Isoelectric Point (pI) Calculator
Calculate the isoelectric point (pI) of amino acids from pKa values.
Includes a preset dropdown for all 20 standard amino acids with pre-filled pKa values.
What Is the Isoelectric Point?
The isoelectric point (pI) is the pH at which an amino acid (or protein) carries zero net charge. At the pI, the molecule has an equal number of positive and negative charges — it is electrically neutral overall.
Why It Matters
- At pH < pI: the molecule is positively charged (cation)
- At pH > pI: the molecule is negatively charged (anion)
- At pH = pI: solubility is at its minimum — proteins tend to precipitate
The pI is essential for:
- Gel electrophoresis (separation by charge)
- Protein purification (isoelectric precipitation)
- Drug design and formulation
- Understanding enzyme activity at different pH values
Formula
For amino acids with no ionizable side chain (e.g. Gly, Ala, Val): pI = (pKa₁ + pKa₂) ÷ 2
For acidic side chains (Asp, Glu — two carboxyl groups): pI = (pKa_α-COOH + pKa_side-chain COOH) ÷ 2
For basic side chains (Lys, Arg, His — two amino/basic groups): pI = (pKa_α-NH₃ + pKa_side-chain basic) ÷ 2
The general rule: average the pKa values of the two groups that flank the neutral (zwitterion) form.
pKa Values of the 20 Standard Amino Acids
| Amino Acid | pKa₁ (α-COOH) | pKa₂ (α-NH₃) | pKa₃ (side chain) | pI |
|---|---|---|---|---|
| Glycine | 2.34 | 9.60 | — | 5.97 |
| Alanine | 2.34 | 9.69 | — | 6.02 |
| Valine | 2.32 | 9.62 | — | 5.97 |
| Leucine | 2.36 | 9.68 | — | 6.02 |
| Isoleucine | 2.36 | 9.68 | — | 6.02 |
| Aspartic Acid | 2.09 | 9.82 | 3.86 | 2.98 |
| Glutamic Acid | 2.19 | 9.67 | 4.25 | 3.22 |
| Lysine | 2.18 | 8.95 | 10.53 | 9.74 |
| Arginine | 2.17 | 9.04 | 12.48 | 10.76 |
| Histidine | 1.82 | 9.17 | 6.00 | 7.59 |