Chemical Structures and Nomenclature

Amino acids are small organic molecules with the general formula shown in Figure 1.

The central carbon atom in this structure is called the a-carbon, and the amino and carboxyl groups attached to it are known as the a-amino group and the a-carboxyl group, respectively. The R groups of the 20 amino acids that can be incorporated into proteins are shown in Table 1; these R groups give the different amino acids their specific chemical and physical properties.

The a-amino group acts as a weak base and is always protonated at physiological pH; similarly, the a-carboxyl group acts as weak acid and at physiological pH is always ionized. Thus, free amino acids in biological material exist as zwitterions, as shown in Figure 2.

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