1848

Chirality

Louis Pasteur (1822–1895), Joseph-Achille Le Bel, (1847–1930), Jacobus Henricus van ’t Hoff (1852–1911)

The discovery of polarized light in the early 1800s led to many experiments designed to determine its nature, and chemists observed various compounds under it to see how they behaved. One such compound was tartaric acid, found in grapes and capable of forming crystals in wine barrels. The French chemist and microbiologist Louis Pasteur found that a solution of tartaric acid crystals from wine could rotate the plane of the polarized light, as if it had been twisted while it passed through. But a solution of factory-made tartaric acid did not show the same behavior. Why did two batches of the same compound behave so differently?

This was a puzzle, because the two samples seemed otherwise identical. When Pasteur examined the crystals (as tartrate salts) under a microscope, he found that the nonrotating sample seemed to be a mixture of two sorts of crystals, mirror images of each other. He separated the two into pure samples and found to his delight that one variety rotated polarized light just like the wine-barrel tartaric acid, while the other rotated the same amount, but in the opposite direction. The original sample showed no rotation because it had been canceled out!

In 1848, the twenty-five-year-old Pasteur theorized that tartaric acid must have a structure that allowed it to exist in “right-handed” and “left-handed” forms, and that the same must be true for other compounds that could rotate polarized light. It wasn’t until the 1870s, though, that Dutch chemist Jacobus Henricus van ’t Hoff and French chemist Joseph-Achille Le Bel were able, independently, to explain how such “chiral” compounds (named later from the Greek word for hand) could exist.

Chirality is now known to be essential to life (proteins and sugars are both chiral), and many important drugs are chiral as well. We now also know that the sorts of mixed crystals Pasteur studied are relatively rare, and that Pasteur was not only very good, he was also very lucky. Still, as he famously said, “Fortune favors the prepared mind.”

SEE ALSO Tetrahedral Carbon Atoms (1874), Fischer and Sugars (1884), Liquid Crystals (1888), Coordination Compounds (1893), Asymmetric Induction (1894), Thalidomide (1960), Resolution and Chiral Chromatography (1960), Murchison Meteorite (1969), B₁₂ Synthesis (1973), Enzyme Stereochemistry (1975), Palytoxin (1994), Shikimic Acid Shortage (2005), Engineered Enzymes (2010)

Crystals of tartaric acid photographed with polarized light shining through them. The colors are produced by their varying orientations and thicknesses.