Question

Please explain the difference between using the pKa value of an amino acid versus using the pI of an amino acid to determine the change and whether it will be protonated or deprotonated at a particular pH.

For example, we say that histidine has a pKa of 6, but the isoelectronic point is 7.59.   If we have a solution with a pH of 7, what would histidine's charge be? If I want to quickly determine or automatically know the charge of the amino acid in any solution, would it be best to know the pI numbers for each amino acid?

Answer 1

The isoelectric point or pI of an amino acid is the pH at which an amino acid has a net charge of zero. While, the pKa value for the amino group on any amino acid specifically refers to the equilibrium between the protonated positive nitrogen and deprotonated neutral nitrogen of zwitter ion of amino acid.

The amino acid will be protonated or deprotonated at a particular pH, depends on the isoelectric point or pI of an amino acid.

• histidine has a pKa of 6, but the isoelectronic point is 7.59. If we have a solution with a pH of 7 (i.e; smaller than pI) the histidine's charge be (+)ve.
• "If I want to quickly determine or automatically know the charge of the amino acid in any solution, would it be best to know the pI numbers for each amino acid " this statement is correct .