What is the isoelectric point?
The isoelectric point (pI) is the pH at which a protein carries no net electrical charge. Below its pI a protein is positively charged; above it, negatively charged. Proteins are often least soluble near their pI.
How it is estimated
The net charge at each pH is computed from the number of ionisable groups (N-terminus, C-terminus, Asp, Glu, Cys, Tyr, His, Lys, Arg) and their pKa values using the Henderson–Hasselbalch equation. The pI is the pH where the net charge crosses zero, found by bisection.
Different tools use different pKa sets, so theoretical pI values typically differ by a few tenths of a unit. Treat the value as an estimate for planning, not a measurement.
Using the pI in the lab
For ion-exchange chromatography, choose a buffer pH about one unit away from the pI: below the pI the protein binds a cation exchanger, above it an anion exchanger.