Isomer

B

Optical Isomers

Another type of stereoisomer occurs when two isomers are mirror images of each other, yet they cannot be superimposed on each other. Scientists call these molecules optical isomers, because the molecules of one isomer will rotate the direction of a beam of plane-polarized light (light that is traveling in the same plane, not in all directions) in the opposite direction that the other isomer will.

Optical isomers are similar to a person’s right and left hand. Even though a person’s hands look very similar and are mirror images of each other, a right hand will not fit into a left-hand glove. While mirror images of many objects can be superimposed on each other (for example, a pushpin can be superimposed on its mirror image), optical isomers cannot be superimposed. For example, the optical isomers of lactic acid are arranged as follows (dark lines point toward the viewer and dashed lines point away):

Just as the right hand and left hand are different, optical isomers (known as enantiomers) are different from each other and can have different properties. For example, muscles produce D-lactic acid when they contract, and a high amount of this compound in muscles causes muscular pain and cramps. On the other hand, sour milk contains L-lactic acid, giving the milk its sour taste.

Enantiomers are often identified with a D- or L- prefix because of the direction they rotate polarized light. Enantiomers that rotate polarized light in a clockwise direction are known as dextrorotatory (right-handed) molecules, and enantiomers that rotate polarized light in a counterclockwise direction are known as levorotatory (left-handed) molecules. A solution containing equal amounts of both enantiomers (known as a racemic mixture) will not rotate polarized light, because the rotations of the two enantiomers cancel each other out.

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When scientists prepare optical isomers in the laboratory, a racemic mixture is usually produced. Companies may produce these mixtures to sell as a drug or food additive, even though only one of two enantiomers has the desired properties. For example, the drug ibuprofen is an optical isomer that pharmaceutical companies sell as a racemic mixture. Only the L- form is an active pain reliever; the D- form is inactive. However, the body converts the D- form to the L- form, so the entire dose becomes effective.

The artificial sweetener aspartame (NutraSweet) is also an optical isomer. Only one of the enantiomers has a sweet taste, while the other tastes bitter, so companies do not produce this sweetener as a racemic mixture (see Sweetener, Artificial). Many simple natural sweeteners like the sugar glucose (C₆H₁₂O₆) are also optical isomers and interact in a similar way to aspartame with the taste buds. The isomer D-glucose, which is a primary source of energy for the body, has a sweet taste. Conversely, L-glucose has no taste and is not even metabolized by the body.

Many optical isomers are found in nature, with one enantiomer often occurring more frequently than the other. For example, all but one of the 20 naturally occurring amino acids (compounds that serve as the building blocks of proteins in organisms) are optical isomers, and all are levorotatory (left-handed) molecules. Conversely, most natural sugar molecules are optical isomers and are dextrorotatory (right-handed) molecules.

In the 1990s, chemists learned how to produce only one enantiomer of an optical isomer at a time in the laboratory. This process is known as biocatalysis. In biocatalysis, chemists use enzymes (complex proteins that catalyze, or encourage, biological reactions) to selectively transform a racemic mixture into the desired enantiomer. Pharmaceuticals produced with this method include an HIV (human immunodeficiency virus) treatment called Ziagen and intermediate products used in making heart drugs.

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