The story · no mathematics

The Handedness of Life

On 7 October 2026 the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry to Henri B. Kagan, of Université Paris-Sud in Orsay, and Kenso Soai, of Tokyo University of Science, “for the discovery of non-linear effects and autocatalysis in asymmetric organic synthesis”.

Both discoveries are about the same awkward fact: molecules can come in left- and right-handed versions that are chemically identical in every way except how they interact with other handed things. Your body is emphatically one of those handed things.

Pasteur, with tweezers

In 1848 Louis Pasteur was looking at crystals of a tartrate salt under a microscope and noticed they came in two shapes, each the mirror image of the other. He separated them by hand with a pair of tweezers, dissolved each pile, and found that one rotated polarised light to the left and the other to the right.

It was the first demonstration that a molecule can be handed, and it set the problem that took the next 178 years. Making a handed molecule is easy. Making only one of the two hands is hard, because nothing in ordinary chemistry has a preference.

Why it stopped being academic

A receptor in your body is built from handed building blocks, so it binds one enantiomer of a drug and not the other. Often the unwanted hand is merely inert. Sometimes it is not.

Thalidomide, prescribed for morning sickness in the late 1950s, is the case everyone cites: one enantiomer sedates, the other causes severe birth defects. The lesson usually drawn — that the drug should have been sold as a single enantiomer — is not quite right, because the two forms interconvert in the body, so a pure sample would not have stayed pure. The real lesson was broader and it stuck: handedness is not a detail, and regulators now treat the two enantiomers of a drug as two different substances.

That turned asymmetric synthesis from a curiosity into an industry. The 2001 chemistry prize went to Knowles, Noyori and Sharpless for catalysts that make one hand preferentially. This year's prize is for two things nobody expected those catalysts to be able to do.

Kagan: more out than in

A chiral catalyst is itself a handed molecule, and making it pure is expensive. The obvious assumption was that a catalyst 80% pure would give a product about 80% pure — you cannot get out what you did not put in.

In 1986 Kagan and his colleagues showed the relationship is not a straight line. The reason turns out to be almost embarrassingly simple: if the catalyst carries two chiral ligands, the minority hand gets locked up in mixed complexes that work badly or not at all, leaving the majority hand to do the chemistry. A catalyst at 50% can deliver a product at 95%. The effect is now routine to exploit, and it is why many industrial processes can tolerate a cheap, impure ligand.

Soai: a reaction that chooses

In 1995 Kenso Soai reported something stranger. He found a reaction whose product is also its own catalyst — and a handed one. Whichever enantiomer happens to be slightly ahead makes more of the catalyst that favours it, so its lead widens with every turn.

Start with an excess too small to measure reliably — parts per million — and after a few rounds the mixture is essentially one hand. Start with nothing at all, just the random fluctuations in how many molecules of each kind happen to form first, and the reaction still ends up almost pure. Which hand wins is then genuinely a coin toss: repeat the experiment and you get the other one about half the time.

The result was received with scepticism until other laboratories reproduced it. It remains the only well-characterised chemical reaction that does this.

The question underneath

Every protein in every living thing is built from left-handed amino acids. Every nucleic acid uses right-handed sugars. There is no chemical reason for either choice, and a world that began racemic should have stayed racemic.

In 1953 Charles Frank showed on paper that it need not: a self-replicating molecule that also suppresses its mirror image makes the balanced state unstable, so any fluctuation, however tiny, grows until one hand holds everything. Soai's reaction is the first laboratory system that behaves this way. It does not prove this is how life chose a hand. It proves the mechanism is real chemistry rather than a theorist's hypothetical.

Sources

• Press release, Nobel Prize in Chemistry 2026, NobelPrize.org.
• Henri B. Kagan and Kenso Soai win Nobel Prize in chemistry, NPR, 7 October 2026.