Using magnets to separate chiral molecules

ChemSci Pick of the Week

A team of scientists in Israel have developed a method for sorting chiral molecules using a magnet – a first that will enable cheaper and more effective drug production in the future.

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Chiral molecules are molecules that can appear in different configurations that are mirror images of each other – the configurations are referred to as left-handed and right-handed enantiomers. In all other respects the enantiomers are identical with each other.

In nature, most molecules only appear in one specific enantiomer. Moreover, the handedness of the enantiomer can have a powerful effect on how that molecule behaves. A famous and tragic example is the drug thalidomide – one enantiomer treated morning sickness while the other caused birth defects.

It is therefore very important to be able to be able to isolate a single pure enantiomer. This is especially so since many chemical reactions in the lab tend to produce a mixture of enantiomers.

Dr Ron Naaman and his team from the Weizmann Institute of Science in Israel have developed a method that can sort enantiomers using a magnet. They place the magnet in a solution containing both enantiomers – one enantiomer crystallises on the north pole of the magnet, and the other enantiomer crystallises on the south pole.

This is possible due to electrons. Electrons have a property called ‘spin’, which can be positive or negative. When a molecule interacts with a surface, it becomes ‘spin-polarised’, meaning that electrons with a particular spin accumulate at one end of the molecule. The direction in which polarisation occurs depends on the chirality of the molecule.

At the pole of a magnet, all the spins are aligned in the same direction. This means that when a chiral molecule approaches the magnet and becomes spin-polarised, the end of the molecule that is pointing to the surface of the magnet can have either the same spin or the opposite spin to the electrons in the magnet. If the spins are opposite the molecule is attracted to the surface. If the spins are the same, the molecule is repelled. In this way, each enantiomer will collect at a different pole of the magnet.

The authors hope that this work will ultimately lead to cheaper and more effective drugs – with fewer side effects – to treat a range of diseases.