One property of amino acids, and proteins more widely, is their ability to ionize. Certain chemical compounds, or rather groups of atoms in those compounds are, depending on the conditions that they are in, are able to form ions. Ions are charged groups, that are formed when a group of atoms donates (or loses) or accepts (or gains) a hydrogen atom (or rather a positively charged hydrogen atom known as an ion) from the surrounding solution. Donating and accepting hydrogen ions changes the charge of the group.
Amino acids are unusual, in that all amino acids contain both carboxyl groups (which can donate hydrogen atoms) and amine groups (which can accept hydrogen atoms. Consequently, amino acids ionize to have different charges in different parts of the amino acid molecule (Figure 1).
This phenomenon gives amino acids some interesting properties. Amino acids can act as buffers, which are chemicals that can maintain the pH of a solution, since they can accept, or donate hydrogen ions to maintain the pH of a solution. This property also applies to chemical groups in the side groups of amino acids, like aspartic and glutamic acids (which have carboxyl groups) and arginine and lysine (which have amine groups). These are also ionized into negatively and positively charged groups, which is one of the mechanisms that allow protein interactions to take place (Figure 2).
This interesting ability for amino acids to form zwitterions is very important for the structure and function of proteins.