Molecular Chirality

Molecular chirality is the property of a molecule that makes it non-superimposable on its own mirror image, the way a left hand and a right hand share the same parts but cannot be laid on top of one another. Louis Pasteur discovered it in 1848 while investigating crystals of sodium ammonium tartrate, a salt of tartaric acid found in wine-making residues: he noticed the crystals grew in two forms, mirror images of each other, and using tweezers under a microscope sorted them by hand into two piles. Dissolved separately, each pile rotated polarized light passing through it, one clockwise and the other by an equal amount counterclockwise, while the original mixed sample had rotated no light at all; Pasteur correctly concluded that the molecules themselves, not just the crystals, existed in two mirror-image forms. The finding founded the field of stereochemistry, extended in 1874 by Jacobus van't Hoff and Joseph Le Bel into the theory that a carbon atom's four bonds point toward the corners of a tetrahedron, which explained why mirror-image molecules were possible at all; Emil Fischer's work on sugar stereochemistry and his lock and key model of enzyme specificity, two decades later, applied the same principle of molecular shape to explain a living process for the first time.