isoelectric point
/ eye-soh-ee-LEK-trik POINT /
Imagine a person carrying an equal number of plus and minus tokens, so that overall they owe nothing and are owed nothing — perfectly balanced. Many molecules can be coaxed into exactly this state by tuning the acidity, and the pH at which it happens is their isoelectric point.
Specifically, the isoelectric point, often written pI, is the pH at which a molecule that can carry both positive and negative charges has zero net charge — its positive and negative groups exactly cancel. For an amino acid or protein this happens at a particular pH set by the pKa values of its acidic and basic groups; below it the molecule is net positive, above it net negative. It is computed roughly as the average of the two pKa values that flank the neutral form.
The isoelectric point matters because a molecule at its pI does not migrate in an electric field and is often least soluble, facts that underpin techniques from protein separation to crystallisation. The honest caveat is that zero net charge does not mean no charge at all — the molecule usually still carries equal and opposite charges as a zwitterion, so it remains chemically reactive even while electrically silent.
The amino acid glycine has a pI near 6.0; in a solution at that pH it sits as a zwitterion with one positive and one negative charge, drifts toward neither electrode, and is at its least soluble.
At the isoelectric point, net charge is zero and migration stops.
Net zero charge is not the same as no charge. A molecule at its pI is usually a zwitterion bearing balanced positive and negative groups, which is why it can be chemically active yet electrically neutral overall.