|Louis Pasteur is best remembered for his work on the germ theory of disease, the development of several vaccines, and the process of pasteurization, all of which laid the foundation for the science of microbiology. However, the renowned French chemist/biologists earliest and probably least-recognized observation was the phenomenon of enantiomerism.
As a graduate student at Pariss École Normale Supérieure in 1848, he was astonished to discover that the sodium ammonium tartrate that precipitated from a concentrated solution could be separated into two crystal forms. Interestingly, these crystals were nonsuperimposable mirror images of each other (1). When exposed to plane-polarized light, Pasteur found that a solution of the one crystal form rotated the light to the right while a solution of the other crystal rotated light to the left. The term dextrorotary (+) describes the right-handed pile, and levorotary () describes the left-handed pile. The components of each of these piles are known as enantiomers.
The solution with which Pasteur began is called a racemic mixture, which is composed of equal parts (+) and () enantiomers. Racemic mixtures are optically inactive because the rotation of plane-polarized light by one enantiomer cancels the equal but opposite rotation by the other enantiomer. Based on his observations, Pasteur predicted that the (+) enantiomer must be a mirror image of the () enantiomer, just as a right hand is a mirror image of a left hand. His observations that optical activity was related to crystalline and molecular structure were lasting contributions to the field of chemistry.