Louis Pasteur and Optical Activity: In Vino, Veritas
Louis Pasteur encountered the phenomenon of optical activity in 1843, during his investigation of the crystalline sediment that accumulated in wine casks (a form of tartaric acid called paratartaric acid—also called racemic acid, from Latin racemus, "bunch of grapes"). He used fine forceps to separate two types of crystals identical in shape but mirror images of each other. Both types proved to have all the chemical properties of tartaric acid, but in solution one type rotated polarized light to the left (levorotatory), the other to the right (dextrorotatory). Pasteur later described the experiment and its interpretation:
In isomeric bodies, the elements and the proportions in which they are combined are the same, only the arrangement of the atoms is different. . . We know, on the one hand, that the molecular arrangements of the two tartaric acids are asymmetric, and, on the other hand, that these arrangements are absolutely identical, excepting that they exhibit asymmetry in opposite directions. Are the atoms of the dextro acid grouped in the form of a right-handed spiral, or are they placed at the apex of an irregular tetrahedron, or are they disposed according to this or that asymmetric arrangement? We do not know.*
Now we do know. X-ray crystallo-graphic studies in 1951 confirmed that the levorotatory and dextrorotatory forms of tartaric acid are mirror images of each other at the molecular level and established the absolute configuration of each (Fig. 1). The same approach has been used to demonstrate that although the amino acid alanine has two stereoisomeric forms (designated d and l), alanine in proteins exists exclusively in one form (the l isomer; see Chapter 3).
(2S,3S)-Tartaric acid (levorotatory)
FIGURE 1 Pasteur separated crystals of two stereoisomers of tartaric acid and showed that solutions of the separated forms rotated polarized light to the same extent but in opposite directions. These dextrorotatory and levorotatory forms were later shown to be the (R,R) and (5,5) isomers represented here. The RS system of nomenclature is explained in the text.
*From Pasteur's lecture to the Société Chimique de Paris in 1883, quoted in DuBos, R. (1976) Louis Pasteur: Free Lance of Science, p. 95, Charles Scribner's Sons, New York.
(2R,3R)-Tartaric acid (dextrorotatory)
is (S) (Latin sinister, "left"). In this way each chiral carbon is designated either (R) or (S), and the inclusion of these designations in the name of the compound provides an unambiguous description of the stereochemistry at each chiral center.
Another naming system for stereoisomers, the d and l system, is described in Chapter 3. A molecule with a single chiral center (glyceraldehydes, for example) can be named unambiguously by either system.
Was this article helpful?