What Is the Isoelectric Point?
Understand how pH affects peptide net charge, how a sequence model estimates pI, and what can shift experimental results.
How pH changes net charge
The isoelectric point, abbreviated pI here, is the pH where net charge is zero. In the sequence model, a peptide is net positive below its pI and net negative above it. Acidic and basic groups can still carry charge at the pI, with their contributions balancing overall.
How a sequence gives an estimate
Assign a pKa to each ionizable group, estimate its fractional charge at a trial pH, and sum the charges. The calculator repeatedly narrows the pH interval until it locates the zero. The model includes the two free termini and the side chains of D, E, C, Y, H, K, and R.
With the same model, PETER gives a pI of 4.53 and INGAR gives 9.75. The prediction depends on the pKa set. Short peptides, chemical modifications, and solvent conditions warrant care when comparing with experimental measurements.