Wide pH range enantioseparation of cyclodextrin silica-based hybrid spheres for high performance liquid chromatography

L-ribose specific recognition surface constructed by pillar[5]arene-based host-guest interaction

In living organisms, chiral molecules have specific chiral conformations that produce different physiological effects. Ribose is one of the components of RNA, which mainly plays a role in regulating biological activity. Inspired by the biological recognition of sugars, functional chiral surfaces for recognizing L-ribose through non-covalent interactions were constructed. In the strategy of this study, a functional chiral gold surface based on host-guest interactions was constructed through the assembly of the host molecule single-function alynyl pillar[5]arene(SAP5) and the guest molecule (S) -mandelate-violet (SMV). The association constant of SMV and SAP5 was calculated to be 2.95×104 M-1, with a binding ratio of 1:1. By impedance and contact angle detection, the constructed functional interface has good detection effect on L-ribose in the range of 1×10-7 M to1× 10-2 M. In addition, CV was disassembled from the aromatic cavity of pillar[5]arene after adding zinc powder and it can repeat five times with good recyclability, thus achieving the organic combination of interface recognition and intelligence.

Homochiral organic molecular cage RCC3-R-modified silica as a new multimodal and multifunctional stationary phase for high-performance liquid chromatography

In this work, homochiral reduced imine cage was covalently bonded to the surface of the silica to prepare a novel high-performance liquid chromatography stationary phase, which was applied for the multiple separation modes such as normal phase, reversed-phase, ion exchange, and hydrophilic interaction chromatography. The successful preparation of the homochiral reduced imine cage bonded silica stationary phase was confirmed by performing a series of methods including X-ray photoelectron spectroscopy, thermogravimetric analysis, and infrared spectroscopy. From the extracted results of the chiral resolution in normal phase and reversed-phase modes, it was demonstrated that seven chiral compounds were successfully separated, among which the resolution of 1-phenylethanol reached the value of 3.97. Moreover, the multifunctional chromatographic performance of the new molecular cage stationary phase was systematically investigated in the modes of reversed-phase, ion exchange, and hydrophilic interaction chromatography for the separation and analysis of a total of 59 compounds in eight classes. This work demonstrated that the homochiral reduced imine cage not only achieved multiseparation modes and multiseparation functions performance with high stability, but also expanded the application of the organic molecular cage in the field of liquid chromatography.

Vacuum-assisted thermal bonding of β-cyclodextrin and its derivatives as chiral stationary phases for high-performance liquid chromatography

In this work, the vacuum-assisted thermal bonding method was proposed for the covalent coupling of β-cyclodextrin (β-CD) (CD-CSP), hexamethylene diisocyanate cross-linked β-CD (HDI-CSP) and 3, 5-dimethylphenyl isocyanate modified β-CD (DMPI-CSP) onto the isocyanate silane modified silica gel. Under vacuum conditions, the side reaction due to the water residue from the organic solvent, air, reaction vessels and silica gel could be avoided, and the optimal temperature and time of vacuum-assisted thermal bonding method were determined as 160°C and 3 h. These three CSPs were characterized by FT-IR, TGA, elemental analysis and the nitrogen adsorption-desorption isotherms. The surface coverage of CD-CSP and HDI-CSP on silica gel was determined as ∼0.2 μmol m^-2, respectively. The chromatographic performances of these three CSPs were systematically evaluated by separating 7 flavanones, 9 triazoles and 6 chiral alcohols enantiomers under the reversed-phase condition. It was found that the chiral resolution ability of CD-CSP, HDI-CSP and DMPI-CSP was complementary to each other. Among them, CD-CSP could separate all 7 flavanones enantiomers with the resolution of 1.09-2.48. HDI-CSP had a good separation performance for triazoles enantiomers with one chiral center. DMPI-CSP showed excellent separation performance for chiral alcohol enantiomers, among which the resolution of trans-1, 3-diphenyl-2-propen-1-ol reached 12.01. Generally, the vacuum-assisted thermal bonding had been demonstrated as a direct and efficient method for the preparation of chiral stationary phases of β-CD and its derivatives.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Cyclodextrin-dendrimers nanocomposites functionalized high performance liquid chromatography stationary phase for efficient separation of aromatic compounds

In this work, novel cyclodextrin-dendrimers nanocomposites functionalized high performance liquid chromatography stationary phases were developed for efficient separation of aromatic compounds. β-cyclodextrin was grafted onto the surface of silica gel matrix with poly (amidoamine) dendrimers as spacers. Scanning electron microscope, fourier transform infrared spectroscopy, element analysis and Brunner-Emmet-Teller measurement proved the successful grafting of cyclodextrin-dendrimers nanocomposites. The obtained stationary phases showed satisfactory separation effects for alkylbenzenes and benzenesulfonic acid substituents in reverse phase liquid chromatography mode. Weak hydrophilic and ion exchange interactions were also confirmed at the same time. Meanwhile, the effects of dendrimers and cyclodextrin on the chromatography performance were discussed. Separation mechanism of the stationary phases were verified by two-dimensional nuclear magnetic resonance technology, and the result implied that the proposed cyclodextrin-dendrimers nanocomposites functionalized stationary phases have significant prospects for separation and determination of more aromatic compounds in future.

Recent progress in the development of chiral stationary phases for high-performance liquid chromatography

Separations and analyses of chiral compounds are important in many fields, including pharmaceutical production, preparation of chemical intermediates, and biochemistry. High-performance liquid chromatography using a chiral stationary phase is regarded as one of the most valuable methods for enantiomeric separation and analysis because it is highly efficient, is broadly applicable, and has powerful separation capability. The focus for development of this method is the identification of novel chiral stationary phases with superior recognition performance and good stability. The present article reviews recent progress in the development of new chiral stationary phases for high-performance liquid chromatography between January 2018 and June 2021. These newly reported chiral stationary phases are divided into three categories: small organic molecule-based (cyclodextrin and its derivatives, macrocyclic antibiotics, cinchona alkaloids, and other low molecular weight chiral molecules), macromolecule-based (cellulose and amylose derivatives, chitin and chitosan derivatives, and synthetic helical polymers) and chiral porous material-based (chiral metal-organic frameworks, chiral covalent organic frameworks, and chiral inorganic mesoporous silicas). Each type of chiral stationary phase is discussed in detail.

Enantioseparation on a new synthetic β-cyclodextrin chemically bonded chiral stationary phase and molecular docking study

A novel β-cyclodextrin derivative chemically bonded chiral stationary phase (EDACD) was synthesized by the reaction of mono-6-ethylenediamine-β-cyclodextrin with the active alkyl isocyanate, anchoring to silica gel. After the successful analysis and characterization using scanning electron microscopy, Fourier transform infrared spectra, solid-state nuclear magnetic resonance spectra, elemental analysis, and thermogravimetric analysis techniques, the enantioselective performance of the as-prepared EDACD column was evaluated by non-steroidal antiinflammatory drugs and flavonoids under the reversed-phase HPLC condition. The factors that affected enantioseparation including mobile phase compositions and buffers were investigated in more detail. As a result, EDACD showed a satisfactory enantioselectivity for the tested drugs. With the mobile phase of acetonitrile and 20-mM ammonium formate adjusted to pH 4.0 using formic acid (85:15, v/v) at the flow rate of 0.6 mL min^-1, the enantiomers of ibuprofen, carprofen, naproxen, indoprofen, ketoprofen, eriocitrin, naringin, and narirutin were separated with the best resolutions of 1.53, 1.64, 3.72, 2.40, 0.50, 0.61, 0.58, and 0.52. To adjust the proportion of acetonitrile to 80% (by volume), the enantiomers of pranoprofen and flurbiprofen were completely resolved with the best resolutions of 1.60 and 1.59. Additionally, by the study of the molecular docking, hydrogen bonding and inclusion complexation were believed to play an important role in chiral recognition. As a new material, EDACD will have a wider application in the analysis of chiral compounds.

Enantioselective Recognition of Chiral Tryptophan with Achiral Glycine through the Strategy of Chirality Transfer

Glycine (Gly), an achiral amino acid, has never been reported for enantioselective recognition owing to the absence of chiral sites. Herein, a facile strategy of chirality transfer is proposed to endow Gly with chirality. Optically active CuO, L-CuO, is first prepared, which can be used for the decoration of Gly through the formation of the Cu(Gly)₂ complex. Successful chirality transfer from L-CuO to Gly is confirmed by circular dichroism (CD) spectra. The formation of the Cu(Gly)₂ complex is further confirmed by Fourier transform infrared spectra and X-ray photoelectron spectroscopy. Next, the resultant L-CuO-Gly is used for chiral analysis of the isomers of tryptophan (Trp). Because of the higher affinity of L-CuO-Gly toward L-Trp than its isomer, the Trp isomers exhibit significant differences in their oxidation peak currents at the L-CuO-Gly-modified glassy carbon electrode (GCE) (I_L-Trp/I_D-Trp = 5.24). Finally, the practicability of the developed L-CuO-Gly/GCE is assessed, and the results indicate that it could be a reliable chiral sensor for the quantitative analysis of Trp isomers in nonracemic mixtures.

Novel chiral stationary phases based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin combining cinchona alkaloid moiety

Novel chiral selectors based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin connecting quinine (QN) or quinidine (QD) moiety were synthesized and immobilized on silica gel. Their chromatographic performances were investigated by comparing to the 3,5-dimethyl phenylcarbamoylated β-cyclodextrin (β-CD) chiral stationary phase (CSP) and 9-O-(tert-butylcarbamoyl)-QN-based CSP (QN-AX). Fmoc-protected amino acids, chiral drug cloprostenol (which has been successfully employed in veterinary medicine), and neutral chiral analytes were evaluated on CSPs, and the results showed that the novel CSPs characterized as both enantioseparation capabilities of CD-based CSP and QN/QD-based CSPs have broader application range than β-CD-based CSP or QN/QD-based CSPs. It was found that QN/QD moieties play a dominant role in the overall enantioseparation process of Fmoc-amino acids accompanied by the synergistic effect of β-CD moiety, which lead to the different enantioseparation of β-CD-QN-based CSP and β-CD-QD-based CSP. Furthermore, new CSPs retain extraordinary enantioseparation of cyclodextrin-based CSP for some neutral analytes on normal phase and even exhibit better enantioseparation than the corresponding β-CD-based CSP for certain samples.

Preparation and evaluation of biselector bonded-type multifunctional chiral stationary phase based on dialdehyde cellulose and 6-monodeoxy-6-monoamino-β-cyclodextrine derivatives

A novel biselector bonded-type multifunctional chiral stationary phase (MCSP) was prepared by covalently crosslinking dialdehyde cellulose (DAC) with 6-monodeoxy-6-monoamino-β-cyclodextrine (CD) via Schiff base reaction. The biselector bonded-type MCSP had good chiral and achiral chromatographic performance in normal phase (NP) and reversed phase (RP) modes. Seven and eight enantiomers were successfully separated on the prepared biselector bonded-type MCSP in NP and RP modes, respectively. The biselector bonded-type MCSP showed enhanced chiral resolution ability compared with single selector chiral stationary phases due to the simultaneous introduction of DAC and 6-monodeoxy-6-monoamino-β-CD on the surface of silica gel. Aromatic compounds including polycyclic aromatic hydrocarbons, anilines, phenols, phenylates, and aromatic acids were choosed as analytes to investigate the achiral chromatographic performance of the biselector bonded-type MCSP in NP and RP modes. Chromatographic evaluation results showed that the above aromatic compounds were essentially capable of achieving baseline separation by hydrophobic interaction, π-π interaction, and π-π electron-donor-acceptor interaction. Moreover, the host-guest inclusion effect of 6-monodeoxy-6-monoamino-β-CD and the multiple interactions made the biselector bonded-type MCSP have good steric selectivity. The preparation method of the biselector bonded-type MCSP was simple and provided a new idea and strategy for the preparation of the subsequent novel biselector MCSP.

Toward High-Efficient Chiral Separation Using Hierarchically Porous HROP@Silica-Gel-Sheet Composite

Separating racemates is still a great challenge for their similarity in chemical structures and physicochemical properties. Despite exhibiting a significant potential in the adsorption separation due to their intrinsic characteristics, hierarchically porous materials utilized in enantioseparation have rarely been reported to date. Furthermore, the molding of such materials together with their hybrid organic-inorganic monoliths is generally required to meet various prerequisites in diverse large-scale industrial applications, but without sacrificing their inherently hierarchical architectures. In this work, a three-dimensional hierarchically porous organic-inorganic composite was simply and feasibly prepared via integrating the micro/meso-porous hyper-cross-linked resin organic polymer (HROP) with macroporous silica gel sheet (SGS), followed by a chiral selector postmodification, named as HROP@SGS. Racemic 1-phenylethanol, ibuprofen, and naproxen could be separated only using such a piece of HROP@SGS as the filler with a solid phase extraction technique. Herein, HROP@SGS exhibited extraordinary chiral resolution performances and succeeded in achieving a complete chiral resolution. Our findings suggest that this simple strategy proposed by us, that is, combining the chiral micro/mesoporous organic materials with macroporous inorganic substrates, can be employed to prepare an unprecedented enantioseparation material, which has a promising potential in large-scale industrial applications, such as fixed-bed and membrane separation.

Graphene quantum dots functionalized β-cyclodextrin and cellulose chiral stationary phases with enhanced enantioseparation performance

Graphene quantum dots (GQD) functionalized β-cyclodextrin (β-CD) and cellulose silica composites were first prepared and applied in HPLC as chiral stationary phases (CSP) to investigate the effect of GQDs on chiral separation. Through comparing the enantioseparation performance of GQDs functionalized β-CD or cellulose CSPs and unmodified β-CD or cellulose CSPs, we found GQDs enhanced the enantioseparation performance of nature β-CD, β-CD-3,5-dimethylphenylcarbamate derivative and cellulose-3,5-dimethylphenylcarbamate derivative. Molecular modeling was applied to understand and theoretically study the enhancement mechanism of GQDs for enantioseparation. According to molecular simulation results, GQDs provide extra interactions such as hydrophobic, hydrogen bond and π-π interaction when chiral selector interacts with enantiomers, which enhances the chiral recognition ability indirectly. The molecular simulation results showed a good agreement with the experimental results. Our work reveals the enhancement performance of GQDs for chiral separation, it can be expected that GQDs-based chiral composites and chiral GQDs have great prospect in chiral separation and other research fields such as asymmetric synthesis, chiral catalysis, chiral recognition and drug delivery.