---
title: "Pyroglutamate and diketopiperazine: N-terminal cyclisation in peptides | Sable Peptides"
description: "Two cyclisation reactions that change peptides at the N-terminus: glutamine or glutamate cyclising to pyroglutamate (−17 or −18 Da), and dipeptide loss as a diketopiperazine. Mechanisms, detection and sequences at risk. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/pyroglutamate-and-diketopiperazine-formation
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# 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.

Source: [Effect of N-terminal glutamic acid and glutamine on fragmentation of peptide ions, J Am Soc Mass Spectrom 2010 (PubMed 20413325)](https://pubmed.ncbi.nlm.nih.gov/20413325/)

## 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)](https://pubmed.ncbi.nlm.nih.gov/35576623/); [Related impurities in peptide medicines, J Pharm Biomed Anal 2014 (PubMed 25044089)](https://pubmed.ncbi.nlm.nih.gov/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

-   [Deamidation and isoaspartate](/guides/deamidation-and-isoaspartate-in-peptides)
-   [Impurity types](/learn/manufacturing/impurity-types)
-   [Terminal capping](/learn/terminal-capping-acetylation-amidation)
-   [Reading extra MS peaks](/guides/esi-charge-states-and-adducts-in-peptide-spectra)

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