Distinguishing Molecular ‘Evil Twins’

A membrane made of 2D layers can catch left-handed molecules while allowing the right-handed ones to be transported away, resulting in high separation efficiency.

AsianScientist (Jul. 11, 2019) – Researchers at the University of Science and Technology of China (USTC) have developed a membrane that can separate a mixture of chiral molecules, molecules that are mirror images of each other but otherwise chemically indistinguishable. Their results have been published in Nature Communications.

In the classic Chinese tale Journey to the West, no one except the Buddha could tell the difference between the real Monkey King and his ‘evil twin’ Six Ears, causing much confusion. Many economically important biomolecules are a mixture of two nearly identical molecules—just like Monkey King and Six Ears. These are the so-called chiral isomers (enantiomers), which have identical chemical formulas but are mirror images of each other and non-superposable.

Despite their chemical similarity, enantiomers may function very differently. For example, the L-enantiomer of amlodipine can treat high blood pressure while the R-enantiomer has no such effect. In biopharmaceutical manufacturing, chiral isomers are often produced at the same time, so the mixture must be separated. However, left-handed and right-handed molecules are extremely difficult to identify and separate.

Polymer membranes to separate chiral molecules have low separation efficiency, and crystalline compounds don’t easily form membranes. Instead, a team of USTC researchers led by Professor Liu Bo used a two-dimensional layered material, tuning its interlayer distance and introducing chiral sites into the interlayer space to create an efficient and stable chiral separation membrane.

“The membrane exhibits high selective permeation efficiency among various enantiomers,” said study first author Mr. Wang Yang, a PhD student at USTC. “It can efficiently separate the R-limonene and retain most of the L-limonene. The separation performance can be further improved when applying a certain pressure.”

This work demonstrates the potential of tuning the chemical environment within interlayer space via electrostatic interaction to fabricate stable membranes that can precisely separate molecules at the sub-nanometer scale. Such membranes could be used for sewage processing and desalination, among other applications.

Currently, the researchers are able to fabricate chiral membranes at the centimeter scale in the lab. The team is increasing the membrane size into meter-scale membranes, aiming to separate chiral drug molecules for the pharmaceutical industry.


The article can be found at: Wang et al. (2019) Graphite Phase Carbon Nitride Based Membrane for Selective Permeation.

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Source: Chinese Academy of Sciences.
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