The isoelectric point (pI) is the pH at which a peptide’s positive and negative charges balance and net charge is zero. It is set by the composition of ionisable groups: the N-terminal amine, the C-terminal carboxyl, and the side chains of aspartate, glutamate, cysteine, histidine, lysine, arginine and tyrosine. Sequences dominated by basic residues have a high pI; acidic sequences have a low one.
Why the isoelectric point matters in peptide research
At its pI a peptide has no electrostatic repulsion between molecules, so it is at its solubility minimum. That single fact explains a large share of reconstitution problems: a peptide that will not clear in near-neutral diluent often has a pI close to 7, and moving the solution one or two pH units away restores solubility. The corollary is a practical rule — acidify a basic peptide, alkalise an acidic one, and only then dilute into the working buffer.
pI also governs analytical behaviour. It determines migration in isoelectric focusing and ion-exchange retention, and it interacts with the counter-ion present, since TFA and acetate forms behave differently. See the solubility guide and reconstitution guide.