The isoelectric point (pI) tells you the pH at which an amino acid, peptide, or protein has no net electrical charge — a number biochemists rely on for everything from predicting how a protein folds to designing an electrophoresis or ion-exchange purification step. This calculator computes pI from pKa values, either for one of the 20 standard amino acids or for a custom peptide you define.
How pI is calculated from pKa values
Every ionizable group on an amino acid — the backbone α-carboxyl, the backbone α-amino, and any ionizable side chain — has its own pKa, the pH at which that group is half protonated and half deprotonated. As pH rises from very acidic to very basic, the molecule's net charge steps down by one unit at every pKa it crosses, moving from fully protonated (most positive) toward fully deprotonated (most negative). The isoelectric point sits in the flat region where net charge crosses zero, and it works out to the average of the two pKa values on either side of that crossing — for glycine, whose only ionizable groups are the backbone carboxyl (pKa 2.34) and amino (pKa 9.60) groups, that's (2.34 + 9.60) / 2 = 5.97.
Why side chains shift pI up or down
Seven of the twenty standard amino acids have a side chain that ionizes near physiological pH: Aspartate and Glutamate carry an extra acidic carboxyl group, Cysteine and Tyrosine carry a weakly acidic thiol or phenol, and Lysine, Arginine, and Histidine carry an extra basic amino, guanidinium, or imidazole group. An extra acidic group adds another low pKa to the mix, pulling the flanking pair — and therefore pI — down toward the acidic end (Aspartate's pI is 2.77, far below glycine's 5.97). An extra basic group does the opposite, pushing pI up toward the basic end (Lysine's pI is 9.74). This is exactly why amino acids are classified as acidic, basic, or neutral by their side chain.
Net charge away from the isoelectric point
Away from pI, a molecule's net charge is not just +1, 0, or −1 — it changes smoothly as each group's protonation state shifts according to the Henderson-Hasselbalch equilibrium. This calculator's Charge at pH tab sums each group's fractional protonation to report a continuous charge (for example +0.85 rather than a flat +1), which is closer to how proteins actually behave in solution and is the basis for techniques like isoelectric focusing, where a mixture of proteins separates by pH because each stops migrating in an electric field exactly at its own pI.