Many chemical compounds important for life exist in two forms, which are mirror images of each other, so-called enantiomers. Although they are almost identical, only one of them matches the function of a biological organism. The second one may be inactive or even harmful. They can be compared to two keys - one opens the door and the other locks it or destroys the lock.
This property of molecules, called chirality, is ubiquitous in nature. All amino acids that make up proteins in our cells have the same "left" version, and sugars in DNA have the same "right" version. In the laboratory, chemical synthesis usually produces a mixture of both versions. For decades, chemists have been wondering how to make a reaction produce only one desired form.
The beginnings of research on chirality date back to the mid-19th century. Louis Pasteur, while studying tartaric acid, a key ingredient in wine, noticed that its crystals existed in two mirror-like forms. He separated them and discovered that each of them twists the plane of polarized light differently. Moreover, tartaric acid-fermenting bacteria "selected" only one of these forms, leaving the other untouched. This was the first evidence that life might prefer one version of chiral molecules.
Over the following decades, chemists tried to understand why nature chose only one of two possible versions. However, in the laboratory a mixture of both enantiomers was always obtained. The breakthrough came in the early 20th century when German chemist Willy Marckwald was the first to carry out a reaction that produced more of one enantiomer than the other. The key turned out to be a chiral catalyst that preferred one of the forms.
In 1953, Charles Frank, a theorist at the University of Bristol, proposed a mathematical model explaining how homochirality, i.e. the predominance of one version of a molecule, could arise. According to him, three conditions are necessary: the presence of a chiral catalyst, an asymmetric reaction and an autocatalysis process in which the reaction product itself becomes a catalyst, enhancing the formation of its own version.
Henri Kagan, working in the 1980s at the University of Paris-Sud, decided to check whether the relationship between the chirality of the catalyst and the product was indeed linear, as previously thought. It turned out not to be the case.
Catalysts composed of two of the same enantiomers worked much faster than those mixed. This effect, called nonlinear asymmetry amplification, made it possible to obtain a much greater advantage of one form of the product than would result from the proportions in the catalyst. This discovery revolutionized the design of asymmetric reactions, especially in the pharmaceutical industry, where drug purity is crucial to patient safety.
Kenso Soai from Tokyo University of Science went a step further. Inspired by the effects described by Kagan, he decided to find a reaction in which the product itself becomes a catalyst, i.e. it meets the condition of autocatalysis in Frank's model. After years of experimentation, he managed to achieve this. In 1995, he described a reaction in which a slight predominance of one enantiomer at the beginning led to almost complete domination of this form in the final product.
In 2003, Soai improved this process, achieving almost 100 percent enantiomeric purity. For the first time in history, it was possible to produce a predominance of one chiral form from a mixture of non-chiral substrates, purely chemically, without the participation of living organisms.
The Soai reaction is considered one of the most elegant experiments in the history of chemistry. It shows that even a minimal predominance of one form at the beginning of the process can, thanks to autocatalysis, lead to the complete domination of this form in the final product. Interestingly, which form will win depends on chance. Sometimes a "left", sometimes a "right" version of the molecule is created.
Kagan and Soai's discoveries are of great practical importance. Thanks to them, chemists have gained new tools for designing reactions in which they can precisely control which version of a chiral molecule will be created. This is crucial in the production of medicines. The story of thalidomide, a 1960s drug where one form was effective while the other caused serious birth defects, is still a cautionary tale for the pharmaceutical industry today.
New methods also allow the production of pure enantiomers in the food and perfume industries or in the production of modern materials. Moreover, these discoveries shed new light on the question of the origins of life on Earth. They show how homochirality, which is the foundation of the biochemistry of all organisms, could have arisen.
