Solubility is minimum at the isoelectric point (pI).
Proteins have minimum solubility at their isoelectric point (pI), where net charge is zero and electrostatic repulsion between molecules is minimized, promoting aggregation. As pH moves away from pI, the protein gains net charge, increasing hydration and electrostatic repulsion, which increases solubility. This principle underlies isoelectric precipitation — a purification technique where pH is adjusted to the target protein's pI. Other factors affecting protein solubility: ionic strength (salting-in at low I, salting-out at high I — the Hofmeister series), temperature, organic solvents, and specific ions. Ammonium sulfate precipitation is the classic salting-out method, exploiting differences in protein solubility at high ionic strength. Understanding solubility is crucial for protein purification, formulation of biologics, and preventing aggregation.