Pasteur's artistic insight changed chemistry

If you have ever had milk, you’re probably familiar with the work of Louis Pasteur, the 19th-century French chemist and biologist. He prevented diseases, developing a process — widely known as pasteurisation — for killing microbes in milk and wine. He also created vaccines for rabies and anthrax. And his ideas led to the acceptance of germ theory, the notion that tiny organisms caused diseases like cholera. “He’s considered the benefactor of mankind,” said Joseph Gal, a chemist and professor emeritus at the University of Colorado, USA. But before all that, Louis was an artist. And without his early creative explorations, he may not have made one of his most monumental, but least talked about, discoveries in science, one with far-reaching implications.

In a paper published last month in Nature Chemistry, Joseph explains how a young Louis fought against the odds to articulate the existence of chirality, or the way that some molecules exist in mirror-image forms capable of producing very different effects. Today we see chirality’s effects in light, in chemistry and in the body — even in the drugs we take. And we might not know a thing about them if it weren’t for the little-known artistic experience of Louis Pasteur, Joseph says. Louis was born in 1822 to a French family of modest means. As a teenager, Pasteur was known for his portraits. But at his father’s urging, he became a scientist. At the age of 24, he discovered chirality.

Molecular secrets in wine

To understand chirality, consider two objects held up before a mirror: a white cue ball from a pool table and your hand. The reflection of the ball is exactly like the original. If you could reach into that mirror, pull out the reflection and cram it inside the original, they’d match up point for point. But if you tried the same thing with your hand, no matter how much you tried, the mirror image would never fit into the original.

At the molecular level some objects are like cue balls, and they are always superimposable. But other things are like hands, and they can never be combined. Hands, like the crystals, Louis would eventually discover, are chiral. And that discovery all came down to an accident in a vat of wine.

During wine-making, a chemical called tartaric acid builds up on vat walls. In the 18th and 19th centuries, makers of medicine and dyes used this acid. In 1819, factory workers boiled wine too long and accidentally produced paratartaric acid, which had unique properties that intrigued scientists like Louis. The study of the acid was related to the study of crystal structures, which at the time seemed like a way to help solve the mystery of how molecules were built. Observing the various ways crystals interacted with light gave scientists clues about their properties.

Earlier in the 19th century, Jean-Baptiste Biot, a French physicist, discovered that tartaric acid was optically active. That is, when Jean-Baptiste shined polarised light through tartaric acid crystals in a solution, they rotated the light clockwise or counterclockwise. But no one knew how the crystals did it. When studying the paratartaric acid, Louis found that it produced two kinds of crystals — one like those found in tartaric acid and another that was the mirror opposite.

Not optically active

The crystals were handed, or what the Greeks call chiral (kheir) for hand. And they were not optically active, like the tartaric acid. Louis concluded that the mirror-image crystals, together as a 50/50 mix in the solution, canceled out each other’s ability to rotate polarised light. And without even knowing how a molecule was built, just eight months after receiving his doctorate, he said that their molecular structure was chiral, too. Chemistry changed forever.

For various reasons, Louis eventually turned to biology. Perhaps he recognised that chirality could play a big role in it, some suggest. We now know that many drugs contain molecules that exist in two chiral forms, and that the two forms can react differently in the body. “Many objects in our universe have this property of chirality,” Joesph said. In the mirror, in a vat of wine heated too long, on a piece of limestone and in your body: the non-superimposable hands of the universe were discovered by a man who wanted to be an artist, but settled for science.