Amino acids contain ionizable groups that gain or lose protons depending on the pH of their environment. Understanding the titration behavior and isoelectric point (pI) of amino acids is fundamental to protein chemistry and separation techniques such as isoelectric focusing.
Ionizable Groups in Amino Acids
The Alpha-Carboxyl Group
Every amino acid has an alpha-carboxyl group with a pKa around 2.2. At pH values below this pKa, the group is protonated (COOH). At pH values above, it is deprotonated (COO-).
The Alpha-Amino Group
Every amino acid has an alpha-amino group with a pKa around 9.4. At pH values below this pKa, the group is protonated (NH3+). At pH values above, it is deprotonated (NH2).
Side Chain Ionizable Groups
Seven amino acids have additional ionizable groups in their side chains: aspartic acid (pKa ~3.9), glutamic acid (pKa ~4.1), histidine (pKa ~6.0), cysteine (pKa ~8.3), tyrosine (pKa ~10.1), lysine (pKa ~10.5), and arginine (pKa ~12.5).
Titration Curve
Titration of a Simple Amino Acid
When alanine (no ionizable side chain) is titrated with base, it shows two buffering regions. At low pH, it is fully protonated with a +1 charge. As base is added, the carboxyl group loses its proton first, producing a zwitterion with no net charge. Further addition deprotonates the amino group, giving a -1 charge.
The Isoelectric Point (pI)
The isoelectric point is the pH at which the amino acid has equal positive and negative charges, resulting in no net charge. For amino acids without ionizable side chains, the pI is the average of the two pKa values: pI = (pKa1 + pKa2) / 2.
pI for Amino Acids with Ionizable Side Chains
For acidic amino acids (aspartic acid, glutamic acid), the pI is the average of the two lowest pKa values. For basic amino acids (lysine, arginine, histidine), the pI is the average of the two highest pKa values.
Applications
The pI is critical for protein purification using isoelectric focusing, a technique that separates proteins based on their pI in a pH gradient. At their pI, proteins have minimal solubility and do not migrate in an electric field.
