Isoelectric Point Calculator (pI) — Amino Acids

Formula

How this is calculated

Every ionisable group on an amino acid either donates or accepts a proton depending on pH. The Henderson-Hasselbalch equation gives the fraction of each group that is protonated at a given pH. For an acidic group such as the alpha-carboxyl (COOH), the protonated form is neutral and the deprotonated form carries −1 charge, so its contribution to net charge is −f_deprotonated = −1/(1+10^(pKa−pH)). For a basic group such as the alpha-amino (NH₃⁺), the protonated form carries +1 and the deprotonated form is neutral, so its contribution is +f_protonated = +1/(1+10^(pH−pKa)). The same logic applies to ionisable side chains.

The isoelectric point pI is the pH at which the sum of all these contributions is exactly zero — the amino acid carries no net electrical charge. This calculator finds pI by binary search over pH 0–14. For a simple amino acid with no side chain the closed-form result is pI = (pKa₁ + pKa₂)/2; with a third ionisable group the binary search is needed for the exact value.

The net-charge vs pH curve shows the titration behaviour: strongly positive charge at low pH (all groups protonated), crossing zero at pI, and increasingly negative above pI. At pH 7 (physiological), most amino acids carry a slight net charge — enter physiological pH to read it from the StatGrid. pKa values vary with temperature, ionic strength, and local environment in folded proteins; the defaults used here are for free amino acids in dilute aqueous solution at 25°C.