Isoelectric Point Calculator

Every amino acid carries both an acid and a base on the same small skeleton, and at one particular pH — its isoelectric point, pI — the positive and negative charges exactly cancel and the molecule sits electrically neutral on average, refusing to migrate in an electric field. This calculator gives the pI for the common amino acids from their standard pKa values, applying the correct averaging rule for each class: the two pKa values that bracket the neutral (zwitterionic) form are the two that count. Select the amino acid and the arithmetic, the pKa trio and the reasoning arrive together.

The Rule: Average the Two pKa Values That Bracket Neutral

An amino acid in water is a small parade of charge states. Fully protonated at low pH it carries +1 (or +2 for the basic residues); as pH rises it sheds protons one pKa at a time, passing through the zwitterion — the form with a deprotonated carboxyl (−) and a still-protonated amino (+) that sum to zero. The isoelectric point is the pH at the middle of that neutral form’s reign, and a standard result of the equilibrium algebra puts it exactly halfway between the two pKa values flanking the neutral species: pI = (pKa_a + pKa_b)/2 for the bracketing pair. For the amino acids with non-ionising side chains the bracket is the backbone pair — the α-carboxyl (pKa ≈ 2) and the α-amino (pKa ≈ 9–10) — which is why their pI values cluster in the 5.5–6.5 neighbourhood: near-neutral molecules, neutral near neutrality. The interesting arithmetic belongs to the five ionisable side chains, and it is a selection puzzle, not a harder formula.

The Three Classes, Worked

Neutral — glycine. pKa values 2.34 (COOH) and 9.60 (NH₃⁺); pI = (2.34 + 9.60)/2 = 5.97, the textbook figure. Acidic — aspartate and glutamate. Their side-chain carboxyls (pKa 3.86 and 4.25) ionise long before the amino group, so the neutral zwitterion is bracketed by the two carboxyl pKa values: glutamate’s pI = (2.19 + 4.25)/2 = 3.22, aspartate’s ≈ 2.97 — these amino acids reach neutrality only in distinctly acidic solution, as their chemistry advertises. Basic — lysine, arginine, histidine. Here two nitrogens compete and the neutral form is bracketed by the two amino-type pKa values: lysine’s pI = (8.95 + 10.53)/2 = 9.74, arginine’s (with its guanidinium at 12.48) = 10.76, histidine’s = (6.00 + 9.17)/2 ≈ 7.59 — the only residue whose side chain (pKa 6.00) ionises near physiological pH, which is why histidine does the buffering and much of the catalytic heavy lifting in proteins. The calculator’s notes name, for each residue, exactly which pair was averaged and which pKa was left out — because choosing the pair is the entire skill.

From Residues to Proteins: pI in the Laboratory

A protein’s pI is the collective property of its hundreds of ionisable groups — not a simple average of residue pI values (the groups titrate in one another’s electrostatic company, and buried residues shift their pKa values substantially) — but the concept and its consequences transfer intact. At its pI a protein’s net charge vanishes, and with the charge goes both its electrophoretic mobility and much of its solubility: proteins are least soluble at their pI, the principle behind isoelectric precipitation (casein curdling from milk at pH 4.6 is the kitchen’s demonstration) and behind isoelectric focusing, the separation technique that parks each protein at the point in a pH gradient equal to its own pI with resolving power fine enough to separate species differing by a single charge. The landmarks help calibrate intuition: casein focuses at pI ≈ 4.6, insulin at ≈ 5.3, while lysozyme — the egg-white antibacterial — sits at the basic extreme near pI ≈ 11, a reminder that protein pI values span the whole pH scale the residues make available. The practical corollaries run through formulation science: keep a protein drug’s buffer at least a pH unit from its pI if you want it soluble and stable; expect aggregation near the pI in storage; and read any protein’s behaviour in ion-exchange chromatography straight off the sign of (pH − pI). The amino-acid arithmetic on this page is the alphabet of that whole literature.

Honest Limits of the Table

The pKa values used here are the standard textbook set, measured for the free amino acids in water at 25 °C. Three qualifications keep them honest. Tables differ slightly between sources (by a few hundredths, occasionally a tenth, reflecting measurement conditions), so a pI quoted to two decimals is a property of a table as much as of a molecule — the calculator shows its inputs precisely so any table can be audited against any other. Residues inside peptides and proteins shift: a neighbouring charge, a hydrophobic pocket or a hydrogen bond can move a side-chain pKa by whole units (enzyme active sites exploit exactly this), which is why protein pI prediction from sequence is an approximation with known error bars. And two residues are quietly complicated — cysteine and tyrosine, whose side chains (thiol ≈ 8.3, phenol ≈ 10.1) ionise near the amino pKa and scramble the simple bracket choice; they are omitted from this calculator’s select list deliberately, with this note as their placeholder, because their pI belongs to the fuller titration treatment rather than the two-value rule. Within its stated scope — the free amino acids, the standard table, the bracketing rule — the arithmetic here is the arithmetic the textbooks print.

Charge at a Glance: Glycine at pH 7

The pI’s averaging rule has a quick corollary worth one worked breath: estimating net charge at any pH by asking, group by group, which side of its pKa the pH sits. Glycine at pH 7: the carboxyl (pKa 2.34) is essentially fully deprotonated — a full −1. The amino group (pKa 9.60) is still protonated in the fraction 1/(1 + 10^(7−9.60)) ≈ 99.75% of molecules — a +0.9975 average. Net charge ≈ −0.0025: not exactly the zwitterion’s zero, because pH 7 sits a unit above glycine’s pI of 5.97, but close enough to show the pI’s meaning in decimal form — the pH where that tiny remainder crosses zero. The same group-by-group tally, summed over a protein’s whole sequence with a proper solver, is how charge-versus-pH curves are actually drawn; the two-group case on this page is the honest miniature of the method.

Related Tools

More from the Chemistry Calculators hub: the Enzyme Activity Calculator for the proteins these residues assemble into, the live Concentration Calculator for preparing the buffers pI work swims in, and the Molar Ratio Calculator for mole arithmetic throughout the hub.

Frequently Asked Questions

Why is the pI an average of pKa values?

Because the neutral form’s population peaks midway, on the logarithmic pH scale, between the two ionisation steps that create and destroy it. Solving the equilibrium expressions for the pH at which the +1 and −1 species are equal gives exactly the midpoint of the bracketing pKa pair — an average in pH is a geometric statement about the underlying constants.

Which two pKa values do I average for an acidic amino acid?

The two lowest — the α-carboxyl and the side-chain carboxyl. The neutral form of aspartate or glutamate exists between losing the first and second carboxyl protons; the amino group (pKa ≈ 9.7–9.8) ionises after neutrality has already been passed, so it plays no part. The calculator’s note for each acidic residue states the pair it used.

What is the charge at physiological pH 7.4?

Compare 7.4 with the pI: below the pI the molecule is net positive, above it net negative. So glycine (pI 5.97) is slightly negative on average at 7.4 (predominantly its zwitterion with a small anionic share — “on average” is doing real work in that sentence), aspartate is firmly negative, and lysine (pI 9.74) is positive, carrying essentially its full +1.

Does the pI change with temperature or salt?

Mildly. pKa values drift with temperature (amino-group pKa values fall about 0.02–0.03 per °C) and with ionic strength, so a pI measured at 37 °C in 0.15 M salt can sit a tenth or two from the table value. For amino-acid identification and teaching the table rules; for formulation work the pI is measured, by focusing or titration, in the actual buffer.

Why is histidine special?

Its imidazole side chain has pKa ≈ 6.0 — the only residue ionising within reach of physiological pH. Histidines therefore switch charge state as cellular pH wobbles, making them nature’s chosen buffer and a recurring catalytic residue (the catalytic triads of proteases turn on exactly this switch). Its pI of about 7.6, computed from the basic-class rule, is the statistical shadow of that versatility.

Can I compute a peptide’s pI the same way?

Not with the two-value rule — a peptide has many ionisable groups (termini plus every Asp, Glu, His, Cys, Tyr, Lys, Arg), and its pI is the pH where the summed charge over all of them crosses zero, found by solving the full titration numerically. The residue pKa table on this page is the input such solvers use, and the bracketing intuition — find the two ionisations flanking neutrality — survives as the way to check their answers by hand.