Average the two pKa values that bracket the zwitterion to get pI.
| pI | – |
Use this calculator for a simple amino acid such as glycine, where pI sits halfway between the carboxyl and ammonium pKa.
The calculation is pI = (pKa1 + pKa2) / 2. For acidic or basic side chains you must pick the two pKa values that surround the zero-net-charge form; this tool does not choose them for you.
This is the two-pKa average only. It is not a full titration-curve fit and it does not use a third side-chain pKa unless you put that pair in the boxes.
pKa1: Lower pKa of the pair around the zwitterion, about 2.3 for a typical α-COOH.
pKa2: Higher pKa of that pair, about 9.6 for a typical α-NH₃⁺.
pI: Isoelectric pH, the average of the two values.
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