How Pasteur’s Artistic Insight Changed Chemistry
Crystals of tartaric acid. Louis Pasteur was studying a version of this byproduct of wine production, paratartaric acid, when he articulated the property of chirality. CreditPasieka/Science Source
If you’ve 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 pasteurization — 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. Pasteur even helped us brew better beer.
“He’s considered the benefactor of mankind,” said Joseph Gal, a chemist and professor emeritus at the University of Colorado.
But before all that, Pasteur 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, Dr. Gal explains how a young Pasteur 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, says Dr. Gal.
Hands and cue balls
Pasteur was born in 1822 to a French family of modest means. His dad was a soldier in Napoleon’s army and a tanner. As a teenager, Pasteur made portraits of his friends, family and dignitaries. But after his father urged him to pursue a more serious profession — one that would feed him — he became a scientist. At the age of 24 he discovered chirality.
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.
Pasteur’s drawing of chiral crystals of paratartaric acid, from 1848. CreditAnnales de Chimie et de Physique
Molecular secrets in wine
During winemaking, 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 Pasteur.
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 Biot shined polarized light (which moves out in only one direction, say vertically or horizontally, rather than all directions) 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, Pasteur found that it produced two kinds of crystals — one like those found in tartaric acid and another that was the mirror opposite. The crystals were handed, or what the Greeks call chiral (kheir) for hand. And they were not optically active, like the tartaric acid.
Pasteur concluded that the mirror-image crystals, together as a 50/50 mix in the solution, canceled out each other’s ability to rotate polarized 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.
“Several famous or much more accomplished scientists, some well along their illustrious careers, studied the same molecules, the same substances,” said Dr. Gal. “Realistically you would think they’d have beaten him to the punch, and yet they missed it.”
A portrait of Pasteur, who lived from 1822 to 1895.CreditCulture Club, via Getty Images
Mirror, mirror, everywhere
For various reasons, Pasteur eventually turned to biology. Perhaps he recognized 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. The most tragic example occurred in the 1950s and ’60s, when doctors prescribed Thalidomide, a drug for morning sickness and other ailments, to pregnant women. The drug also contained a chiral molecule that caused disastrous side effects in many babies.
Today, pharmaceutical companies work harder to separate the active and inactive forms of molecules, and the Food and Drug Administration issued rules to crack down on many chiral drugs in the 1990s. But not all are dangerous, and some were grandfathered in. For example, the pain reliever ibuprofen, as formulated in the United States, contains a 50/50 mix of chiral molecules: one that reduces headaches and its mirror image, which does not appear to be harmful.
“Many objects in our universe have this property of chirality,” said Dr. Gal.
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.
Source: https://www.nytimes.com/2017/06/14/science/louis-pasteur-chirality-chemistry.html?rref=collection%2Ftimestopic%2FChemistry&action=click&contentCollection=science®ion=stream&module=stream_unit&version=latest&contentPlacement=8&pgtype=collection
