By Maria Bregman
How chemistry learned to choose a mirror image
On 7 October 2026, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry 2026 to Henri B. Kagan of Université Paris-Sud, France, and Kenso Soai of Tokyo University of Science, Japan, “for the discovery of non-linear effects and autocatalysis in asymmetric organic synthesis”. The prize amount is 12 million Swedish kronor, to be shared equally between the laureates, with a prize share of 1/2 each.
Some molecules, such as amino acids, exist as two variants that are each other’s mirror image. Life, however, uses only one of them. The Academy calls life’s chemistry homochiral, after the Greek words for “same” and “hand”: like hands, all amino acids exist as two mirrored variants, but only one is found in the proteins in your cells, and the same is true of the sugar molecules in DNA. For a long time chemists wondered how that could emerge. When they experimented with reactions that can form two mirrored molecules, they always obtained equal proportions of both in the test tube.
Why it matters outside the laboratory
The Academy’s popular background compares the problem to a locksmith who makes keys to order and always ends up with two mirrored variants, only one of which fits the lock. Many drug molecules occur in two variants that are each other’s non-identical mirror image: one has the therapeutic effect, while the other can cause unnecessary and sometimes harmful side effects. The importance of producing pure mirror images, the Academy recalls, was shown by the thalidomide scandal in the early 1960s, when thousands of children were affected by birth defects caused by the sedative; researchers who analysed what had happened realised that it was the active substance’s mirror image that caused the harm.
According to the Academy, the discoveries now honoured are decisive for chemists who design reactions for the manufacture of pharmaceuticals, and also of flavours, scents and new materials. The non-linear effects, it adds, are vital for every company that makes substances intended to interact with living beings, including agricultural chemicals.
A long chain of ideas
The Academy describes the prize as the result of a long chain of ideas, of which the first link was forged in the mid-19th century by Louis Pasteur, working on tartaric acid. Pasteur’s crystals came in two mirrored forms, one bending polarised light to the right and the other to the left. In the early 1900s the German chemist Willy Marckwald carried out the first successful asymmetric reaction, using a chiral catalyst; the difference was very small, but its existence was a breakthrough. In 1953 the theoretical physicist Charles Frank of the University of Bristol set out a model of a reaction that could lead to homochirality, with three conditions: a chiral catalyst and an asymmetric reaction; one mirror image enhanced and the other dampened; and a catalyst that forms itself, which is called autocatalysis. The 2026 prize recognises the fulfilment of the last two conditions.
Kagan, 1986
Kagan worked at Université Paris-Sud and was, in the Academy’s words, one of all the chemists refining asymmetric reactions in the early 1980s. Researchers commonly used catalysts with two components, a metal atom and a chiral substance, and assumed a linear relationship: if the catalyst contained a mix of mirror images, the product would reflect the same proportions. Kagan began to question this. He reasoned that the metal would attract at least two of the chiral molecules, so that a mixed catalyst would exist in three forms: right-right, left-right and left-left. The left-right form drove the reaction far more slowly than the other two. When he plotted his results, the graph was curved rather than straight, and the excess of one mirror image in the product was greater than the excess in the catalyst.
“In 1986, he described no fewer than three different asymmetric reactions that displayed what chemists call non-linear effects,” the Academy writes, calling it a historic breakthrough. The Academy also notes that non-linear effects have become an important tool: that a reaction is non-linear tells chemists something about how it occurs, and the information can be used to obtain the purest possible mirror images.
Soai, 1995 and 2003
Soai, at Tokyo University of Science, studied an asymmetric reaction with a major non-linear effect. Noticing similarities between its catalyst and its product, he wondered whether he could design a reaction in which the catalyst formed itself. By experimenting with many molecules he found a chiral substance, 5-pyrimidyl alkanol, that could create itself. In a publication from 1995 he described starting with a two per cent excess of one mirror image and ending with an excess of 87 per cent. The reaction was self-reinforcing and met all of Frank’s criteria, but did not reach the 100 per cent purity of life.
Eight more years of searching led to 2003, when Soai presented a reaction that formed an excess of one mirror image which then formed copies of itself. In one experiment, after substrate and reagent were added three times, one mirror image made up more than 99.5 per cent of the product. Chance decided the outcome: both forms were created at the start, slightly more of one, and that tiny excess took over the whole reaction. When Soai repeated the reaction, the other mirror image could take over instead. “Other than life itself, no one had previously achieved this feat,” the press release says.
Heiner Linke, chair of the Nobel Committee for Chemistry, said: “Henri Kagan and Kenso Soai have provided a solution to a chemical mystery that is over a century old: how homochirality can emerge spontaneously. The chemical reactions they have developed are spectacular.”
What the prize does not claim
The Academy is plain that the Soai reaction is artificial and different to the chemistry of life. It did, however, awaken new enthusiasm among chemists who want to understand life’s origins, and researchers around the world are now trying to repeat Soai’s achievement with the aim of producing homochiral amino acids and sugars. The prize does not say how life on Earth acquired its handedness.
The laureates
Kagan was born in 1930 in Boulogne-Billancourt, France, took his PhD in 1960 at the Collège de France, and is Professor Emeritus at the then Université Paris-Sud. Soai was born in 1950 in Hiroshima, Japan, took his PhD in 1979 at the University of Tokyo, and is Professor Emeritus at Tokyo University of Science.
Henri B. Kagan. Photo: Ecole polytechnique from Paris / Wikimedia Commons, CC BY-SA 2.0.
