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).
Figure 1: Amino acids, such as glycine, are able to become ionized. These chemical groups can become ionized by donating and accepting hydrogen ions respectively, so that the carboxyl group becomes COO- and the amine group becomes NH3+. This process results in the formation of a zwitterion, a type of charged molecule which has both positive and negatively charged parts of the same molecule.
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).
Figure 2: Amino acids which contain amine or carboxyl groups in their in their R groups, such as arginine and aspartic acid (but also lysine and glutamic acid) ionize to form positively and negatively charged groups. Due to the attraction of opposite charges, this causes the amino acids to be attracted to each other to form strong, specific interactions between the amino acids.
This interesting ability for amino acids to form zwitterions is very important for the structure and function of proteins.