Henri Kagan and Kenso Soai won the 2026 Nobel Prize in Chemistry for showing how a tiny imbalance between mirror-image molecules can grow into near-total dominance.
Hold up your hands. They are made of the same parts in the same arrangement, yet no amount of turning will make a left hand fit a right-hand glove. Each is the mirror image of the other, and the two cannot be superimposed.
Many molecules share this property. Built from the same atoms, joined in the same order, they still come in two mirror-image forms that chemists call enantiomers. Such molecules are called chiral, from the Greek word for hand, as Heiner Linke, chair of the Nobel Committee for Chemistry, reminded reporters when the 2026 prize was announced. It went to Henri B. Kagan of France and Kenso Soai of Japan, “for the discovery of non-linear effects and autocatalysis in asymmetric organic synthesis”.
Seen first in polarised light
The story of molecular handedness begins with light. In the early nineteenth century, the French physicist Jean-Baptiste Biot noticed that some substances, sugar solutions among them, rotate the plane of polarised light. Some turned it to the right, others to the left.
In 1848 a young Louis Pasteur examined crystals of a salt of tartaric acid, a by-product of winemaking. Under a lens he saw that they came in two shapes, mirror images of each other. He separated them by hand, dissolved each pile and found that one solution rotated polarised light to the right and the other to the left by the same amount. The handedness of the crystals reflected a handedness in the molecules themselves. Polarimetry, the measurement of that rotation, is still used in laboratories today.
Life picks a side
As biochemistry matured, a striking pattern emerged. Living things are almost entirely one-handed. The amino acids that make up proteins are, with rare exceptions, of the form chemists label L, the “left-handed” series. The sugars in DNA and RNA belong to the D, or “right-handed”, series. This one-handedness is called homochirality.
It is not a curiosity. Proteins fold into precise shapes because all their building blocks have the same handedness. Enzymes recognise their targets the way a glove recognises a hand. A mixture of both forms would make the precise architecture of life impossible. As the Nobel Committee’s scientific background puts it, without that uniform chirality, life as we know it could not exist.
Yet ordinary chemistry has no such preference. Make a chiral molecule from ingredients that are not themselves chiral, with nothing to bias the reaction, and you get the two mirror-image forms in equal amounts, a 50:50 mixture called a racemate. So how could a one-handed chemistry ever arise from a world that, chemically, treats left and right alike?
A theory waiting for an experiment
In 1953 the physicist F. C. Frank proposed an answer on paper. Imagine a molecule that helps make more of itself, and at the same time hinders the production of its mirror image. Start with a tiny chance excess of one form, and that form would accelerate its own production while suppressing its rival. Over time, the small excess would grow until one hand dominated completely.
It was an elegant model, but for decades no one could show real chemistry behaving that way.
Kagan’s surprise
Henri Kagan, born in 1930 in Boulogne-Billancourt, was already one of the founders of asymmetric catalysis, the art of using chiral catalysts to make one enantiomer selectively. In 1986 he noticed something that broke a basic assumption of the field.
Chemists expected the purity of a product to mirror the purity of the catalyst used to make it: a catalyst that was 50 per cent enriched in one form should give a product about 50 per cent enriched. Kagan found reactions where it did not. A catalyst only modestly enriched in one form could produce a product far richer in that form than the catalyst itself. He called this a non-linear effect.
The explanation lies in partnerships between catalyst molecules. In solution, they can pair up. A pair made of one left and one right molecule may be stable and nearly inactive, which takes the minority form out of play. What is left to do the work is the excess form, so the product comes out purer than the catalyst. The effect can also run the other way, depleting purity, which made Kagan’s non-linear effect a powerful tool for working out how catalysts behave.
Soai’s self-copying reaction
Kenso Soai, born in 1950 in Hiroshima and later professor at Tokyo University of Science, took the next step. He went looking for a reaction whose product would be its own catalyst, copying its own handedness. He found one: a reaction in which a particular chiral alcohol, once formed, speeds the formation of more molecules of the same handedness.
In repeated rounds of this reaction, Soai’s group took a mixture with an almost imperceptible excess of one form and amplified it to near-complete purity. Frank’s thought experiment had been realised in a flask. The Nobel Committee noted that, apart from life itself, nobody had achieved this before. Peter Somfai, a member of the committee, called the Soai reaction probably the coolest experiment in organic chemistry.
Kagan showed that small imbalances can be magnified. Soai showed that they can magnify themselves. Together, Linke said, they solved a chemical mystery more than a century old: how homochirality can emerge spontaneously.
– Dr Chandrasekhar Namburi
