Isoelectric Point
In acidic solutions, an amino acid exist as a positive ion and migrates toward the cathode. In basic solution, the amino acid exist as a negative ion and migrates towards the anode. At a certain pH, the amino acid molecule would not migrate to either electrode and exist as a neutral dipolar ion. This pH is called Isoelectric Point of that amino acid. At this pH, amino acids exist as zwitterions or dipolar or inner salts. In other words, it is the pH at which the amount of negative charge on an amino acid exactly balances the amount of positive charge.
Isoelectric pont = pH at which there is no net charge.
All amino acids do not have the same isoelectric point. The pH of the isoelectric point depends upon other functional groups in the amino acid structure. Neutral amino acids have isoelectric points from pH 5.5 to 6.3. Acidic amino acids have isoelectric points at a high pH around 10. The isoelectric points of some amio acids are given below in table.
| Isoelectric Points of Amino Acids | |
|---|---|
| Amino Acids | Isoelectric Points |
| Alanine | 6.1 |
| Valine | 6.0 |
| Serine | 5.7 |
| Threonine | 5.6 |
| Aspartic Acid | 2.8 |
| Glutamic Acid | 3.2 |
| Lysine | 9.7 |
| Arginine | 10.8 |
| Proline | 6.29 |
| Phenylalanine | 5.48 |
| Glycine | 5.97 |
| Histidine | 7.58 |
| Cystine | 5.0 |
| Tryptophan | 5.9 |
Amino acids have minimum aqueous solubility at their isoelectric points. This fact has been made use in the separation of α-amini acids from protein hydrolysates.
How to Calculate Isoelectric Point of Neutral Amino Acids
For an amino acid with two pKa values, the isoelectric point can be calculate simply by averaging the two pKa values.
Example: For glycine with pKa values of 2.34 and 9.60
Isoelectric Point = (2.34 + 9.60)/2 = 5.97
