The amino acid composition of proteins The common property of all proteins is that they consist of long chains of α-amino (alpha amino) acids. The general structure of α-amino acids is shown in . The α-amino acids are so called because the α-carbon atom in the molecule carries an amino group (―NH2); the α-carbon atom also carries a carboxyl group (―COOH). In acidic solutions, when the pH is less than 4, the ―COO groups combine with hydrogen ions (H+) and are thus converted into the uncharged form (―COOH). In alkaline solutions, at pH above 9, the ammonium groups (―NH+3) lose a hydrogen ion and are converted into amino groups (―NH2). In the pH range between 4 and 8, amino acids carry both a positive and a negative charge and therefore do not migrate in an electrical field. Such structures have been designated as dipolar ions, or zwitterions (i.e., hybrid ions). Although more than 100 amino acids occur in nature, particularly in plants, only 20 types are commonly found in most proteins....
