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Isoelectric point (pI) and peptide charge calculator
Type an amino acid or a short peptide, like Gly-Lys-Asp or GKD. Get the isoelectric point or the net charge at any pH, worked out group by group.
Three-letter (Gly-Lys-Asp) or one-letter (GKD), up to 30 residues
Leave empty to use the textbook table. If you give any, give them for every group.
How to find the isoelectric point
The pI is the pH where the molecule's net charge is zero. To find it by hand:
- List every group that can gain or lose an H+: the free amino end, the free carboxyl end, and side chains of Asp, Glu, His, Cys, Tyr, Lys and Arg.
- Order them from lowest pKa to highest. At very low pH every group is protonated, so the charge equals the number of basic groups.
- Move up the list. Each group loses an H+ and drops the charge by one. Find the two groups on either side of the neutral form.
- Average those two pKa values. That is the pI.
The calculator also checks the shortcut by finding where the summed charge actually crosses zero.
Mistakes that cost points
Averaging the wrong pairList the groups from lowest pKa to highest and find where the charge goes from plus one to zero to minus one. Only those two pKa values belong in the average.
Counting His as neutralIts side chain can be partly positive, especially when the pH is near or below its pKa.
Forgetting the endsThe free amino end is a base and can carry +1. The free carboxyl end is an acid and can carry -1. Both count.
Practice problems
Answers use the textbook table this tool defaults to. Your course may use slightly different pKa values.
What is the pI of glycine?
pI = 6.07, the average of the carboxyl end (2.35) and the amino end (9.78).
What is the pI of aspartate?
pI = 2.95, the average of the carboxyl end (1.99) and the side chain (3.90). The amino end is not in the pair.
What is the pI of lysine?
pI = 9.80, the average of the amino end (9.06) and the side chain (10.54).
What is the pI of histidine?
pI = 7.69, the average of the side chain (6.04) and the amino end (9.33).
What is the net charge of Gly-Lys-Asp at pH 7.0?
About 0.00: the amino end and Lys are near +1, the carboxyl end and Asp are near -1.
What is the pI of Ala-His-Glu-Lys?
pI = 7.96, the average of His (6.04) and the amino end next to Ala (9.87).
Practice it until it sticks
Novis turns problems like these into a spaced-repetition deck, checks your own written work one step at a time, and tells you which step went wrong. Novis opens October 20.