pI

Ampholytes and polyampholytes contain groups whose electrostatic charge depends on the pH of the solution.

Each molecule has a distinct pH (called the pI or isoelectric point) at which the net average charge of all the groups adds up to zero. If acidic groups predominate, the pI will be low. If basic groups predominate, the pI will be high.

For ampholytes, like glycine, which have only a single acidic group and a single basic group, the pI can be determined by averaging the pK as of the two groups (from Equation 2.18) For example, the pK a values of the carboxylate and amino groups on glycine are 2.3 and 9.6, respectively. Thus,

> pI = (2.3 + 9.6)/2 = 5.95 (See Figure 2.19)

Molecules with larger numbers of acidic and basic groups typically do not have pIs that can be as straightforwardly determined. This is because the pK a of a group on a molecule with many ionizing groups is influenced by the state of ionization of the others and will not behave identically to the same group alone on another molecule.

See also: Ampholytes