Understanding Peptide Isoelectric Point
The isoelectric point, often called pI, is the pH where a peptide has no net electrical charge. At this pH, positive and negative charge contributions balance. The peptide may still contain charged groups. The total sum is near zero.
Why pI Matters
Peptide pI helps explain solubility, migration, and purification behavior. A peptide often shows lower solubility near its pI. It may move slowly in an electric field because the net charge is minimal. Researchers use pI during electrophoresis planning, ion exchange method selection, buffer design, and basic stability review.
Charge Sources
Every peptide has an amino terminus and a carboxyl terminus. Some side chains also ionize. Aspartic acid, glutamic acid, cysteine, and tyrosine can lose protons. Histidine, lysine, arginine, and the amino terminus can gain protons. Each group responds to pH according to its pKa value. Different pKa tables give slightly different results.
Calculation Method
This tool estimates net charge over a pH scale. It applies Henderson Hasselbalch style fractions for each ionizable group. Positive groups contribute fractional positive charge. Acidic groups contribute fractional negative charge. A bisection search then finds the pH where net charge is closest to zero. The result is an estimate, not a measured lab value.
Good Input Practice
Enter a sequence with one letter amino acid codes. Spaces and line breaks are removed. Unknown symbols are ignored and reported. Choose a pKa set that matches your workflow. Use custom pKa values when your method requires a different table. Review the charge profile if you need behavior away from the pI.
Limits
Real peptides may behave differently in salts, solvents, strong denaturants, or complex buffers. Terminal modifications, phosphorylation, labeling, and unusual residues can shift pI. Treat the value as a planning guide. Confirm critical decisions with experimental data when precision matters.
Interpreting Output
The summary shows the cleaned sequence, residue count, estimated pI, and charge at the selected pH. A charge near zero means the pH is close to the predicted pI. Positive charge suggests the peptide is below its pI. Negative charge suggests it is above its pI. The profile table helps compare several buffer choices before exporting results. Save raw sequences with reports for later method review as well.