Abstract:

Capillary electrochromatography for enantioseparation has received considerable research attention in the past decades, because it integrates the advantages of classical electrophoresis and modern micro-column separation. Chirality is a fundamental feature of compounds found in nature and is also a major concern in the modern pharmaceutical industry. Porous organic cages (POCs) are defined as a class of porous materials with permanent ordered three-dimensional cavity structures that are different from those of porous materials, such as zeolite, metal-organic frameworks, covalent organic frameworks, and mesoporous silica. POCs have good solubility in general organic solvents and can be used as a chromatographic stationary phase conveniently coated inside a standard capillary column. Homochiral POCs with hydroxyl groups on the cage molecules were synthesized by imine-linked condensation of 2-hydroxy-1,3,5-triformylbenzene with (1R,2R)-1,2-diphenylethylenediamine. The thus-synthesized POCs were characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD) analysis, etc. In the FT-IR spectra, the absorption peaks at 1602, 1489, and 1458 cm^-1 were attributed to the C=C-H and C=C tensile vibrations in the benzene ring. The strong characteristic absorption peak at 1636 cm^-1 was attributed to the imine bond (C=N) stretching, the two peaks at about 2900 cm^-1 were attributed to C-H bond vibration, and the absorption peak at 3420 cm^-1 was attributed to the O-H pulling vibration. In the XRD patterns, the powder diffraction peaks of the POCs were consistent with the simulated data. These results indicated that POCs were successfully synthesized. Thermogravimetric analysis was performed in the temperature range of 25-800 ℃ (10 ℃/min), and the POCs were found to be stable up to 380 ℃. Dichloromethane was used as solvent to uniformly coat POCs on the capillary wall to prepare an electrochromatography column. Joule heat generated in electrophoresis was negligible under the experiment condition used for the open-tubular column. Four chiral compounds, viz. dihydroflavone, praziquantel, naproxen, and 3,5-dinitro-N-(1-phenylethyl)benzamide, were used as test compounds, and the electrochromatography separation conditions were optimized such that the best separations were obtained. The voltage was applied to separate the selected enantiomers in the range of 10-20 kV. Considering the good separation and appropriate migration time simultaneously, applied voltages of 13 kV and 12 kV were recommended for dihydroflavones and 3,5-dinitro-N-(1-phenylethyl)benzamide, respectively, as well as 14 kV for praziquantel and naproxen. The concentration of the buffer solution for dihydroflavonoids was 0.075 mol/L, and those for praziquantel, naproxen, and 3,5-dinitro-N-(1-phenylethyl)benzamide were 0.100 mol/L. The pH was 3.51 for all four substances. Resolutions of 2.99, 2.10, 2.58, and 3.59 were achieved on a POC chiral column for dihydroflavonoids, praziquantel, naproxen, and 3,5-dinitro-N-(1-phenylethyl)benzamide, respectively. Two positional isomers, viz. o,m,p-nitrophenol and o,m,p-nitrophenilamine, were also successfully separated with 0.100 mol/L Tris-H₃PO₄ at pH 3.51. Therefore, the chiral electrochromatography column showed good chiral recognition ability and the POC is an excellent separation material with excellent application prospect in chiral electrochromatography.

Key words: capillary electrochromatography (CEC), chiral separation, chiral compounds, porous organic cages (POCs)