Isoelectric Point Calculator

Typical: ~2.0–2.5
Typical: ~8.5–10.5
e.g., Asp=3.65, Glu=4.25, Lys=10.5, Arg=12.5, His=6.0, Cys=8.3, Tyr=10.1
Chemistry

Estimate the isoelectric point (pI) of a peptide or protein from the pKa values of its ionizable groups.

Formula

pI = (pKa1 + pKa2) / 2 for adjacent ionizable groups
  • For a simple amino acid (no ionizable side chain): pI = (pKa_COOH + pKa_NH3) / 2.
  • For acidic amino acids: pI = (pKa_COOH + pKa_side) / 2.
  • For basic amino acids: pI = (pKa_NH3 + pKa_side) / 2.
  • At pI, the molecule has zero net charge and minimum solubility.

Glycine (pKa 2.35 and 9.78)

Inputs
  • pKa α-Carboxyl (C-terminus): 2.35
  • pKa α-Amino (N-terminus): 9.78
  • pKa Side Chain (0 if none): 0
  • Side Chain Type: none

pI = (2.35 + 9.78) / 2 = 6.07.

Frequently asked questions

What is pI?
The pH at which a molecule carries no net electrical charge. At this pH, it has minimum solubility and does not migrate in an electric field.
How is pI used in lab?
In isoelectric focusing (IEF) and protein purification to separate proteins by their pI.