Optically Active Substance
Optically Active Substance
a medium exhibiting natural optical activity. There are two types of optically active substances. Substances of the first type, for example, sugars, camphor, and tartaric acid, are optically active in any state of aggregation. Substances of the second type, for example, quartz and cinnabar, are active only in the crystal phase.
The optical activity of substances of the first type is due to the asymmetric structure of the substances’ molecules. For substances of the second type optical activity is the result of a specific orientation of the molecules, or ions, in the unit cells of the crystal—that is, it is the result of the asymmetry of the force field binding the particles in the crystal lattice. The crystals of optically active substances always exist in both right-handed and left-handed forms. The lattice of a right-handed crystal and the lattice of a left-handed crystal are nonsuperimposable mirror images of each other: they are enantiomorphs. The optical activity of right-handed and that of left-handed forms of optically active substances of the second type have different signs and are equal in absolute value, external conditions being equal. For this reason the forms are called optical antipodes, a term sometimes applied to crystals of optically active substances of the first type as well.
Molecules of right-handed and left-handed optically active substances of the first type are optical isomers; that is, they are mirror images of each other in structure. They can be distinguished from each other, while the molecules of optical antipodes—optically active substances of the second type—are quite indistinguishable. The physical and chemical properties of pure optical isomers are the same in the absence of an asymmetric agent that reacts to the mirror asymmetry of the molecules. The product of a chemical reaction not involving such an agent is always a mixture of equal amounts of optical isomers and is called a racemic substance. The physical properties of a racemic substance and of pure optical isomers are often different. For example, the melting point of a racemic substance is somewhat lower than that of a pure isomer. A racemic substance is separated into pure isomers by collecting enantiomorphic crystals, by a chemical reaction involving an asymmetric agent (a pure isomer or an asymmetric catalyst), or by microbiological means. The last method shows the existence of asymmetric agents in biological processes and is connected with a special property of animate nature, a property that has not yet received a satisfactory explanation: nature constructs proteins from left-handed optical isomers of amino acids, and 19 of the 20 amino acids important for life are optically active.
It should be noted with respect to optically active substances of the first type that the terms “left-handed” and “right-handed” —Land D, respectively—are arbitrary in that they do not correspond directly to the direction of rotation of the plane of polarization. The situation is different for the same terms—l and d— applied to optically active substances of the second kind or for the terms “levorotatory” and “dextrorotatory.”
Optical isomers often exhibit quite different physiological and biochemical effects. For example, proteins artificially synthesized from D-amino acids are not assimilated by an organism; bacteria ferment only one of the isomers without touching the other; and L-nicotine is several times more poisonous than D-nicotine. The surprising phenomenon of the preeminent role of only one of the optical isomers in biological processes may be of fundamental importance in explaining the origin and evolution of life on earth. Scientists’ special interest in optically active substances is due to the abundance of the substances in nature and to the substances’ great role in the processes of life. The high sensitivity of research methods based on the dispersion of optical activity is also a factor in the interest shown in optically active substances.
S. G. PRZHIBEL’SKII