Polysiloxane assisted fabrication of chiral crystal sponge coated capillary column for chiral gas chromatographic separation

Chiral ionic organic single-crystal and its exfoliated two-dimensional nanosheets with enhanced enantioseparation

A chiral ionic organic single-crystal (CIOC) was prepared for the first time through ionic self-assembly using bipyridium chiral ionic liquid (CIL) and 4,4'-biphenyldisulfonic acid (BDA). The CIOC can be ultrasonically exfoliated to produce two-dimensional nanosheets (2D-NSs). The 2D-NSs presented enhanced enantioseparation compared to the CIOC and CIL when used as gas chromatography stationary phase, which may be due to the exfoliated 2D-NSs exhibiting greater exposure of functional groups. Additionally, better resolution of other organic compounds such as positional isomers, n-alkanes and n-alkanols, Grob mixture, phenols and anilines was obtained in 2D-NSs than CIOC and CIL. This work not only provides a reference for the preparation of chiral ionic organic single-crystals and two-dimensional nanosheets for chiral separation, but also stimulates the preparation of such new ionic organic single-crystals via self-assembly for other potential applications.

Grafting (S)-2-Phenylpropionic Acid on Coordinatively Unsaturated Metal Centers of MIL-101(Al) Metal-Organic Frameworks for Improved Enantioseparation

Chiral metal-organic frameworks (cMOFs) are emerging chiral stationary phases for enantioseparation owing to their porosity and designability. However, a great number of cMOF materials show poor separation performance for chiral drugs in high-performance liquid chromatography (HPLC). The possible reasons might be the irregular shapes of MOFs and the low grafting degree of chiral ligands. Herein, MIL-101-Ppa@SiO₂ was synthesized by a simple coordination post-synthetic modification method using (S)-(+)-2-Phenylpropionic acid and applied as the chiral stationary phase to separate chiral compounds by HPLC. NH₂-MIL-101-Ppa@SiO₂ prepared via covalent post-synthetic modification was used for comparison. The results showed that the chiral ligand density of MIL-101-Ppa@SiO₂ was higher than that of NH₂-MIL-101-Ppa@SiO₂, and the MIL-101-Ppa@SiO₂ column exhibited better chiral separation performance and structural stability. The binding affinities between MIL-101-Ppa@SiO₂ and chiral compounds were simulated to prove the mechanism of the molecular interactions during HPLC. These results revealed that cMOFs prepared by coordination post-synthetic modification could increase the grafting degree and enhance the separation performance. This method can provide ideas for the synthesis of cMOFs.

In situ fabrication of chiral covalent triazine frameworks membranes for enantiomer separation

Rapid and high-flux enantiomer separation is significant for drug development. Membrane separation technology provides promising approaches for enantiomer separations. Porous membrane with good selectivity and high permeability is an ideal choice for enantiomer separations. Herein, we demonstrate the preparation of a novel two-dimensional chiral covalent triazine frameworks (CCTF) membrane by "in situ growth" method. Inheriting the strong chirality and specific interactions from CCTF, the CCTF membranes exhibited good enantioselectivity for drug intermediates and drug, including (R)/(S)-1-phenylethanol, (R)/(S)-1,1'-binaphthol and (R)/(S)-ibuprofen. Under optimal separation conditions, the enantiomeric excess value (e.e %) was above 21.7 % for (R)/(S)-1-phenylethanol, 12.0% for (R)/(S)-1,1'-binaphthol and 9.7 % for (R)/(S)-ibuprofen. The mechanism of the CCTF recognizing enantiomers were simulated by quantum mechanical calculations. In addition, the mechanism was also proved by the separation of enantiomers using this CCTF-modified silica column in liquid chromatography. The CCTF membrane may bring about the potentially application for large-scale production of chiral compounds. Meanwhile, this work provides a theoretical guidance for the application of CCOFs in enantiomer separation.

Amphiphilic tocopheryl polyethylene glycol succinate as gas chromatographic stationary phase for high-resolution separations of challenging isomers and analysis of lavender essential oil

This work presents the investigation of using tocopheryl polyethylene glycol succinate as the stationary phase for gas chromatography separations of isomers with different varieties and gas chromatography-mass spectrometry analysis of a wide range of components in lavender essential oil. Its capillary column exhibited moderate polarity and column efficiency of 4000 plates/m determined by n-dodecane at 120°C. As demonstrated, it showed outstanding separation performance toward challenging isomers such as xylenes, alkanes, phenols, and anilines and a wide range of components in essential oils with distinct advantages over the commercial polyethylene glycol and polysiloxane columns. Moreover, its capillary columns displayed excellent repeatability and reproducibility with the RSD values of the retention times in the range of 0.02-0.07% for run-to-run, 0.14-0.22% for day-to-day, and 2.5-4.3% for column-to-column. Its application to gas chromatography-mass spectrometry analysis of the lavender essential oil proved its good potential for practical gas chromatography analyses. To our knowledge, this work presents the first example of employing tocopheryl polyethylene glycol succinate for chromatographic analyses and demonstrates its promising future in this field.

Recent advances of the ionic chiral selectors for chiral resolution by chromatography, spectroscopy and electrochemistry

Ionic chiral selectors have been received much attention in the field of asymmetric catalysis, chiral recognition, and preparative separation. It has been shown that the addition of ionic chiral selectors can enhance the recognition efficiency dramatically due to the presence of multiple intermolecular interactions, including hydrogen bond, π-π interaction, van der Waals force, electrostatic ion-pairing interaction, and ionic-hydrogen bond. In the initial research stage of the ionic chiral selectors, most of work center on the application in chromatographic separation (capillary electrophoresis, high-performance liquid chromatography, and gas chromatography). Differently, more and more attention has been paid on the spectroscopy (nuclear magnetic resonance, fluorescence, ultraviolet and visible absorption spectrum, and circular dichroism spectrum) and electrochemistry in recent years. In this tutorial review as regards the ionic chiral selectors, we discuss in detail the structural features, properties, and their application in chromatography, spectroscopy, and electrochemistry.

Polydopamine-Assisted Rapid One-Step Immobilization of L-Arginine in Capillary as Immobilized Chiral Ligands for Enantioseparation of Dansyl Amino Acids by Chiral Ligand Exchange Capillary Electrochromatography

Herein, a novel L-arginine (L-Arg)-modified polydopamine (PDA)-coated capillary (PDA/L-Arg@capillary) was firstly fabricated via the basic amino-acid-induced PDA co-deposition strategy and employed to constitute a new chiral ligand exchange capillary electrochromatography (CLE-CEC) method for the high-performance enantioseparation of D,L-amino acids (D,L-AAs) with L-Arg as the immobilized chiral ligand coordinating with the central metal ion Zn(II) as running buffer. Assisted by hydrothermal treatment, the robust immobilization of L-Arg on the capillary inner wall could be facilely achieved within 1 h, prominently improving the synthesis efficiency and simplifying the preparation procedure. The successful preparation of PDA/L-Arg coatings in the capillary was systematically characterized and confirmed using several methods. In comparison with bare and PDA-functionalized capillaries, the enantioseparation capability of the presented CLE-CEC system was significantly enhanced. Eight D,L-AAs were completely separated and three pairs were partially separated under the optimal conditions. The prepared PDA/L-Arg@capillary showed good repeatability and stability. The potential mechanism of the greatly enhanced enantioseparation performance obtained by PDA/L-Arg@capillary was also explored. Moreover, the proposed method was further utilized for studying the enzyme kinetics of L-glutamic dehydrogenase, exhibiting its promising prospects in enzyme assays and other related applications.

The selectivity of a polydimethylsiloxane-based triblock copolymer as the stationary phase for capillary gas chromatography

A triblock copolymer (PCL-PDMS-PCL) constructed from polydimethylsiloxane (PDMS) and poly(ε-caprolactone) (PCL) chains was synthesized and used as the stationary phase for capillary gas chromatography (GC).

A chiral metal-organic cage used as the stationary phase for gas chromatography separations

Chiral metal-organic cages (MOCs) are a new type of porous materials with unique molecular recognition ability, which have received research attention as a chiral stationary phase (CSP) for gas chromatography (GC). Herein, we report the detailed investigation of a chiral MOC ([Cu₁₂(L_PA)₁₂(H₂O)₁₂], PA = L-phenylalanine, MOC-PA) as a novel stationary phase for GC separations. The MOC-PA capillary column exhibited a high-resolution performance for a wide range of analytes, including n-alkanes, n-alcohols, esters, aromatic compounds and the Grob mixture, positional isomers and racemates. In particular, MOC-PA coated column displayed good resolution and performance for amino acid derivatives. Moreover, the MOC-PA column showed excellent separation repeatability and reproducibility. The relative standard deviation (RSD) values for the retention times were in the range of 0.16-0.30% for run to run (n = 3), 0.31-0.77% for day-to-day (n = 3), and 3.6-4.7% for column-to-column (n = 3), respectively. The experimental results showed that MOC-PA had great potential as a GC stationary phase.

A hydroxyl-functionalized homochiral porous organic cage for gas chromatographic separations

A hydroxyl-functionalized homochiral porous organic cage (POC) was synthesized and characterized by FTIR, NMR, thermogravimetric analysis (TGA), MALDI-TOF-MS, and elemental analysis. The synthesized homochiral POC was used as stationary phase to prepare a capillary gas chromatography (GC) column by a static coating method. The fabricated column shows excellent selectivity not only for the separation of positional isomers but also for the resolution of various racemates. Thirty-nine racemates have been resolved on the column, including alcohols, diols, halohydrocarbons, epoxides, esters, lactones, ketones, ethers, and organic acids. Compared to the commercial β-DEX 120 column and previously reported chiral POCs (CC3-R, CC9, and CC10)-coated columns, there are 11, 10, 24, and 15 tested racemates that cannot be resolved on β-DEX 120 column, CC3-R column, CC9 column, and CC10 column, respectively. This reveals that the fabricated column has prominent complementarity or superior separation performance to these columns in enantioseparation. Besides, the fabricated column can achieve some enantioseparations which are not possible using all previously reported chiral POC-based columns. Some positional isomers (xylenes, dichlorobenzenes, dibromobenzenes, nitrochlorobenzenes, and nitrobromobenzenes) were also separated with high-resolution values. The column exhibits good repeatability, reproducibility, and stability. The relative standard deviation (RSD) values of retention times were 0.03-0.18%, 0.11-0.92%, and 2.1-6.6% for run-to-run (n = 5), day-to-day (n = 5), and column-to-column (n = 3), respectively. The experimental results demonstrate the great potential of POCs for practical application in GC. Graphical Abstract A hydroxyl-functionalized homochiral porous organic cage was used as stationary phase for gas chromatography separation of racemates and positional isomers. The resolution of racemates mainly depended on hydrogen bonding, π-interaction, host-guest inclusion, steric fit, etc., while separation of positional isomers by shape-selective guest binding.