Aspartimide formation in Fmoc peptide synthesis: the side reaction behind isoAsp impurities

Prepared by: Research Desk. Sources are linked for verification.

Published 24 October 2026

At a glance

• What it is: a side reaction at aspartic acid (Asp) residues in which the peptide backbone closes onto the Asp side chain to form a five-membered ring, the aspartimide
• When it happens: mainly during the repeated basic steps that remove the Fmoc protecting group in Fmoc solid-phase peptide synthesis (SPPS)
• Why it matters: the ring opens again to give a mixture of products, some with exactly the same mass as the target peptide
• Hot spots: sequences where Asp is followed by glycine (Asp-Gly) are a classic problem, and elevated temperature makes it worse
• Status in the literature: a 2025 review in ChemBioChem calls it "a persistent challenge" and surveys the main prevention strategies (Kong et al., 2025)

Where aspartimide comes from

In Fmoc SPPS, the peptide chain is built on a resin one amino acid at a time. Each new residue carries an Fmoc group on its amine, which is removed with a base, usually piperidine, before the next coupling. Side chains carry acid-labile protecting groups; for aspartic acid the standard choice is a tert-butyl ester, written Asp(OtBu). The overall approach is described in reviews by Fields and Noble (1990) and Behrendt, White and Offer (2016), the latter noting that Fmoc SPPS is now "the method of choice for peptide synthesis".

The weakness is that the base used to remove Fmoc can also act on the Asp side chain. The nitrogen of the next residue's backbone amide attacks the protected side-chain carbonyl, the ester group leaves, and a five-membered succinimide ring forms. Because Fmoc removal is repeated for every residue added after the Asp, a single aspartic acid near the C-terminus of a long sequence is exposed to base many times. Kong et al. (2025) describe the process as base-mediated, which is why it is chiefly discussed as an Fmoc-chemistry problem. For how Fmoc and Boc strategies differ, see the guide to how research peptides are made and tested.

What the ring turns into

The aspartimide is itself an impurity, but it is also reactive. It can open in several ways, and each gives a different product.

The ring's alpha carbon can also lose its stereochemistry, so even the "correct" alpha-linked product may come back partly as D-aspartic acid. Behrendt et al. (2016) report aspartimide suppression together with suppression of aspartic acid epimerisation for this reason.

The last row was characterised in detail by Samson et al. (2019), who observed significant chain termination at an N-terminal Xaa-Asp-Yaa motif. Using NMR, they showed that only the six-membered piperazine-2,5-dione ring forms, and concluded that hindering aspartimide formation reduces or completely suppresses this termination.

Which sequences are most at risk

Risk depends on the residue after the aspartic acid and on the conditions.

• Asp-Gly. Glycine has no side chain to block the attacking amide, so Asp-Gly is the standard worst case. The model peptide VKDGYI, a fragment of scorpion toxin II, is widely used to test new methods (Behrendt et al., 2016).
• Other neighbours. Behrendt et al. (2015) also tested variants of the same model with asparagine and arginine after the Asp.
• Heat. Microwave and high-temperature synthesis speed up coupling and deprotection, but they also speed up side reactions. Palasek et al. (2007) reported aspartimide formation and racemisation during microwave Fmoc SPPS, and Personne et al. (2023) studied the problem at 60 °C.
• Long, Asp-rich targets. Behrendt et al. (2016) used teduglutide, a GLP-2 analogue whose synthesis is complicated by extensive aspartimide formation, as a real-world test case.

How chemists suppress it

The 2025 review by Kong et al. groups prevention strategies into four families. The table links each to an example study.

In the 2016 Asp-Gly study, Behrendt et al. compared their bulky ester building blocks directly with the common practice of adding acids to the Fmoc deprotection solution, and reported an almost complete reduction in aspartimide formation with the ester approach. No single method is described as universal; Kong et al. conclude that compatible, scalable tactics are still needed for both research and industry.

Reading HPLC and MS data with aspartimide in mind

These by-products behave differently in the two standard tests on a certificate of analysis (COA).

• Mass spectrometry. The aspartimide itself (18 Da lighter) and piperidides (67 Da heavier) show up as extra masses. The alpha- and beta-Asp products and the D-epimer have the same mass as the target, so a mass spectrum alone cannot rule them out. Our article on mass spectrometry on a peptide COA explains why same-mass impurities are a blind spot.
• RP-HPLC. Isomers often elute close to, but not always at the same time as, the main peak. A shoulder or a small neighbouring peak in an Asp-containing sequence is consistent with these products, though it does not identify them. Separating them can require optimised gradients or orthogonal methods.
• Peptide mapping and NMR. Distinguishing isoAsp from Asp or confirming the epimer typically needs more specialised work, such as enzymatic digestion with LC-MS/MS or the NMR analysis used by Samson et al.

For the wider list of impurity types and the tests that find them, see common impurities in synthetic peptides. For how a purity figure differs from content by weight, see HPLC purity vs net peptide content.

Limitations of the evidence

• Most studies use short model peptides such as VKDGYI, and the size of the effect varies with sequence, resin, temperature and reagents.
• Several of the prevention papers come from authors at companies that develop and sell peptide building blocks, as stated in their affiliations.
• Reported reductions are measured under each paper's own test conditions and are not directly comparable between studies.
• A COA rarely states which synthesis strategy was used, so a reader usually cannot tell which measures were taken for a given batch.

Key terms

• Fmoc. 9-Fluorenylmethoxycarbonyl, an amine protecting group removed with base in each cycle of Fmoc SPPS.
• Aspartimide. A five-membered succinimide ring formed between an Asp side chain and the next backbone nitrogen.
• IsoAsp (beta-Asp). An aspartic acid residue linked into the backbone through its side-chain carboxyl instead of its alpha carboxyl.
• Epimerisation. A change in configuration at one stereocentre, here turning L-aspartic acid into D-aspartic acid.
• Piperidide. An adduct formed when piperidine opens the aspartimide ring.

Supplier documentation

Purita is a store that sells peptides for laboratory research use only. It publishes a certificates page where reports are available for some products; reports are not yet available for every item, so check the page for the specific material.

Purita certificates page

Affiliate disclosure: this is a paid link. DesignPeptide earns a commission if a purchase is made through it, at no extra cost to the buyer. A listing is not an endorsement or a statement about product quality. See the affiliate disclosure.

Related pages

• Peptide design, synthesis and testing, explained
• About DesignPeptide
• How research peptides are made and tested: from synthesis to COA
• Common impurities in synthetic peptides and how labs detect them
• Mass spectrometry on a peptide COA: how ESI-MS and MALDI-TOF confirm identity
• HPLC purity vs net peptide content: two numbers that are often confused