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The 2026 Nobel Prize in chemistry honors Henri Kagan and Kenso Soai for discovering how chemistry can favor one version of a molecule over its mirror image, known as chirality. Chiral objects, like spiral galaxies or human hands, have a reflection that cannot be perfectly aligned with the original. While most living things are built from only one specific form of molecules—a state called homochirality—early Earth likely had a balanced mix of both forms. Kagan found in the 1980s that a catalyst does not need to be entirely pure to produce a single-handed product, sometimes creating a purer result than itself. Soai discovered in the 1990s a molecule that copies itself and amplifies small imbalances into overwhelmingly one-handed mixtures, offering a possible explanation for how life chose its specific molecular handedness.

This property is critical in medicine because human receptors are built from single-handed proteins. If a drug enters the body as a 50:50 mixture of both forms, one version might act as a therapeutic key while the other causes harm or interacts with wrong enzymes. Thalidomide illustrates this danger; sold as a mixture to treat morning sickness, its S-form caused severe limb deformities in over 10,000 children after birth. Although only the S-form was initially identified as the culprit, the drug’s flexibility allowed it to convert between forms inside the body, meaning even a pure version would likely have been unsafe.

Other drugs show different outcomes. Ibuprofen contains an active S-form and an inactive R-form, but the body converts some of the R-form into the active type, making the mixture effective. L-DOPA treats Parkinson’s disease by crossing the blood-brain barrier to become dopamine; its mirror image, D-DOPA, cannot do this. William Knowles solved the challenge of producing pure L-DOPA using a special catalyst in the 1970s, earning him the 2001 Nobel Prize. Similarly, cells use only D-glucose for energy, while L-glucose provides no calories despite tasting sweet, highlighting how chirality dictates biological function.