The isoelectric point (pI) is the pH at which a peptide has no net electrical charge. Enter any sequence to calculate pI, plot the pH-charge curve, and explore ionization at any pH.
At its pI, a peptide carries no net electric charge and will not migrate in an electric field — a property exploited in isoelectric focusing (IEF) for protein separation. Below the pI the peptide is positively charged; above it, negatively charged.
Isoelectric focusing (IEF) separates peptides within a pH gradient gel — each molecule migrates until it reaches its pI, where it stops. Combined with SDS-PAGE (2D-PAGE), it produces the high-resolution protein maps used in proteomics. The technique can resolve proteins differing by as little as 0.01 pH units in pI — enough to detect a single charge-altering amino acid substitution.
Near its pI, a peptide has minimal charge repulsion between molecules, promoting aggregation. The practical rule: keep buffer pH at least 1–2 units away from the pI to maintain solubility. For example, insulin (pI ~5.3) is formulated at pH 7.4 in most injectables. Conversely, pI-based precipitation is deliberately used in purification — adjusting buffer pH to the pI selectively crashes a target protein out of solution.
A reference range of well-characterised proteins and peptides: