The side chains of the amino acids play an essential role in determining the properties of proteins. Proline shares many properties with the aliphatic group. The rigidity of the ring plays a critical role in protein structure, as we shall see later. The side chain sulfhydryl groups of cysteine can undergo a reversible oxidation reaction reaction to form cystine, which contains a disulfide bond . The presence of disulfide bonds in proteins is often a critical structural feature.
Proteins contain several classes of weak acid groups.
The ionization state of the side chain weak acid groups controls the charge on the protein. Tryptophan has the highest molar absorptivity, followed by tyrosine, with phenylalanine making only a small contribution. In some proteins the side chains of a few amino acids are modified after they are biosynthesized. If this process is repeated many times, then a long linear chain of amino acids is produced - a polypeptide. As we shall see later, the planarity of the these elements has important consequences for the three dimensional structure of proteins.