Explain how pH, pKa and pI can be used to predict the mobility of an amino acid, in a direct-current electrical field.

Answer 1

An amino acid has both an amino group -NH2 and a carboxyl group -COOH. Depending on the pH, the NH2 group and/or the COOH group may be protonated or not. If the NH2 is protonated, it becomes -NH3^+ and if the -COOH group becomes de-protonated, it becomes -COO^-. So, you see that depending on the pH (and the pKa of the different groups), the amino acid may have a net positive charge, a net negative charge, or it may be neutral (the pI, or isoelectric point). When it is neutral, it is referred to as a zwitter ion. The net charge will ultimately determine the mobility of the amino acid when placed in a direct current electric field.