separation of chiral enantiomers of drugs has been gaining the interest and attention of researchers. In 1991, the first guidelines for development of chiral drugs were firstly released by the US-FDA. Since then, the development in chromatographic enantioseparation tools has been fast and variable, aiming at creating a suitable environment where the physically and chemically identical enantiomers can be separated. Among those tools, the immobilization of chiral selectors (CS) on different stationary phases and the chiral mobile phase additives (CMPA) which have been progressed and studied extensively. This review article highlights the major advances in immobilization of CS together with their different recognition mechanisms as well as CMPA as a cheaper and successful alternative for chiral stationary phases. Moreover, the role of molecular modeling tool as a pre-step in the choice of CS for evaluating possible interactions with different ligands has been pointed up. Illustrations of reported methods and updates for immobilized CS and CMPA have been included.

Enantioseparation of planar chiral ferrocenes on cellulose-based chiral stationary phases: Benzoate versus carbamate pendant groups

In this study, the enantioseparation of 14 planar chiral ferrocenes containing halogen atoms, and methyl, iodoethynyl, phenyl, and 2-naphthyl groups, as substituents, was explored with a cellulose tris(4-methylbenzoate) (CMB)-based chiral column under multimodal elution conditions. n-Hexane/2-propanol (2-PrOH) 95:5 v/v, pure methanol (MeOH), and MeOH/water 90:10 v/v were used as mobile phases (MPs). With CMB, baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.24 to 1.77, whereas only three analytes could be enantioseparated with 1.14 ≤ α ≤ 1.51 on a cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC)-based column, used as a reference for comparison, under the same elution conditions. Pendant group-dependent reversal of the enantiomer elution order was observed in several cases by changing CMB to CDMPC. The impact of analyte and chiral stationary phase (CSP) structure, and MP polarity on the enantioseparation, was evaluated. The two cellulose-based CSPs featured by different pendant groups were also compared in terms of thermodynamics. For this purpose, enthalpy (ΔΔH°), entropy (ΔΔS°) and free energy (ΔΔG°) differences, isoenantioselective temperatures (T_iso ), and enthalpy/entropy ratios (Q), associated with the enantioseparations, were derived from van 't Hoff plots by using n-hexane/2-PrOH 95:5 v/v and methanol/water 90:10 v/v as MPs. With the aim to disclose the functions of the different substituents in mechanisms and noncovalent interactions underlying analyte-selector complex formation at molecular level, electrostatic potential (V) analysis and molecular dynamics simulations were used as computational techniques. On this basis, enantioseparations and related mechanisms were investigated by integrating theoretical and experimental data.

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.

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.

Enantioselective Separation of Chiral N1-Substituted-1H-pyrazoles: Greenness Profile Assessment and Chiral Recognition Analysis

Two commercialized polysaccharide-based chiral stationary phases, Lux cellulose-2 and Lux amylose-2, were examined for their chiral recognition ability on a set of 18 biologically active racemic 4,5-dihydro-1H-pyrazole derivatives by applying normal and polar organic elution modes. The results showed that all compounds were baseline-resolved with at least one of the used elution modes. The cellulose-based column was superior using polar organic mobile-phase compositions with analysis times close to 5 min and resolutions up to 18, while the enantiomer-resolving ability of amylose-based columns was greater using the normal elution mode with analysis times close to 30 min and resolutions up to 30. The competition between the analytes and the mobile phase constituents on H-bond interactions with the stationary phase has been discussed, and the impact of this competition on chiral recognition has been investigated. It was found that the polar organic mode is very beneficial for short run times and sharp peaks. The developed enantioselective high-performance liquid chromatography (HPLC) methods will be applied to monitor the stereoselective synthesis of compounds 1-18 or to develop preparative HPLC techniques for compounds 1-18, followed by stereospecific pharmacological studies for each enantiomer separately. Greenness profile assessment of the different elution solvents was carried out using the AGREE metric approach.

Enantioseparations of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases and molecular dynamics simulations of selector-selectand interactions

2'-(4-Pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs (TCIBP = 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridyl) are chiral compounds that showed interesting inhibition activity against transthyretin fibrillation in vitro. We became interested in their enantioseparation since we noticed that the M-stereoisomer is more effective than the P-enantiomer. Based thereon, we recently reported the enantioseparation of 2'-substituted TCIBP derivatives with amylose-based chiral columns. Following this study, herein we describe the comparative enantioseparation of both 2'-(4-pyridyl)- and 2'-(4-hydroxyphenyl)-TCIBPs on four cellulose phenylcarbamate-based chiral columns aiming to explore the effect of the polymer backbone, as well as the nature and position of substituents on the side groups on the enantioseparability of these compounds. In the frame of this project, the impact of subtle variations of analyte and polysaccharide structures, and mobile phase (MP) polarity on retention and selectivity was evaluated. The effect of temperature on retention and selectivity was also considered, and overall thermodynamic parameters associated with the analyte adsorption onto the CSP surface were derived from van 't Hoff plots. Interesting cases of enantiomer elution order (EEO) reversal were observed. In particular, the EEO was shown to be dependent on polysaccharide backbone, the elution sequence of the two analytes being P-M and M-P on cellulose and amylose tris(3,5-dimethylphenylcarbamate), respectively. In this regard, a theoretical investigation based on molecular dynamics (MD) simulations was performed by using amylose and cellulose tris(3,5-dimethylphenylcarbamate) nonamers as virtual models of the polysaccharide-based selectors. This exploration at the molecular level shed light on the origin of the enantiodiscrimination processes.

Enantioseparation of 5,5'-Dibromo-2,2'-Dichloro-3-Selanyl-4,4'-Bipyridines on Polysaccharide-Based Chiral Stationary Phases: Exploring Chalcogen Bonds in Liquid-Phase Chromatography

The chalcogen bond (ChB) is a noncovalent interaction based on electrophilic features of regions of electron charge density depletion (σ-holes) located on bound atoms of group VI. The σ-holes of sulfur and heavy chalcogen atoms (Se, Te) (donors) can interact through their positive electrostatic potential (V) with nucleophilic partners such as lone pairs, π-clouds, and anions (acceptors). In the last few years, promising applications of ChBs in catalysis, crystal engineering, molecular biology, and supramolecular chemistry have been reported. Recently, we explored the high-performance liquid chromatography (HPLC) enantioseparation of fluorinated 3-arylthio-4,4′-bipyridines containing sulfur atoms as ChB donors. Following this study, herein we describe the comparative enantioseparation of three 5,5′-dibromo-2,2′-dichloro-3-selanyl-4,4′-bipyridines on polysaccharide-based chiral stationary phases (CSPs) aiming to understand function and potentialities of selenium σ-holes in the enantiodiscrimination process. The impact of the chalcogen substituent on enantioseparation was explored by using sulfur and non-chalcogen derivatives as reference substances for comparison. Our investigation also focused on the function of the perfluorinated aromatic ring as a π-hole donor recognition site. Thermodynamic quantities associated with the enantioseparation were derived from van’t Hoff plots and local electron charge density of specific molecular regions of the interacting partners were inspected in terms of calculated V. On this basis, by correlating theoretical data and experimental results, the participation of ChBs and π-hole bonds in the enantiodiscrimination process was reasonably confirmed.