The 2026 Nobel Prize in Chemistry went to Henri B. Kagan and Kenso Soai for showing how chemistry can choose one mirror image of a molecule, the way life does. Many molecules come in two forms that are mirror images of each other, like your left and right hands, and life uses only one of them. In a test tube, chemists always made an equal mix of both. Kagan found that a mixture can behave in a surprising, curved way, and Soai found a reaction that copies itself until one hand takes over almost completely.
Hold up your hands: however you turn them, they never line up. Many molecules are the same, and life uses only one hand.
On Wednesday 7 October 2026 the Royal Swedish Academy of Sciences announced the prize. It went to Henri B. Kagan of Université Paris-Sud in France and Kenso Soai of Tokyo University of Science in Japan, “for the discovery of non-linear effects and autocatalysis in asymmetric organic synthesis”. That is a mouthful. The committee’s own picture helps: imagine a locksmith whose keys always come out as two mirror-image twins. Only one fits the lock, the other can damage it, and the two are very hard to tell apart.
Pasteur and the tweezers
In the mid-nineteenth century Louis Pasteur was studying tartaric acid, a substance important in wine-making. His colleagues said it sometimes bent polarised light to the right, and at other times did not affect the light at all.
Pasteur grew crystals of tartaric acid and looked at them under his microscope. They came in two variants, each the mirror image of the other. With tweezers he sorted them into two little piles and dissolved each pile separately. One solution bent polarised light to the right, the other to the left. Mixed together, they let the light pass straight through.
Substances like this came to be called chiral, and the two mirror-image molecules enantiomers: like your two hands, they cannot be laid on top of each other.
In 1857 Pasteur let bacteria ferment the two forms. They willingly fermented the form found in grapes, the one that bends light to the right, and left its mirror image completely untouched. It was the first hint that life’s chemistry takes sides.
Later, researchers found the same in life’s building blocks. Amino acids come in two mirrored forms, but only one of them is found in the proteins in your cells. The sugars in DNA come in two forms as well, and life uses only one. Chemists call this homochirality, from the Greek words for same and hand.
Yet in a flask, chemists always got an even fifty-fifty mix of both hands. So how could life end up one-handed?
Marckwald, thalidomide and Frank’s three conditions
Some chemists claimed that one-handed chemistry was unique to life. Then, in the early 1900s, the German chemist Willy Marckwald carried out the first successful asymmetric reaction: more of one hand was made than the other. His key was a catalyst, a substance that drives a reaction without being used up, and his catalyst was itself chiral, so it nudged the reaction toward one mirror image. The difference was tiny, but it was a breakthrough.
Why does making only one hand matter so much? The answer came in the early 1960s, when thousands of children were affected by birth defects caused by a sedative called thalidomide. When researchers analysed what had happened, they realised that it was the mirror image of the active substance that caused the harm. Medicines have the locksmith’s problem: one mirror image heals, while the other can cause harmful side effects.
Now step back to 1953 and Charles Frank, a theoretical physicist at the University of Bristol. Chemists still did not understand how life’s one-handedness arose, so Frank wrote down a model with three conditions:
- A chiral catalyst and an asymmetric reaction.
- The formation of one mirror image is enhanced, and the other dampened.
- The reaction makes its own catalyst. That is autocatalysis, and it lets one hand grow exponentially.
Frank ended his paper with a wonderfully dry line:
A laboratory demonstration may not be impossible. — F. C. Frank, “On spontaneous asymmetric synthesis”, 1953
Marckwald had already met the first condition, and the pioneers who made such reactions truly useful won the Nobel Prize in Chemistry in 2001 and 2021. But the other two boxes stayed empty for decades.
Kagan’s curved graph
In the early 1980s Henri Kagan, at Université Paris-Sud, was one of many chemists chasing one pure hand. Their catalysts had two parts: a metal atom, the motor of the reaction, and a chiral molecule, which made the reaction one-handed. Everyone assumed that if you mixed both hands in the catalyst, the product would simply copy those proportions. Plot one against the other and you would get a straight line.
Kagan questioned the assumption. After all, nobody actually knew how the catalyst worked. Metal atoms usually hold several molecules at once, so he reasoned that the metal grabbed at least two of the chiral molecules. With both hands in the mix, that makes three kinds of catalyst: right-right, left-left and left-right. The first two make products that are mirror images of each other. The odd one out, the joker in the pack, was left-right, which drove the reaction far more slowly than the other two.
So when Kagan plotted his results in 1986, the line was not straight. It curved. The product had more of one hand than the catalyst did: a purer product than the catalyst that made it, which until then had been unthinkable. Chemists call this a non-linear effect. That same year he described no fewer than three different asymmetric reactions that showed it, and that put a tick in the second box of Frank’s model. One hand was enhanced, the other dampened.
Soai’s reaction: chance picks a hand
Many chemists began to explore the strange new curve, and one of them, Kenso Soai at Tokyo University of Science, was studying a reaction with a big non-linear effect. He noticed that the catalyst and the product were very alike, which sparked an idea: could a reaction make its own catalyst?
Reactions that made their own catalyst were already known in the early 1990s, but none of them chose a hand. Soai tried molecule after molecule, by trial and error, until he found one that could create copies of itself. In 1995, in the journal Nature, he described the experiment. He started with a two per cent excess of one hand of this molecule and ended with an excess of 87 per cent. Each new copy helped to make more copies of the same hand, and the reaction fed itself.
That met all three of Frank’s conditions, but it did not reach the 100 per cent purity of life, so for eight more years Soai kept searching. In 2003 he presented a reaction that started without any one-handed ingredient at all. Both hands formed, but by chance slightly more of one appeared, and that tiny excess was enough to take over the whole reaction. In the end, one hand can make up 99.99 per cent of the product.
And here is the strangest part. Run it again, and the other hand can win instead. What you get at the end depends on what happened by chance at the start. In one series of 37 experiments, 18 ended with one hand and 19 with the other: very nearly a coin toss. Other than life itself, no one had ever achieved this before.
What the prize is for, and what it is not
A non-linear effect tells chemists how a reaction actually works, which helps them tune it to make the purest possible single hand. That matters for anything meant to interact with living things: pharmaceuticals, flavours, scents and agricultural chemicals.
Then there is the big question. Soai’s reaction is artificial, quite different from the chemistry of life, but it reawakened the search for life’s origins. Around the world, researchers are now trying to repeat his achievement with amino acids and sugars.
A word on what this prize is not. It is not the answer to how life became one-handed. That happened about three and a half to four billion years ago, and we will probably never have a definitive answer. Frank’s model, made real by Kagan and Soai, is one possible route among several. Nor was this the first one-handed reaction: that honour belongs to Marckwald, early in the 1900s.
Kagan was born in 1930, and Soai in 1950 in Hiroshima. The prize will be handed over in Stockholm on 10 December. The Short on the same prize tells it in under a minute; the video animates the whole trail, from Pasteur’s tweezers to a coin toss inside a flask.
Turn your hands however you like. They will never line up. Many molecules have a mirror twin, and life settled on one.
