Optical Isomers

When a molecule can be assembled such that all the atom connections are the same, but with arrangements in space such that one isomer is the nonsuperimposable mirror image of the other, the isomers are called optical isomers. Most of the chemical and physical properties of optical isomers are identical, but the two isomers interact with polarized light differently, hence the name. Many molecules of biochemical relevance are optical isomers, with one isomer biologically active, the other inactive. The amino acid alanine (amino acids are the building blocks of proteins), shown in Figure 1.1, has two mirror image isomers.

Figure 1.1. Optical isomers of the amino acid alanine.

A tetrahedral atom with four different groups bonded to it, as is the case with alanine, always results in a pair of optical isomers. Although there are other ways of arranging atoms as mirror images, the case of a tetrahedral C with four different groups is by far the most common.