Article — Isoelectric Point Calculator (pI from pKa values)
Isoelectric Point Calculator: pI of Amino Acids and Proteins
The isoelectric point (pI) is the pH at which a molecule carries no net electric charge. For amino acids without ionizable side chains, pI = (pKa₁ + pKa₂)/2 — about 6.0 for glycine and alanine. For proteins, pI is found by iteration: the pH where the sum of positive and negative charges from all ionizable groups equals zero.
This calculator has two modes. The amino acid mode covers all 20 standard residues with their classical pKa values. The protein mode takes counts of Asp, Glu, His, Cys, Tyr, Lys, Arg, and the N-terminus, then runs a bisection search across pH 0 to 14 using the EMBOSS pKa table.
What is isoelectric point?
Isoelectric point is the pH where a polyprotic molecule (amino acid, peptide, protein, or any other multi-pKa species) has zero net charge. Below the pI the molecule carries a net positive charge; above the pI it carries a net negative charge. The transition is gradual — the charge passes through zero rather than flipping abruptly.
pI emerges from the simultaneous equilibria of every ionizable group. Each group has its own pKa, and at any pH the fraction protonated is set by the Henderson-Hasselbalch equation. Summing the contributions gives the net charge q(pH). The pI is just the root of q(pH) = 0.
The isoelectric point formula
Plain AA pI = (pKa₁ + pKa₂)/2Acidic side pI = (pKa_COOH + pKa_side)/2Basic side pI = (pKa_NH₃⁺ + pKa_side)/2Protein solve q(pH) = 0The averaging rule for single amino acids is exact at the pI condition (zero net charge). For glycine: pI = (2.34 + 9.60)/2 = 5.97. For aspartate the two relevant pKa values are α-COOH (2.09) and side chain COOH (3.86), giving pI = (2.09 + 3.86)/2 = 2.98. For lysine they are α-NH₃⁺ (8.95) and ε-NH₃⁺ (10.53), giving pI = (8.95 + 10.53)/2 = 9.74.
Isoelectric point of the 20 amino acids
Standard pI values for free amino acids at 25 °C:
- Aspartate (D) = 2.77 (most acidic)
- Glutamate (E) = 3.22
- Asparagine (N), Glutamine (Q) = 5.4 to 5.6
- Phenylalanine (F) = 5.48
- Tyrosine (Y) = 5.66
- Cysteine (C) = 5.07
- Glycine (G), Alanine (A), Valine (V) = 5.97 to 6.02 (near neutral)
- Methionine (M), Tryptophan (W) = 5.74 to 5.89
- Histidine (H) = 7.59
- Lysine (K) = 9.74
- Arginine (R) = 10.76 (most basic)
Isoelectric point of common proteins
Casein from milk has pI 4.6 — which is why milk curdles at pH 4.6 to 4.7 (lactic acid bacteria during yogurt-making, or vinegar in paneer). Hemoglobin sits near 6.8, insulin near 5.3, ribonuclease A near 9.5. Most extracellular proteins cluster around pH 5 to 6; nuclear proteins (histones) and many ribosomal proteins are highly basic, pI 10 to 12.
The average pI of all human proteins, weighted by abundance, is around 6.5 — slightly acidic. The cytosol holds pH ≈ 7.2, so most proteins carry a small net negative charge there, which contributes to keeping them soluble and from sticking to each other.
Practical uses of isoelectric point
pI is one of the most useful single numbers in protein chemistry. It drives every separation technique that exploits charge:
- Isoelectric focusing (IEF) — proteins migrate in a pH gradient and stop at their pI
- Ion exchange chromatography — choose anion exchanger above pI, cation exchanger below
- Isoelectric precipitation — adjust buffer to pI to crash proteins out of solution
- 2D gel electrophoresis — IEF in one direction, SDS-PAGE in the other; spots a unique protein
- Drug formulation — protein solubility is minimum at pI; buffers are chosen well away
- Diagnostic electrophoresis — hemoglobin variants (HbS, HbC) have different pI and separate cleanly
Computed pI ignores local pKa shifts. A glutamate buried near a positive lysine in the folded structure has its pKa raised by 1 to 2 units. Disulfide bonds remove Cys residues from the ionizable pool. The true pI of intact protein can differ from the sequence-based calculation by 0.5 pH or more.
Why different pI models give different answers
Three pKa tables dominate published calculations:
EMBOSS — round values widely used in introductory texts and online tools. Asp 3.65, Glu 4.25, His 6.0, Cys 8.33, Tyr 10.07, Lys 10.53, Arg 12.48. Used by the protein mode in this calculator.
Bjellqvist — derived from 2D gel mobility data. Adjusts pKa for residues near termini (Asp near N-terminus drops to 3.57). Used by the Expasy ProtParam server.
Lehninger / Stryer — textbook values that vary modestly between editions. Often quoted in introductory biochemistry problems.
For most proteins the three agree within 0.5 pH units. Disagreements widen for histidine-rich or extreme-pI proteins where small pKa shifts matter.
For purification, compute pI from sequence then choose buffer pH at least 1 unit away. Above pI, the protein is negatively charged and binds anion exchangers (Q, DEAE). Below pI, it binds cation exchangers (SP, CM). At pI itself, solubility drops and the protein may precipitate.
Common isoelectric point mistakes
The frequent errors:
- Wrong pKa pair — for acidic amino acids use COOH and side, not COOH and amino
- Missing termini — free N and C-termini contribute one charge each per chain
- Counting disulfide cysteines — Cys in a disulfide bond is not ionizable; subtract from count
- Ignoring blocked termini — N-acetylation removes the N-terminal amine
- Confusing isoelectric point with isoionic point — these differ at non-zero ionic strength
- Treating peptides like full proteins — short peptides need exact pKa, not averaged textbook values
Two-dimensional gel electrophoresis was the workhorse of early proteomics (1975 to 2005). The first dimension is isoelectric focusing in an immobilized pH gradient — proteins migrate to their pI and stop. The second dimension is SDS-PAGE by size. The combination resolves up to 10,000 individual proteins per gel, each appearing as a single spot at coordinates (pI, MW) unique to that polypeptide. Mass spectrometry has largely replaced 2D gels, but the IEF dimension is still essential when separating intact proteoforms.