Isoelectric Point Calculator

The isoelectric point, usually abbreviated pI, is the pH at which a molecule has zero average net electrical charge. For peptides and proteins, this value depends mainly on the ionizable amino and carboxyl termini and on ionizable side chains. A molecule at its pI can still contain positive and negative groups, but their average charges balance. The calculator estimates that balance directly from a one-letter amino acid sequence. Charge changes continuously with pH. At low pH, protonation favors positively charged amino groups and suppresses negative carboxylate groups. At high pH, deprotonation reduces positive charge and increases negative charge. The Henderson-Hasselbalch relationship converts each group's pKa and the selected pH into a fractional charge. Summing those fractional contributions gives the estimated net charge rather than forcing every residue into a fully charged or uncharged state. This implementation uses a standard educational pKa set: 9.69 for the amino terminus, 2.34 for the carboxyl terminus, 3.86 for aspartate, 4.25 for glutamate, 6.00 for histidine, 8.33 for cysteine, 10.07 for tyrosine, 10.50 for lysine, and 12.40 for arginine. A binary search from pH 0 to 14 finds the point where the calculated net charge crosses zero. The displayed result is rounded to two decimal places. Experimental pI values may differ because a residue's local environment can shift its pKa. Neighboring charges, solvent exposure, hydrogen bonding, protein folding, disulfide formation, terminal modifications, phosphorylation, and other post-translational modifications all matter. Different published pKa tables also produce slightly different predictions. Therefore, sequence-based pI is a useful estimate for planning and comparison, not an exact substitute for isoelectric focusing or another experimental measurement. Isoelectric point influences protein solubility, electrophoretic migration, purification, crystallization, and formulation. Proteins often have reduced solubility near their pI because electrostatic repulsion is minimized. Ion-exchange chromatography commonly chooses a buffer pH relative to pI: below the pI a protein tends to be net positive, while above the pI it tends to be net negative. The calculated charge at a chosen pH can help select an initial exchanger and buffer condition. Enter only standard amino acid letters; spaces and letter case are normalized. Ambiguous codes and chemical modifications are not modeled. For best interpretation, compare predictions made with the same pKa convention and verify sensitive laboratory decisions experimentally.