Structure of a Typical Amino Acid
Amino acids in solution at neutral pH exist predominantly as dipolar ions (also called zwitterions). Amino acids can exist as zwitterions – substances containing equal numbers of positive and negative charge due to their carboxyl and amine groups, which can be negatively and positively charged, respectively. In the dipolar form, the amino group is protonated (NH3 +) and the carboxyl group is deprotonated (COO–). The ionization state of an amino acid varies with pH (Figure 4.4). They differ from each other in their side chains, or R groups, which vary in structure, size, and electric charge, and which influence the solubility of the amino acids in water.
