Louis Pasteur is buried in an elaborate mausoleum at the Institute Pasteur in Paris where mosaic tiles on the tomb commemorate various aspects of the great man's scientific life. A flock of sheep, for example, represents his work on the anthrax vaccine and a dog reminds us of his conquest of rabies. But almost hidden in the ornate tableau is the phrase "une dissymmetry dans les molecules" or, in English, "molecular dissymmetry."
Contrary to popular belief, Louis Pasteur was not a doctor or a biologist, he was a chemist. His early fascination was with crystals, particularly with salts of tartaric acid which since antiquity had been known to form during the fermentation of grape juice. One day in 1848 he was examining a sample of such crystals under a magnifying glass and made a remarkable observation. There were actually two kinds of crystals, and they were mirror images of each other! Pasteur laboriously separated the crystals with tweezers and discovered that they had exactly the same physical properties, save for one. When they were dissolved in water and placed in the beam of a special kind of light, known as plane polarized light, they rotated the beam in opposite directions. Since the crystals had been dissolved in water, Pasteur hypothesized that this behavior was actually due to the individual molecules, and that the two types of crystal must consist of mirror image molecules!
This was really advanced thinking, but Pasteur could push it no further because at the time practically nothing was known about the structure of molecules. Twenty-five years later, Jacobus van't Hoff correctly interpreted Pasteur's finding by proposing that carbon atoms in molecules connected to other atoms in a specific three-dimensional pattern. This "tetrahedral" pattern could give rise to molecules which were alike in every respect except that, that like our hands, they were non-superimposable mirror images of each other. This explanation laid one of the cornerstones to the building of modern organic chemistry and van't Hoff was duly rewarded with the first ever Nobel Prize in chemistry in 1901.
All of this sounds highly theoretical, but the concept of mirror image molecules has a great deal of practical importance. About half of all the drugs marketed today are 鈥渃hiral,鈥� meaning that they can exist in mirror image forms each of which is called an 鈥渆nantiomer. 鈥淭he enantiomers are referred to as 鈥淪鈥� or 鈥淩鈥� depending on the direction they rotate polarized light. If both enantiomers are present to an equal extent, which is usually the case when the drug is produced in the lab, we have a 鈥渞acemic鈥� mixture. In some cases, one enantiomer is therapeutically more active than the other, yet both give rise to side effects equally. This suggests that marketing only the active form would mean that the effective dose could be cut in half, and side effects could be greatly reduced.
Take, for example, one of the most common over-the counter pain killers, ibuprofen (Advil.) The S-enantiomer is therapeutically much more active than the other, yet both give rise to side effects equally. However, there is no point in separating the enantiomers, generally a costly process, because it turns out that in the body some of the S-enantiomer is converted to the R form. The situation is different for bupivacaine, a commonly used local anesthetic for nerve blocks and epidurals. While both enantiomers can depress the ability of the heart to pump blood, the R-enantiomer does this to a greater extent meaning that the S-enantiomer is safer to use. In this case then the separation of the enantiomers is of practical importance.
The story extends to other types of chemicals as well. Many pesticides are mixtures of mirror image forms with only one type exhibiting activity. Dichlorprop is a commonly used herbicide but only the R form has herbicidal activity so there is no point in releasing the S form into the environment and consequently only the R enantiomer is approved for use.
Obviously then, that reference to dissymmetric molecules on Pasteur's tomb is most appropriate. His pioneering studies put us on the path to the understanding of molecular structure which is fundamental to chemical progress.
