What is the isoelectric point (pI)?

The pI is the pH at which an amino acid carries zero net charge. At pH below pI the molecule is positively charged; above pI it is negatively charged.

How is pI calculated?

For amino acids without an ionisable side chain, pI = (pKa1 + pKa2) / 2, where pKa1 is the α-carboxyl and pKa2 is the α-amino pKa. For amino acids with an ionisable side chain the two pKa values that flank the neutral zwitterion are averaged.

Why does histidine have a higher pI than most amino acids?

Histidine's imidazole side chain has a pKa around 6.0, which is close to physiological pH. This extra ionisable group raises the pI to ~7.6 compared to ~5–6 for non-ionisable amino acids.

Are the pKa values exact?

The values shown are standard reference values used in most biochemistry textbooks (Lehninger, Stryer). Actual pKa values shift slightly depending on temperature, ionic strength, and whether the residue is in a peptide chain.

Isoelectric Point of Amino Acids

Name: Isoelectric Point of Amino Acids
Availability: InStock
Author: Nham Vu

Look up or calculate the isoelectric point (pI) of all 20 standard amino acids from their pKa values; see charge state at any pH and the charge curve across the full pH spectrum.

Select Amino Acid

pKa Values & Isoelectric Point

Select an amino acid above.

Net Charge at a Given pH

pH value:

Enter a pH and click Calculate.

Charge vs. pH Curve

Net charge from pH 0 to 14. The curve crosses zero at the pI.

Select an amino acid to see the curve.

All 20 Standard Amino Acids — pI Reference

Amino Acid	Code	pKa1 (α-COOH)	pKa2 (α-NH₃)	pKaR (side chain)	pI

Summary

Look up or calculate the isoelectric point (pI) of all 20 standard amino acids from their pKa values; see charge state at any pH and the charge curve across the full pH spectrum.

How it works

Select an amino acid from the dropdown.
The tool displays pKa values for the α-amino group, α-carboxyl group, and side chain (where applicable).
The isoelectric point (pI) is computed as the average of the two pKa values that bracket the neutral form.
Enter any pH value to see the calculated net charge at that pH using the Henderson–Hasselbalch equation for each ionisable group.
The charge-vs-pH curve plots net charge from pH 0 to 14 so you can see the titration profile at a glance.

Use cases

Predicting the net charge of an amino acid at physiological pH (7.4).
Understanding isoelectric focusing (IEF) separation of amino acids.
Designing buffer conditions where an amino acid carries a specific charge.
Teaching biochemistry students how pKa values determine pI.
Cross-checking textbook pI values during exam preparation.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu