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Web Date: January 13, 2014

Metal-Organic Framework Used For Chiral Chromatography

Analytical Chemistry: A column loaded with a MOF can separate enantiomers of pharmaceuticals via high-performance liquid chromatography.
Department: Science & Technology
News Channels: Analytical SCENE, Organic SCENE, Materials SCENE
Keywords: metal-organic framework, HPLC, chiral separation, enantiomers
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Mirror Image Separator
A metal-organic framework, made from zinc (turquoise pyramids) and the chiral ligand salen, can separate mixtures of enantiomers (green and purple molecules, top left) when the molecules adsorb into the MOF’s pores.
Credit: Anal. Chem.
20140113lnj1-chiralmof
 
Mirror Image Separator
A metal-organic framework, made from zinc (turquoise pyramids) and the chiral ligand salen, can separate mixtures of enantiomers (green and purple molecules, top left) when the molecules adsorb into the MOF’s pores.
Credit: Anal. Chem.

Researchers have demonstrated that a chiral metal-organic framework (MOF) can separate enantiomers of drug molecules using traditional chromatography (Anal. Chem. 2013, DOI: 10.1021/ac403674p). Although the study marks a new application for the materials, some experts warn that the MOFs may not outperform standard materials used for chiral separations.

Separating mixtures of enantiomers is important in the pharmaceutical industry because enantiomers of drug molecules may have different biological effects. Currently, chemists isolate enantiomers using high-performance liquid chromatography (HPLC) through columns packed with a stationary phase that includes chiral materials. As the mixture moves through the column, each enantiomer interacts differently with the stationary phase, causing the molecules to travel at different speeds through the column. As a result, one compound will flow off of the column earlier than the second one.

Bo Tang at Shandong Normal University in China, and his colleagues wondered if a chiral MOF could be used as a stationary phase. MOFs are three-dimensional structures easily assembled using metals and ligands. When chemists use chiral ligands, they produce MOFs with chiral pores. Since individual enantiomers would have different affinities for these asymmetric pores, Tang thought that the chiral MOFs could be used to separate the isomers.

The researchers synthesized a MOF using zinc and a chiral salen ligand functionalized with pyridine. They packed crystals of the MOF into an HPLC column and used the column to separate enantiomers of ibuprofen, phenylethylamine, and benzoin. While the column easily separated those three mixtures, it could not separate enantiomers of ketoprofen and naproxen because the molecules are larger than the diameter of the MOF’s pores.

Though using MOFs for chiral separations is a new application of the materials, says Yoshio Okamoto, an emeritus professor at Nagoya University in Japan and a researcher at Harbin Engineering University in China, wonders if this MOF offers any advantage over the ubiquitous polysaccharide columns currently used in chiral separations. He also notes that, like ketoprofen and naproxen, many drug molecules are larger than the approximate 9.8 Å pore size of this MOF. Yu Ma, a researcher who worked on the project, says the pore size can easily be changed by varying the metal and ligand used to make the MOF.

 
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