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Glossary

Isoelectric Point (pI)

At its isoelectric point a peptide carries no net charge and is least soluble. Using the pI to fix a cloudy vial after reconstitution and plan purification.

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 terminal amine and carboxyl groups plus ionisable side chains (Asp, Glu, Cys, His, Lys, Arg, Tyr). Many basic residues push the pI up; many acidic residues pull it down.

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, then transfer the dissolved peptide into the final assay 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 its effect depends on the counter-ion, since acetate and TFA salts do not behave alike. Our guides on solubility and on reconstitution give practical detail.

hydrophilic · lipophilic · molecular weight

All glossary terms

Further reading

Guides that reference this term

Molecule guides

Cagrilintide: Acylated Amylin Analogue Structure, Receptor Logic and Research Use

Why the amylin axis needs its own reference compound — cagrilintide's aggregation-resistant backbone, its non-selective receptor profile, and the handling it demands.

Molecule guides

IGF-1 LR3: Escaping the Binding Proteins, and What That Means at the Bench

Two modifications aimed at the binding-protein interface rather than the receptor — why apparent potency rises, how to dissolve a recombinant growth factor, and what its certificate should prove.

Methods

Peptide Solubility and pH: Net Charge, Isoelectric Point and Picking a Solvent

Solubility is predictable from composition. Count the charges, find the pI, pick the solvent — plus the co-solvent maths and the sequences that need special care.

Comparisons

Bacteriostatic or Sterile Water: Choosing a Diluent for Peptide Reconstitution

A single preservative is the only thing that sets the two apart — and it determines whether a dissolved stock may be sampled again and again or has to be finished in one sitting.