ABSTRACT
Amino acids all possess a carboxylic acid group and an amino group, both linked to a single carbon atom called the a-carbon (Figure 1.1). The differences between amino acids result from the side chain attached to the a-carbon atom, which can be
aliphatic or aromatic in nature and can include extra amino, imino, or carboxylic acid functional groups (Figure 1.1). All amino acids except glycine can exist as optical isomers in D-and L-forms (Figure 1.1), but only L-forms are found in living organisms (with the exception of D amino acids in certain bacterial cell wall proteins).6 The chemical versatility provided by the 20 common amino acids is critically important to the function of proteins. Five of the 20 amino acids have side chains that can form ions in solution and impart polar and hydrophilic properties to the protein.
