However, while he was still at the École Normale Supérieure in Paris, he gained some notoriety in the scientific community by solving a problem that had been tackled by many researchers before him.

Louis Pasteur studying at the École Normale Supérieure in Paris.

At that time, Pasteur was studying the chemical structure of crystals. The crystalline state is a type of arrangement adopted by (almost) all solid materials. In a crystal, the atoms that make up the substance are close to each other, interacting with one another in a regular manner, which ultimately generates a highly ordered molecular structure that follows a repetitive pattern (symmetry). This arrangement not only gives the crystal its traditional appearance (with flat faces and specific orientations) but also determines its physical, chemical, and biological properties.

In particular, Pasteur studied tartaric acid, and also another substance known at the time as racemic acid, both products present during wine production. Tartaric acid was known for spontaneously crystallizing and accumulating as a crust on the walls of fermentation vessels or cork stoppers.

At that time, it was understood that although both acids had the same chemical composition—having equal amounts of carbon, hydrogen, and oxygen atoms— they reacted differently in certain experiments. When polarized light passed through a solution prepared with each of these compounds, the light behaved quite differently upon exiting. In the case of tartaric acid solution, the plane of polarization of the light was deflected to the right. In contrast, the polarized light passing through the racemic acid solution was unaffected and exited without changing direction.

Pasteur hypothesized that if the two molecules were composed of the same quantity and type of atoms but exhibited different physical properties, the difference must lie in the arrangement of the atoms.

What he discovered was simple, yet fascinating. When crystallizing racemic acid, the small crystals observed under the microscope were not all identical: about half of them had facets oriented in one direction, while the others appeared to be "inverted," with the same facets facing the opposite way, as if one set were the mirror image of the other. This was similar to what happens with our hands: the right hand is like the left, but they cannot be superimposed.

From the observation of this 'specular' characteristic of the molecules, Pasteur had discovered the asymmetry of molecules. He took the two types of crystals from racemic acid and dissolved them in water to measure the rotation of polarized light. One type of crystal exhibited the typical behavior of tartaric acid (rotation to the right, or dextrorotatory), while the other rotated the light in the opposite direction, to the left (levorotatory). By mixing both substances in equal parts, he was able to reconstitute the optically inactive mixture, which from then on was called a racemic mixture. This laid the groundwork for a discipline known as stereochemistry, which developed significantly in the following decades.

His experiments allowed him to demonstrate that the asymmetry of molecules granted them different optical properties, establishing the foundations of 'molecular chirality' (1948). Derived from the Greek word kheir (hand), chirality in chemistry refers to the property of an object that cannot be superimposed on its mirror image. This is fundamentally due to the spatial arrangement of its atoms.

Thus, a chiral molecule exists in two forms depending on the direction in which it rotates polarized light: to the left or to the right. Based on this, there are two forms of chiral molecules: levorotatory (light rotates to the left) and dextrorotatory (to the right). These forms are known as enantiomers and are identified by preceding the molecule's name with the letters R- and S-, derived from the Latin rectus and sinister.

From Chemistry to Pharmacology and Food

Although all these concepts may seem very specific and technical, the discovery of molecular chirality has been crucial for the advancement of science in various fields. For example, chirality is particularly relevant in the pharmaceutical industry, as often only one of the enantiomers of a compound produces an effect (it is biologically active) while the other does not, or they may even have opposing effects.

This is the case with ibuprofen, the traditional anti-inflammatory: while the S-enantiomer of ibuprofen can relieve pain or reduce fever, its mirror image —the R-enantiomer— has no therapeutic effect whatsoever.

The same occurs with vitamin C, chemically known as ascorbic acid. In this case, R-ascorbic acid has very low biological activity, is not easily metabolized by the body, and can even be somewhat toxic. Meanwhile, S-ascorbic acid (the one found in traditional vitamin C supplements) is an antioxidant compound that helps fight infections, heal wounds, and maintain healthy tissues.

A more complex case is thalidomide, which in its R-enantiomer helps reduce nausea in pregnant women, while consuming the S-enantiomer can cause fetal malformations.

Beyond pharmacology, another chiral molecule with curious effects is limonene, which is extracted from the oil present in citrus peels and gives the characteristic smell. The R-enantiomer of limonene is found in lemon peels and produces its unique scent; in contrast, the S-enantiomer gives the distinctive smell of orange.

Similarly, in the food industry, aspartame tastes sweet if it is the S-enantiomer, or bitter if it is the R-enantiomer.

None of this was known when Pasteur analyzed those tartaric and racemic acid crystals, and it would still be several years before further knowledge would be developed. However, the significance of his discovery was already evident, earning the 26-year-old scientist the Legion of Honor from France. This is, to this day, the most recognized distinction from the government of that country, established by Emperor Napoleon I in 1804.