Work out isoelectric point instantly with clear inputs, formula shown and shareable results.
Sums the Henderson-Hasselbalch charge contributions of ionisable side chains and termini, then finds the pH at which net charge is zero.
Net charge and pI
Charge(pH) = sum of basic groups / (1 + 10^(pH - pKa)) - sum of acidic groups / (1 + 10^(pKa - pH)); the pI is the pH where the sum is zero
Approximate side-chain values: aspartate and glutamate 4.1, histidine 6.0, lysine and arginine 11.5, with termini at 3.6 and 9.1.
Local structure shifts side-chain pKa values by a unit or more, so sequence-based estimates are approximate.