The 95-year-old who helped explain why life is left-handed

Kenso Soai was shopping near his home in Hiroshima when the phone rang to tell him he had just won the Nobel Prize in Chemistry. It was one of the most exciting days of his life, he later told the committee. But the real surprise was not the call. It was the age of his co-recipient. Henri B. Kagan is 95 years old. He has been working on this specific problem of molecular asymmetry since the 1980s, and now the Royal Swedish Academy of Sciences has finally recognized the full weight of their combined work.

The prize marks a rare moment in chemistry where two distinct discoveries, separated by decades and different continents, have been stitched together to explain a fundamental feature of biology. Why do the essential molecules of life overwhelmingly exist in just one of two possible mirror-image forms? It is a question that has haunted scientists for over a century. Kagan and Soai did not just answer it. They provided the chemical mechanism that makes the answer possible.

The mystery of the mirror image

To understand the prize, you have to understand chirality. The word comes from the Greek for hand. Your left and right hands are mirror images, but they are not identical. You cannot superimpose them. Many molecules behave the same way. They exist as enantiomers. One form might smell like mint, while its mirror image smells like caraway. In biology, this distinction is not just about smell. It is about survival. Living organisms use amino acids that are all left-handed. If the right-handed version were dominant, life as we know it likely would not exist.

For decades, chemists could create these mirror images in the lab, but they could not explain how nature chose one side over the other. It was a gap in our understanding of the origin of life. We knew the result. We did not know the mechanism. How does a tiny imbalance in the early universe, or in the primordial soup, become the overwhelming dominance of one form? That was the puzzle Kagan and Soai finally solved.

Kagan’s nonlinear leap

Henri Kagan, a French chemist affiliated with Université Paris-Sud, cracked the first part of the code in the 1980s. He discovered that chiral catalysts could produce one molecular form preferentially, but the effect was not linear. A small initial difference in the starting materials could produce a much larger difference in the final chemical mixture. This non-linear amplification was the key. It meant that a tiny preference, perhaps from a random fluctuation, could be magnified into a dominant outcome.

This was a breakthrough because it showed that chemistry does not always behave in simple, proportional ways. It can amplify small signals. Kagan’s work provided the theoretical foundation for how a system could pick a side. But it was still a controlled reaction. It required an external catalyst to drive the preference. The missing piece was a way for the reaction to sustain and amplify itself without constant external input.

Soai’s self-replicating reaction

Kenso Soai, a Japanese chemist at the Tokyo University of Science, took that idea and pushed it into the realm of the self-sustaining. He discovered asymmetric autocatalysis. In the Soai reaction, the product of the chemical reaction is also the catalyst for the next step. The molecules produced help create more of themselves, specifically the version with the same handedness. A tiny initial imbalance can therefore become greatly amplified, eventually producing an overwhelming majority of one molecular form. It is a chemical version of a snowball rolling down a hill, gathering more snow as it goes.

Peter Somfai, a member of the Nobel Committee for Chemistry, called it probably the coolest experiment in organic chemistry. He noted that the discoveries reshaped our understanding of molecular chirality, including how it is created, amplified, and transmitted. The Soai reaction is not just a laboratory curiosity. It is a proof of concept for how life might have locked in its molecular preference. It shows that chemistry has an inherent tendency to amplify asymmetry if given the right conditions.

Why this matters for medicine

This is not just abstract theory. It has direct implications for pharmacology. More than half of clinically employed drugs are chiral. The two mirror images of a drug can have vastly different effects on the body. One might treat a cold, while the other makes the sniffles worse. In some cases, one form is therapeutic, and the other is toxic. Understanding how to control and amplify chirality is essential for designing safer, more effective medicines.

Soai and Kagan’s work provides the tools to design reactions that produce the desired mirror image with high purity. This is critical for drug development. As we design new treatments for complex diseases, we need to be choosy about which molecular form we are delivering to patients. The Nobel Prize recognizes that this is not just a fundamental scientific question. It is a practical necessity for modern medicine.

A legacy of collaboration

The award also highlights the power of long-term collaboration and the persistence of scientific inquiry. Kagan is 95, and Soai is 76. Their work spans several decades, and the recognition comes at the very end of their careers. It is a reminder that some of the most profound discoveries in science take a long time to be fully appreciated. The mechanism they discovered is elegant, but its implications for our understanding of life’s origins are vast.

As we continue to explore the frontiers of chemistry and biology, the work of Kagan and Soai will serve as a foundation. They showed us that small differences can lead to big outcomes. They showed us that chemistry can amplify asymmetry. And they showed us that the mystery of life’s handedness is not just a philosophical question. It is a chemical reality that we can now understand, and perhaps even replicate, in the lab.