How this calculator works
Identify the two consecutive acid dissociation steps that enclose the species with zero net charge. Their pKa average estimates the isoelectric pH for a simple amino acid. For an acidic or basic side chain, these are not necessarily the terminal carboxyl and amino pKa values. Both entries are pKa values; a pKb must first be converted using the appropriate pKw. A whole protein needs a charge model covering all ionizable groups.
Formula
pI = (pKa,lower + pKa,upper)/2.
Worked example
Using 2.34 and 9.60 for glycine gives pI 5.97. A lower pKa of 2 and a pI of 6 imply an upper pKa of 10.
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Frequently asked questions
Can I enter pKb as the second value?
No. Enter the conjugate acid pKa, calculated as pKw - pKb at the same temperature.
Does this predict a protein sequence pI?
No. Proteins contain many titratable residues and require a net-charge calculation.
Why does pI depend on conditions?
Temperature, ionic strength and the molecular environment can shift pKa values.