Glossary
Isoelectric Point (pI)
The pI is the pH at which a peptide has zero net charge, which is also where it dissolves least. How to use it when a reconstituted vial will not turn clear.
A peptide’s isoelectric point (pI) is the pH at which its positive and negative charges cancel out, giving a net charge of zero. It depends on the mix of ionisable groups present: the amine at the N-terminus, the carboxyl at the C-terminus, and the side chains of aspartate, glutamate, cysteine, histidine, lysine, arginine and tyrosine. Sequences rich in basic residues have a high pI; acidic sequences have a low one.
The pI and reconstitution problems
At the pI there is no electrostatic repulsion keeping the molecules apart, so solubility is at its lowest. This alone accounts for many reconstitution difficulties: a peptide that stays cloudy in a roughly neutral solvent frequently has a pI near 7, and shifting the pH by one or two units restores solubility. That leads to a simple working rule — acidify basic peptides, make acidic ones slightly alkaline, and only afterwards dilute into the working buffer.
The pI also controls analytical behaviour. It sets how a peptide migrates in isoelectric focusing and how it is retained on ion-exchange media, and it interacts with the counter-ion, because TFA and acetate salts behave differently. See the solubility guide and reconstitution guide.
Related terms
hydrophilic · lipophilic · molecular weight