The 2026 Nobel Prize in Chemistry was awarded to Henri Kagan and Kensō Soai for discovering a solution to the enigma of nature’s chemical asymmetry, a finding that opened up new avenues for manipulating the reactions used in manufacturing pharmaceuticals and other materials.
Some molecules are chiral, meaning a molecule has two forms that are mirror images of each other but not identical, like your left and right hand. One of the most mysterious chemical phenomena in biology is that living organisms contain almost exclusively one of the two forms. This is known as homochirality.
Both versions of the molecules—each called an enantiomer—have similar physical properties but can behave differently. The Nobel Committee used a simple analogy to explain this: A locksmith can create two mirror-image keys, but “only one of them fits the lock—and the lock can be damaged if your customers try to unlock it with the other key.”
In the development of new drugs that use, for example, amino acids or sugar molecules as a basis, homochirality poses a problem: While one of the enantiomers of these chiral molecules produces the desired therapeutic effect, the other can cause unnecessary and, at times, harmful side effects. Scientists can’t always easily control which version of the molecule they get out of their synthesis.
The discovery of chiral molecules dates back to the time of Pasteur, who found two enantiomers that reacted very differently when exposed to bacteria. Since then, various researchers have attempted to replicate homochirality in the laboratory with the goal of understanding its spontaneous origin and applying this knowledge to the design of controlled reactions for various purposes. However, early experiments yielded equal amounts of the two variants, failing to reproduce the selectivity observed in nature.
At the beginning of the last century, the German chemist Willy Marckwald designed an asymmetric reaction capable of producing a slightly greater amount of one of the two versions. He achieved this using a chiral catalyst—a substance that facilitates a chemical reaction without being consumed and favors the formation of one of the enantiomers. The ratio between the two variants, however, only changed a little.
Later, in 1953, theoretical physicist Charles Frank proposed a mathematical model that outlined three conditions for reproducing homochirality. First, there must be an asymmetric reaction capable of favoring one of the two versions; that imbalance has to be amplified; and the reaction itself has to produce the catalyst that drives it. This last characteristic is called autocatalysis and generates a kind of positive feedback loop, in which a small initial advantage can cause one of the variants to assemble at an ever-faster rate until it dominates the reaction.
This framework formed the basis of Henri Kagan’s research. Beginning in the early 1980s, he focused on refining asymmetric reactions through a detailed study of the catalyst. At that time, scientists typically used catalysts consisting of a metal atom that acted as the reaction’s driving force and a chiral substance that ensured the process was asymmetric. They assumed that if they used a mixture containing equal amounts of the two versions of that chiral molecule, the result would also be a balanced mixture.
Kagan began to question that idea. He realized that the metal atom likely did not interact with just one enantiomer but with both at the same time. This meant that, by mixing versions A and B, three different combinations could form: A-A, B-B, and B-A. The first two would produce molecules that were mirror images of each other. But the A-B combination could behave differently during the reaction.
The A-B combination, the scientist discovered, could slow down the reaction, indirectly favoring the formation of one of the two versions of the final molecule. The phenomenon was called the “nonlinear effect” and represented a new way to manipulate chemical reactions.
“In addition to their practical interest, nonlinear effects are a very valuable tool for studying reaction mechanisms and the nature of catalytically active species,” explained José María Andrés García, a professor in the Department of Organic Chemistry at the University of Valladolid, in a statement reported by SMC Spain.
Inspired by Kagan’s findings, Kensō Soai sought to create a reaction in which the product itself would act as a catalyst and help produce more molecules identical to itself—one of those theoretical conditions for the homochirality. He conducted a test with a substance known as 5-pyrimidylalkanol. At the beginning of the experiment, one version was just 2 percent more abundant than the other version of the molecule. By the end of the reaction, that advantage had jumped to 87 percent.
In the years that followed, Soai worked toward achieving reactions that resulted in the near-absolute purity of one enantiomer, replicating the molecular state inside living organisms. Finally, in 2003, he engineered a reaction in which the starting substances had no defined orientation, but as they reacted, a slightly greater amount of one enantiomer appeared by chance. That tiny difference then produced many more molecules of the same variant. Gradually, that form came to dominate the reaction, accounting for up to 99.99 percent of the final product.
In other words, the reaction formed only one of the two possible enantiomers. As the Nobel Committee stated, “Other than life itself, no one had previously achieved this feat.”
This story originally appeared on WIRED en Español and has been translated from Spanish.
