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An important feature of amino acids is that they are chiral. The exception to this is glycine. The best, and most iconic, way to explain this is with your hands. Place your left hand on a table palm down. Now place your right hand on top of it, palm down. They don't exactly overlap. Your left hand's thumb sticks out to the right, your right hand's thumb to the left. That is because they are mirror images. They are non-superimposable (Figure 4.6).

Figure 4.6 Chirality illustrated with hands and the generic structure for amino acids.

Source: Reproduced with permission of wikicommons, https://commons.wikimedia.org/wiki/File:Chirality_with_hands.svg.

Given four different side chains or more, molecules too can be assembled into left- and right-handed forms. The reason why glycine is not chiral is that the central (alpha) carbon in the molecule has two identical H atoms attached to it, which does not allow for two distinctive left- and right-handed forms of the molecule. Chiral forms of a molecule are said to be isomers, which are chemical compounds with the same chemical formula but different structures. The conventional way to classify chiral molecules is based on the direction that polarized light, when shone at the molecules, is rotated (Figure 4.7). The different mirror images tend to rotate it one way or the other. If it is rotated to the left, we call it a levorotatory molecule or the “L” form. If it is to the right, or dextrorotatory, we call it the “D” form. When we have an equal mixture of both L and D molecules, we say that the mixture is racemic. Specifically, we use the term enantiomer to refer to one of the two chiral forms of the molecule.

Figure 4.7 Chiral molecules rotate polarized light in particular directions. Schematically, the rotation of light to the left (levorotation) is shown here with the amino acid, L-alanine.