Ampholytes

Ampholytes are molecules containing both acidic and basic groups.

All of the common amino acids found in proteins (Figure 5.3) are ampholytes because they contain a carboxyl group (-COOH) that acts as an acid and an amino group (-NH 2) that acts as a base.

As free amino acids, each amino acid has at least two pK a values (some have more because they have additional acidic or basic groups).

The titration of an ampholyte generates a more complex plot of pH versus moles of acid (or base) added than are obtained for a simple buffer with only a single ionizing species because the ionization of each acidic and basic group of the ampholyte is represented by a step in the titration curve. In Figure 2.18, for example, there are two steps in the titration curve of the ampholyte glycine, whereas in Figure 2.17 the titration curves of NH 4+ and HCOOH have only one step each. Figure 2.19 shows the fraction of each molecular species of glycine present in the solution as a function of pH.

The presence of both acidic and basic groups in a single molecule means the molecule may exist in several different charged states. For example, glycine can have a charge of +1, 0, or -1, depending on the pH of the solution in which it is dissolved. The state at a net charge of zero arises when the basic amine group is charged +1 and the acidic carboxyl group is charged -1.

Molecules containing a mixture of charges that result in the molecule having an overall charge of 0 are called Zwitterions. The zwitterion form of the amino acid, glycine, is as follows:

See also: Ionic Equilibria and pH, Henderson-Hasselbalch Equation, pI, Zwitterions