Isoelectric Point (pI) Calculator
Calculate the isoelectric point (pI) of an amino acid from its pKa values, by solving for the pH where net charge is zero.
Includes 20 presets.
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, so 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 and Glu, which carry two carboxyl groups): pI = (pKa_α-COOH + pKa_side-chain COOH) ÷ 2
For basic side chains (Lys, Arg and His, which carry two 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.
That shortcut is what textbooks teach, and it works for most amino acids. It breaks down where the three pKa values interleave awkwardly, which is exactly what happens with tyrosine (its phenol sits above the amino group) and cysteine (its thiol sits below). Rather than guess which pair to average, this calculator solves directly for the pH at which the net charge on the molecule is zero. For simple cases the two methods agree to the second decimal; where they differ, the net-charge answer is the correct one.
pKa Values of 10 Common 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 | — | 6.01 |
| 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 | 2.97 |
| 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 (basic) | 7.59 |
| Cysteine | 1.96 | 10.28 | 8.18 (acidic) | 5.07 |
| Tyrosine | 2.20 | 9.11 | 10.07 (acidic) | 5.63 |
Two of those sit exactly on a rounding boundary. Alanine works out at 6.015 and aspartic acid at 2.975, so you will find 6.02 and 2.98 printed in plenty of textbooks. Same number, rounded the other way, and nothing turns on which you use.
Acidic, basic, neutral: what the label actually means
An amino acid is called acidic or basic because of its side chain, not because of where its pI happens to fall. Glycine and alanine have pI values just under 6 and are neutral amino acids: they have no ionizable side chain at all, and their pI is simply the midpoint of the carboxyl and amino groups. Aspartic acid is acidic because it carries a second COOH group, and that is what drags its pI down to 2.97.
The number that actually tells you how a molecule will behave in a gel or on a column is not the classification, it is the net charge at your working pH. The result panel gives you that directly.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.