How this instrument works
A simple amino acid in solution exists as a zwitterion — a single molecule carrying both a positively charged group (the protonated amino group) and a negatively charged group (the deprotonated carboxyl group) at the same time. Which groups are actually protonated shifts with pH, and there's exactly one pH where the molecule's total charge, averaged across the population, comes out to zero: that pH is the isoelectric point, pI.
For an amino acid with no ionizable side chain, pI sits exactly halfway between the molecule's two relevant pKa values — the acidic dissociation constant for the carboxyl group and the basic one for the amino group: pI = (pKa + pKb) ⁄ 2. That midpoint makes sense once you picture the titration curve: below pI, the molecule carries excess positive charge (both groups tend toward protonated); above it, excess negative charge (both groups tend toward deprotonated); at the exact midpoint between the two transitions, the positive and negative populations balance.
The isoelectric point isn't just a bookkeeping number — it's what protein purification and separation techniques like isoelectric focusing and ion-exchange chromatography are built around. A protein or amino acid has essentially zero net charge, and therefore the least electrostatic repulsion holding it in solution, exactly at its pI, which is why proteins tend to precipitate out of solution most readily when the surrounding pH is adjusted to match their isoelectric point.
- Enter the acidic dissociation constant, pKa, for the molecule's acidic (carboxyl) group.
- Enter the basic dissociation constant, pKb, for the molecule's basic (amino) group.
- Read the isoelectric point, pI — the pH at which the molecule's average net charge is zero.
Worked example — glycine, the simplest amino acid
Glycine, the simplest of the twenty standard amino acids, has a carboxyl group with pKa = 2.34 and an amino group with pKb = 9.60 — both real, widely tabulated biochemistry reference values. Averaging them: pI = (2.34 + 9.60) ⁄ 2 = 5.97. That figure matches glycine's real, experimentally measured isoelectric point almost exactly, which sits right around pH 6.
That means at pH 5.97, a population of glycine molecules in solution carries, on average, no net charge — roughly equal numbers exist as the fully protonated cation and the fully deprotonated anion, canceling out overall, alongside the neutral zwitterion form. Shift the surrounding pH just slightly below 5.97 and the population tips measurably positive; shift it above and the population tips measurably negative — which is exactly the sensitivity isoelectric focusing techniques rely on to separate different proteins by their distinct pI values.
Questions
What is the isoelectric point (pI) of an amino acid?
It's the specific pH at which an amino acid's average net electrical charge is zero — the positively charged (protonated) and negatively charged (deprotonated) forms of the molecule are present in populations that exactly balance each other out. For a simple amino acid with no ionizable side chain, it's calculated as the average of the molecule's two relevant pKa values, one for the carboxyl group and one for the amino group.
Why is pI the average of two pKa values rather than just one?
Because a simple amino acid has two separate ionizable groups — an acidic carboxyl group and a basic amino group — each with its own dissociation constant, and the molecule's net charge depends on the ionization state of both simultaneously. The pH exactly midway between the two pKa values is where the molecule transitions from net-positive to net-negative, since it's equidistant from the pH at which each individual group is 50% ionized.
Does this formula work for amino acids with an ionizable side chain, like lysine or aspartic acid?
Not directly. Amino acids with a third ionizable group on their side chain — like lysine's extra amino group or aspartic acid's extra carboxyl group — have three pKa values instead of two, and their pI is calculated as the average of the two pKa values that flank the neutral zwitterion form, not simply the average of all three. This calculator, which takes exactly one pKa and one pKb, is built for the thirteen or so amino acids whose side chains don't ionize.
Why does protein purification care about the isoelectric point?
A protein carries essentially no net charge at its isoelectric point, which means it experiences the least electrostatic repulsion from other copies of itself in solution and is generally at its least soluble there — proteins commonly precipitate out of solution when the pH is adjusted to match their pI. This effect underlies isoelectric precipitation as a purification technique, and it's also the operating principle behind isoelectric focusing, an electrophoresis technique that separates a mixture of proteins by driving each one to the specific pH matching its own distinct pI.
Is a lower pI more acidic or more basic than a higher pI?
A lower pI means the amino acid or protein reaches its zero-net-charge point at a more acidic pH — generally because it's more acidic overall, dominated by acidic (carboxyl-bearing) groups. A higher pI means it takes a more basic pH to reach that same zero-net-charge balance, generally reflecting a molecule dominated by basic (amino-bearing) groups. Comparing pI values across different proteins is a standard way to predict how they'll behave differently during pH-dependent separation techniques.