Chiral Analytes Research Articles

Dependence of enantioseparation performance on structure of chiral selectors derived from N-cycloalkylcarbonyl chitosan

In order to study dependence of enantioseparation capability on structure of chiral selectors derived from N-cycloalkylcarbonyl chitosan, nine chitosan 3,6-bis(arylcarbamate)-2-(cyclopentylformamide)s were synthesized. The corresponding chiral stationary phases (CSPs) were prepared with the chitosan derivatives. Enantioseparation capability of the CSPs was evaluated under the same conditions with the same chiral analytes. Different from CSPs derived from cyclopropylcarbonyl, cyclobutylcarbonyl and cyclohexylcarbonyl chitosans, all the CSPs with halo and methyl substituents in the present work showed good enantioseparation capability. Therefore, N-cyclopentylcarbonyl chitosan is more preferable to be used to develop enantiomeric separation materials among N-cycloalkylcarbonyl chitosans. Besides, the chitosan derivatives in this study and cellulose tris(3,5-dimethylphenylcarbamate) are complementary in enantioseparation for some chiral analytes. It was also observed that the chitosan derivative with 3,4-dichlorophenyl substituent baseline separated the most chiral analytes, thus revealing that this substituent can be applied as a building block to construct enantiomeric separation materials.

Separation of brombuterol enantiomers in capillary electrophoresis with cyclodextrin-type chiral selectors and investigation of structure of selector-selectand complexes using nuclear magnetic resonance spectroscopy.

The major goal of this study was to determine the affinity pattern of brombuterol (BB) enantiomers toward various cyclodextrins (CD) and to evaluate the potential of NMR spectroscopy for understanding fine mechanisms of interactions between CDs and BB enantiomers. Separation of BB enantiomers was performed in a fused-silica capillary using a phosphate buffer, pH 2.5, at the room temperature in the normal polarity mode. It was shown once again that CE in combination with NMR spectroscopy represents a very sensitive tool for studies of affinity patterns and structure of CD complexes with chiral guests. Although opposite affinity patterns of BB enantiomers were observed toward native β- and γ-CDs, no significant differences between the structures of the complexes of these two CDs with BB were detected by NMR spectroscopy. In contrary to this, the opposite affinity pattern of BB enantiomers toward β-CD and its two sulfated derivatives, heptakis (2,3-O-diacetyl-6-sulfo)-β-CD (HDAS-β-CD) and heptakis (2-O-methyl-3,6-di-O-sulfo)-β-CD (HMDS-β-CD) was associated with major differences in the structure of the complexes. In addition, it was shown again that HMDS-β-CD provides separation of enantiomers without formation of inclusion-type complex with the chiral analyte.

Click regulation of cyclodextrin primary face for the preparation of novel chiral stationary phases.

In this study, a series of novel CD chiral stationary phases were fabricated by immobilization of mono-6A -deoxy-N3 -cyclodextrin onto silica surfaces followed by click regulation of CD primary face with 4-pentynoic acid (acidic moiety), 2-propynylamine (alkaline moiety) and L-propargylglycine (chiral amino acid moiety), respectively. Enantioseparations of various kinds of racemates including dansyl-amino acids, chiral lactides and diketones were conducted in reversed phase modes on these chiral stationary phases, where nearly forty diketones and chiral lactides were firstly separated on cyclodextrin stationary phases. 4-Pentynoic acid moiety can make the retention ability decline while amine moiety significantly enhanced the retention ability of the stationary phases. For most of the studied analytes, the chiral amino acid moiety had the most positive effects on both the retention time and the resolution. The inclusion complexation between chiral analytes and cyclodextrins were also investigated by fluorescence method.

The degree of substitution affects the enantioselectivity of sulfobutylether-β-cyclodextrin chiral stationary phases.

Three chiral stationary phases were prepared by dynamic coating of sulfobutylether-β-cyclodextrin (SBE-β-CD) with different degrees of substitution, onto strong anion-exchange stationary phases. The enantioselective potential and stability of newly prepared chiral stationary phases were examined using a set of structurally different chiral analytes. Measurements were performed in RP-HPLC. Mobile phases consisted of methanol/formic acid, pH 2.10, and methanol/10 mM ammonium acetate buffer, pH 4.00, in various volume ratios. SBE-β-CDs with degrees of substitution (DS) 4, 6.3, and 10 proved suitable for the enantioseparation of 14, 11, and 8 analytes, respectively. The SBE-β-CD DS 4 based chiral stationary phase enabled the enantioseparation of the nearly all basic and neutral compounds. Chiral stationary phases with higher sulfobutylether-β-cyclodextrin substitution (especially DS 10) yielded higher enantioresolution values for acidic compounds.

Recognition Mechanisms of Chiral Selectors: An Overview.

Stereospecific recognition of chiral molecules plays an important role in nature as the basis of the interaction of chiral bioactive compounds with the chiral target structures. In separation sciences such as chromatographic and capillary electromigration techniques, interactions between chiral analytes and chiral selectors, i.e., the formation of transient diastereomeric complexes in thermodynamic equilibria, are the basis for chiral separations. Due to the large structural variety of chiral selectors, different structural features contribute to the overall chiral recognition process. This introductory chapter briefly summarizes the present understanding of the structural enantioselective recognition processes for various types of chiral selectors.

Performance evaluation of enantioseparation materials based on chitosan isobutylurea derivatives

Efficient resolution of chiral analytes was achieved using chiral stationary phases with chitosan isobutylurea derivatives as the chiral selector.

Enrichment and Separation of Cationic, Neutral, and Chiral Analytes by Micelle to Cyclodextrin Stacking-Micellar Electrokinetic Chromatography.

Analyte focusing by micelle to cyclodextrin stacking (MCDS) in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) and fused silica capillaries is demonstrated for neutral, cationic, and chiral analytes. The stacking was at a dynamic boundary formed between the injected charged SDS micelles and neutral γ-cyclodextrin (γ-CD) zones, where the analytes bound inside micelles were released due to the formation of stable SDS-CD inclusion complexes. The complex formation reduced or eliminated the affinity of the analytes to the micellar phase. There was reversal (for charged) or nulling (for neutrals) of the analyte's effective electrophoretic mobility that caused the analytes to accumulate at the boundary. Under the conditions where the SDS micelles velocity is faster than the electroosmotic flow (using acidic buffer), MCDS was conducted by injection of a long plug of sample in a micellar diluent after injection of a CD solution plug into a capillary that was filled with MEKC background solution. By simply extending the length of the CD plug, chiral separations of chlorpheniramine and phenoxyacid herbicides were achieved without optimizing the MEKC conditions. The analytical figures of merit including linearity and repeatability for the tested compounds were found acceptable, and the sensitivity enhancement factors were up to 171. The stacking strategy in MEKC was applied to metabolic stability studies of small molecules with HepG2 cell line, where the samples were only treated with acetonitrile and then diluted with the micellar diluent (demonstrating the reduction of tedious sample preparation requirements for biological samples prior to chemical analysis).

Rapid and mild fabrication of protein membrane coated capillary based on supramolecular assemble for chiral separation in capillary electrochromatography

Exploration of the simple and stable coating methods is of great significance in capillary electrochromatography (CEC). In this work, a lysozyme assemble supramolecular membrane coated capillary column was developed for CEC chiral separation. Taking advantage of phase transformation of lysozyme, the coating process was achieved within 1.0 h thus to a large extent reduced the capillary preparation time and simplified the coating procedure. The successful fabrication of the supramolecular membrane coated capillary was verified by scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), fluorescence imaging, and electroosmotic flow (EOF). The separation capacity of the coated capillary was evaluated by analysis of different chiral analytes, including chiral amine drug and neurotransmitters, and good enantioseparation efficiency was achieved for the three pairs of enantiomers. For three consecutive runs, the relative standard deviations (RSD) for the migration time of the analytes for intra-day (n = 3), inter-day (n = 3) and column-to-column (n = 3) were in the range of 0.7–1.5%, 2.7–3.6%, and 4.5–5.8%, respectively. Additionally, the supramolecular membrane coated capillary column could run consecutively 100 times without observable change in the separation efficiency, proving the feasibility of the coating method based on the adhesion of the protein-based supramolecular membrane.

The preparation of poly-levodopa coated capillary column for capillary electrochromatography enantioseparation

Levodopa (L-DOPA) is promising as chiral stationary phase for open tubular capillary electrochromatography (OT-CEC) enantioseparation owing to its self-polymerization adhesive property and chiral recognition potential. In this work, CuSO4/H2O2 was used as a trigger agent to accelerate the polymerization process of L-DOPA and the poly-levodopa (poly-(L-DOPA)) coated column was successfully prepared for the first time by depositing it on the inner wall of fused silica capillary via the rapid and in-situ approach at room temperature. The performance of the poly-(L-DOPA) coated capillary was validated by the separation of different chiral analytes, including chiral amine drugs, neurotransmitters and amino acids, and the good enantioseparation efficiencies were achieved. For five consecutive runs, the relative standard deviations (RSDs) for the migration time of the analytes for intra-day, inter-day and column-to-column were in the range of 0.19–1.33%, 0.96–4.47%, and 2.21–7.79%, respectively. Additionally, the poly-(L-DOPA) coated capillary column could be successively used over 250 runs without observable change in the separation efficiency.

Performances comparison of enantiomeric separation materials prepared from shrimp and crab shells

Previously reported studies demonstrate that many chitin/chitosan derivatives are promising for enantioseparation of chiral compounds. The aim of the present study is to investigate influence of the chitin sources on performances of the chitosan type enantiomeric separation materials. Therefore, the chitosans were prepared from crab and shrimp shells, from which two sets of chiral selector, i.e. chitosan bis(3,5-dimethylphenylcarbamate)-(octanamide)s and chitosan bis(3,5-dichlorophenylcarbamate)-(octanamide)s were synthesized. The chitosan bis(3,5-dimethylphenylcarbamate)-(octanamide)s, respectively, derived from crab and shrimp shells were close in swelling capacity and enantioseparation capability, and the same feature was found for the other two chiral selectors of chitosan bis(3,5-dichlorophenylcarbamate)-(octanamide). On the other hand, although most of the chiral analytes were eluted out in the same elution order, there were two analytes were in reversed elution orders when separated by the two chiral stationary phases of chitosan bis(3,5-dichlorophenylcarbamate)-(octanamide). Based on the observed results, we conclude that enantiomeric separation materials may be developed either with shrimp chitin or crab chitin, depending on the source accessibility.

Last ten years (2008-2018) of chiral ligand-exchange chromatography in HPLC: An updated review.

Chiral ligand-exchange chromatography is one of the elective strategies for the direct enantioresolution of small chelating compounds: amino acids, diamines, amino alcohols, diols, small peptides, etc. Unlike other methods, the interaction between chiral selector and analyte enantiomers is mediated by a cation, thus producing diastereomeric ternary complexes. Two main approaches are conventionally applied in chiral ligand-exchange chromatography. The first relies upon chiral stationary phases where the chiral selector is either covalently immobilized or physically adsorbed onto suitable packing materials (coated phases). In the second approach, chiral molecules are added to the eluent, thus generating chiral eluent systems. Among the advantages of chiral ligand-exchange chromatography, the generation of UV/vis-active metal complexes, and the use of commercially available or easy-to-synthesize chiral selectors, in combination to rather inexpensive achiral columns for coated phases and chiral eluents, are noteworthy. Besides amino acids and amino alcohols, other species have proven suitable for chiral ligand-exchange chromatography applications. Recently, the use of either chiral ionic liquids or micellar liquid chromatography systems as well as the successful off-column formation of diastereomeric complexes have expanded the selectivity profiles and application fields. All of these issues are touched in the review, shedding light to the contributions appeared in the last decade.

On our way to sub-second separations of enantiomers in high-performance liquid chromatography

In this study our preliminary attempt for obtaining fast and highly efficient separations of enantiomers in high-performance liquid chromatography with slightly modified state-of-the-art commercial instrumentation is described. In order to reach this goal after careful selection of chiral analytes, the preparation of chiral stationary phase (CSP), mobile phase composition and column dimensions were optimized. The concept of segmented chiral-achiral column was introduced. As the result of these optimizations baseline separation of enantiomers was achieved with the analysis time between 1–2 s.

MALDI coupled to modified traveling wave ion mobility mass spectrometry for fast enantiomeric determination.

In this work, the use of MALDI traveling wave ion mobility spectrometry-mass spectrometry (MALDI-TWIMS-MS) for stereoselective structural analysis of direct cleavage and identification of 2-substituted piperidines obtained through solid-phase asymmetric synthesis by using heterogeneous 8-phenylmenthyl-based chiral auxiliary resins. A strategy for gas-phase chiral and structural characterization of small molecular weight molecules by using MALDI-IMS-MS technique is discussed. Because both MALDI and IMS do not directly offer chiral resolution, an easy methodology by adding a chiral phase is described to carry out in situ online ion/molecule complexation with different chiral analytes inside the mass spectrometer. Piperidine enantiomers were resolved, and separation obtained shows dependence of surface areas. To corroborate this assumption and elucidate the separation mechanism to accomplish an analytical technique by which fast determination of the chirality of molecules may be determined for a wide range organic compound applications, it was performed DFT calculations to determine the cross-sectional areas of proton-bound dimer complexes. Drift times are affected by cross-sectional areas, correlating bigger times with bigger molecular volumes during the ion mobility experiments of proton-bound dimer complexes.

Surface-enhanced chiroptical spectroscopy with superchiral surface waves.

We study the chiroptical properties of one-dimensional photonic crystals supporting superchiral surface waves by introducing a simple formalism based on the Fresnel reflection matrix. We show that the proposed framework provides useful insights on the behavior of all the relevant chiroptical quantities, allowing for a deeper understanding of surface-enhanced chiral sensing platforms based on one-dimensional photonic crystals. Finally, we analyze and discuss the limitations of such platforms as the surface concentration of the target chiral analytes is gradually increased.

Chiral Separations Using a Modified Water Stationary Phase in Supercritical Fluid Chromatography

A novel means of achieving chiral separations in supercritical fluid chromatography (SFC) using a water stationary phase is presented. By adding various chiral selectors to the phase, different chiral analytes can be readily separated using neat CO2 as a mobile phase. For example, by adding β-cyclodextrin, it is found that certain flavanone enantiomers can be separated, while using the antibiotic vancomycin as a selector provides separation of some chiral phenoxypropionic acids. Other additives such as sodium chloride and triethylamine are also explored and found to enhance certain separations when also present in the water phase. While column pressure has a moderate impact on chiral analyte retention and separation in this SFC method, column temperature has a comparatively larger influence. In particular, relatively cooler temperatures below about 5 °C are found to markedly increase resolution and selectivity. For instance, notably large resolution of 4.7 is achieved for a phenoxypropionic acid pair at 0 °C and 150 atm CO2. Since the method does not require modifier to elute such polar species, it is also readily compatible with FID detection and does not generate organic waste. Therefore, results indicate that this approach could be a potentially simple and flexible means of achieving chiral separations in SFC.

Separation of enantiomers of chiral sulfoxides in high-performance liquid chromatography with cellulose-based chiral selectors using methanol and methanol-water mixtures as mobile phases

The interplay between structural details of chiral analytes and selectors in the separation of 14 chiral sulfoxides was systematically studied on 18 different polysaccharide-based chiral columns. Retention and enantioselectivity of a set of chiral sulfoxides were of primary interest. Several of chiral columns studied exhibited quite powerful chiral recognition ability in pure methanol. With addition of water to the mobile phase retention increased in the most cases and the separation factor improved. However, several exceptions were also noted. Of monosubstituted phenylcarbamates of cellulose as chiral selectors, chlorosubstituted ones did not show better enantiomer resolving ability compared to unsubstituted cellulose tris(phenylcarbamate). Out of disubstituted phenylcarbamates of cellulose the ones with methylsubstituents showed higher enantiomer resolving ability compared to chloro-substituted ones and substitution in positions 3 and 5 of the phenyl moiety was clearly advantageous. From disubstituted derivatives those possessing a combination of methyl- and chloro-substituents were advantageous compared to the ones having dimethyl- or dichloro-substituents. Interesting examples of reversal in enantiomer elution order (EEO) were observed on cellulose tris(4-chloro-3-methylphenylcarbamate)- and cellulose tris(3-chloro-4-methylphenylcarbamate)-based chiral stationary phases (CSPs) function of the water content in the mobile phase.

NMR Spectroscopic Determination of Enantiomeric Excess Using Small Prochiral Molecules.

The use of chiral auxiliaries, which derivatize enantiomers to diastereomers, is an established technique for NMR spectroscopic analysis of chirality and enantiomeric excess ( ee). Here we report that some small prochiral molecules exhibit ee-dependent splitting of 1H NMR signals at room temperature based on acid/base interactions with chiral analytes, especially when either a chiral or prochiral acid contains a phenoxy group at the α-position of the carboxylic acid. As a representative case, the benzylamine (BA)/2-phenoxylpropionic acid (PPA) complex was comprehensively investigated by using various methods. Notably, X-ray crystallographic analysis shows that there are multipoint interactions in the BA/PPA complex, implying that "fixing" of molecular conformation is critical for efficient intermolecular transfer of magnetic anisotropy. Our results suggest that a wide range of prochiral molecules are available for NMR determination of ee when intermolecular interactions between prochiral molecules and chiral analytes are adequately designed.

Unusual retention behavior of omeprazole and its chiral impurities B and E on the amylose tris (3-chloro-5-methylphenylcarbamate) chiral stationary phase in polar organic mode

Recent reports have demonstrated that the new commercially available immobilized-type chiral stationary phases (CSPs) containing amylose tris(3-chloro-5-methylphenylcarbamate) (ACMPC) as a selector exhibit not only an exceptionally high enantioselectivity in high-performance liquid chromatography (HPLC) but they are also applicable to a wide range of chiral analytes. Herein, we report the results obtained in the HPLC analysis of omeprazole and its impurities B and E on the ACMPC-based Chiralpak IG-3 CSP (CSP) under polar organic conditions. A systematic evaluation of the retention characteristics of the selected benzimidazole chiral probes was carried out by changing the composition of the mobile phase and the column temperature. It is worth emphasizing that the high affinity of both enantiomers of all analytes recorded in pure methanol mode dramatically decreased incorporating small volumes of either basic or acid additives in the mobile phase. Unspecified sites of the IG-3 CSP presumably involved in strong and non-stereoselective H-bonding contacts with chiral analytes are assumed responsible for the unproductive retention process.

Chirality Sensing, Halochromism and Solvatochromism in a Conjugated Calix[4]Pyrrole

The conjugated calix[4]pyrrole tetrakis(3,5-di-t-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) is a highly coloured macrocycle with excellent potential as a sensing element in various applications. The first of these is chiral sensing where guest binding at the macrocyclic NH groups allows for estimation of enantiomeric excesses in non-racemic mixtures of chiral analytes. This feature is permitted by the relatively weak binding of the guest resulting in its rapid exchange so that the molecule essentially acts as a probe of average solution chirality. Chirality sensing in OxP is complicated by tautomeric processes when an acid analyte is used. This has allowed us also to develop an acidity sensor for use in non-polar media. More recently, we have designed a series of OxP type molecules that undergo modulation of halochromic (sensitivity to acid) properties and the development of colour scales based on the blocking of OxP binding sites and involving allied protic tautomerization of the OxP chromophore. We will discuss the sensing properties of OxP in relation to chirality, tautomeric processes and the complicating factor of solvatochromism.

Determination of enantiomeric excess of some amino acids by second-order calibration of kinetic-fluorescence data

In this investigation a new non-separative kinetic-spectroflourimetric method is proposed for the determination of lysine (lys), leucine (leu) and phenylalanine (phe) enantiomers as their o-phthaldialdehyde (OPA) derivatives in the presence of an optically active chiral thiol compound, 1-mercapto-2-propanol (MP). At ambient temperature and in the borate buffer media of pH 9.6, MP, OPA, as highly selective fluorogenic reagents, and amino acid (AA) enantiomers reacts with each other to yield two fluorescent diasteriomers of D and L-AA with maximum difference in fluorescence intensity at about 450 nm. To achieve information from the small spectral changes, the data are analyzed by Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) method. Linear calibration curves are achieved to distinct D and L-lys, leu and phe in different mole ratios by applying appropriate constraints in MCR-ALS procedures. This is the first application of MCR-ALS in determination of enantiomeric excess (ee) using OPA/MP adduct as chiral reagent, which benefits from direct time dependent-fluorescence spectral analysis and does not require prior separation of chiral analytes. Both the cross-validated correlation coefficient (Q2) and root mean squares error of prediction (RMSEP) indicated satisfactory prediction ability of this method.