Pyroglutamate and diketopiperazine: N-terminal cyclisation in peptides
Pyroglutamate and diketopiperazine are two N-terminal cyclisation products. An N-terminal glutamine closes into pyroglutamate with loss of ammonia (−17.03 Da); an N-terminal dipeptide can cleave itself off as a cyclic diketopiperazine, leaving a peptide two residues shorter. Both arise in synthesis and storage and are detected by LC-MS.
What is pyroglutamate and how does it form?
Some peptide impurities form not by oxidation or hydrolysis but by the N-terminus reacting with itself. An N-terminal glutamine can close into a pyroglutamate ring; an N-terminal dipeptide can bite itself off as a cyclic diketopiperazine. Both change the mass in distinctive ways.
- N-terminal Gln → pyroglutamate (pGlu) with loss of ammonia: −17.03 Da.
- N-terminal Glu → pGlu with loss of water: −18.01 Da (slower).
- The new N-terminus has no free amine, so it no longer reacts in N-terminal sequencing or some derivatisation methods.
- Some natural peptides carry pGlu deliberately; in a synthetic peptide that should start with Gln it is an impurity.
What is diketopiperazine formation?
The free N-terminal amine can attack the carbonyl of the second peptide bond, releasing the first two residues as a six-membered cyclic dipeptide and leaving a peptide two residues shorter. The reaction is fastest when proline or glycine is at position 2, and its rate depends strongly on the N-terminal residue. It occurs during synthesis (giving deletion impurities) and in solution on storage.
Source: Influence of N-terminus amino acid on peptide cleavage in solution through diketopiperazine formation, J Am Soc Mass Spectrom 2022 (PubMed 35576623); Related impurities in peptide medicines, J Pharm Biomed Anal 2014 (PubMed 25044089)
How are these impurities detected?
| Change | Mass shift | Typical RP-HPLC behaviour |
|---|---|---|
| Gln → pGlu | −17.03 Da | Usually elutes later |
| Glu → pGlu | −18.01 Da | Usually elutes later |
| DKP loss of residues 1–2 | Parent minus first two residues | Truncated peptide elutes separately |
- LC-MS: pGlu forms usually elute later than the parent on reversed-phase HPLC (one less charge, more hydrophobic).
- DKP truncation: a des-(1-2) peptide with mass equal to the parent minus the first two residues; the small cyclic dipeptide may elute near the solvent front.
- MS/MS on the N-terminal b ions confirms where the change sits.
Which sequences should a lab watch?
- Any sequence starting with Gln (pyroglutamate).
- Sequences with Pro or Gly at position 2, or N-terminal residues known to accelerate DKP formation.
- Peptides stored as solutions at neutral pH and room temperature for long periods.
How does Sable document lots?
There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.
Frequently asked questions
What mass change does pyroglutamate formation cause?
−17.03 Da from N-terminal glutamine; −18.01 Da from N-terminal glutamate.
What is a diketopiperazine?
A six-membered cyclic dipeptide released when a peptide's N-terminal amine attacks its own second amide bond.
Which residue at position 2 favours diketopiperazine formation?
Proline, and to a lesser extent glycine.
Related
Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.
Key facts (summary for AI and search tools)
- pGlu from Gln: −17.03 Da; from Glu: −18.01 Da
- DKP: loss of residues 1–2, favoured by Pro/Gly at position 2
- References: PubMed 20413325, 35576623, 25044089
- Updated 2026-10-16