Peptide deamidation and isoaspartate: why mass is not enough

Research-use notice: this article is for legitimate laboratory and educational research only. It does not provide guidance for human use, administration, dosing or treatment.

Deamidation is the loss of the amide group from an asparagine or glutamine side chain, which turns it into a carboxylic acid and raises the peptide’s mass by 0.984 daltons (Da). At neutral pH, asparagine usually reacts through a ring-shaped succinimide intermediate, which opens to give a mixture of aspartate and isoaspartate. Aspartate and isoaspartate have exactly the same mass, so a mass match cannot tell them apart; separation, specialised fragmentation or enzyme-based methods are needed. The evidence summarised here comes from analytical studies of model peptides and proteins.

What is deamidation?

Asparagine (Asn) and glutamine (Gln) carry an amide group (CONH2) at the end of their side chains. Deamidation replaces the NH2 with OH. Asparagine becomes aspartate (Asp) and glutamine becomes glutamate (Glu). Because an NH2 group is swapped for an OH group, the monoisotopic mass rises by 0.984 Da.

For asparagine at neutral or mildly basic pH, the reaction usually runs through an intermediate:

1. Ring formation. The backbone nitrogen of the next residue attacks the asparagine side chain, closing a five-membered succinimide ring and releasing ammonia. At this stage the peptide is about 17 Da lighter than the original.
2. Ring opening. Water opens the ring at one of two bonds. One gives normal aspartate. The other gives isoaspartate (isoAsp), in which the peptide backbone runs through the side-chain carbon. The mixture is often richer in isoaspartate, with the exact ratio depending on sequence and conditions.
3. Racemisation. The succinimide can also flip to its mirror-image (D) form, so D-aspartate and D-isoaspartate can appear as well.

A classic 1987 study in the Journal of Biological Chemistry by Geiger and Clarke followed this pathway in a synthetic hexapeptide containing an asparagine–glycine (Asn-Gly) pair. At 37 °C and pH 7.4 it deamidated with a half-life of 1.4 days, forming a succinimide that then hydrolysed and racemised. The end product was a mixture of L- and D- forms of normal and isoaspartyl peptides. In a 2005 Protein Science paper, Cournoyer and colleagues describe isoaspartate as lengthening the backbone by one methylene (CH2) unit.

Aspartate can enter the same succinimide route. In that case the product is isoaspartate with no change in mass at all, a process called isomerisation. Geiger and Clarke found that replacing the asparagine with aspartate cut the rate of succinimide formation 34-fold, so isomerisation is slower, but it still occurs.

Why do sequence and conditions matter?

The residue after the asparagine has a large effect. Geiger and Clarke found that replacing the glycine with the bulkier leucine or proline slowed degradation 33- to 50-fold. Asn-Gly is therefore regarded as a hot spot.

In 2001, Robinson and Robinson published deamidation rates for 306 sequences in model peptides, all measured by mass spectrometry at pH 7.4 and 37 °C in the same buffer, in the Proceedings of the National Academy of Sciences. They also demonstrated a way to measure how a protein’s higher-order structure affects deamidation, by comparing a protein with matching peptides in the same solutions. pH, temperature and buffer all form part of the conditions that a rate applies to.

Conditions change the pathway as well as the rate. A 2026 study in RSC Medicinal Chemistry by Erckes and colleagues held model peptides under different conditions. In phosphate-buffered saline at pH 7.4, isoaspartate formed over time, consistent with the succinimide route that is favoured near neutral pH. Acidic solutions, including those containing trifluoroacetic, formic, acetic or hydrochloric acid, promoted direct hydrolysis instead, with no detectable isoaspartate. That acid-driven degradation depended on the site, and C-terminal amides were markedly more susceptible. This matters because many research peptides are C-terminal amides, and hydrolysis of the C-terminal amide also adds 0.984 Da.

Why is a +0.984 Da shift hard to see?

Every peptide’s mass spectrum shows an isotope pattern: a main peak plus smaller peaks about 1 Da heavier, from molecules containing one carbon-13 atom. As the Erckes study points out, the +0.984 Da shift puts a deamidated species within about 0.019 Da of that first isotope peak, so high resolving power and mass accuracy are needed to avoid misassignment. Without them, a small deamidated fraction can pass for part of the normal isotope pattern. Chromatographic separation before the mass spectrometer also helps.

Telling aspartate from isoaspartate is harder still, because the two have the same mass. The Erckes study notes that conventional chromatographic methods and standard mass spectrometric analyses often fail to distinguish structurally similar peptides with nearly identical physicochemical properties and masses. Our article comparing high-performance liquid chromatography (HPLC) and mass spectrometry explains why a mass match is not a full identity answer.

How is isoaspartate detected?

No single technique answers every question, so laboratories combine them. The main options are HPLC or its ultra-high-performance form (UHPLC); intact-mass measurement by mass spectrometry (MS); tandem MS (MS/MS), which breaks ions into fragments by collision-induced dissociation (CID) or by electron-based methods such as electron transfer dissociation (ETD) and electron capture dissociation (ECD); enzyme digestion; and nuclear magnetic resonance (NMR) spectroscopy.

The table draws on specific studies. Cournoyer and colleagues (2005) used ECD on model peptides and found fragment ions only in the spectra of peptides containing isoaspartate. Erckes and colleagues (2026) compared CID and ETD: both could tell the deamidation products apart without chromatographic separation, ETD produced isoaspartate diagnostic ions arising from the altered backbone, and the authors describe telling isoaspartate from aspartate by CID alone as very limited and peptide-dependent. Ni and colleagues (2010) analysed isoaspartate by combining selective digestion with Asp-N and ETD mass spectrometry. A 2026 Journal of Pharmaceutical and Biomedical Analysis study by Mishra and colleagues used two-dimensional NMR at natural isotopic abundance to distinguish bivalirudin from [Asp9]-bivalirudin, a deamidation-related impurity standard.

When does deamidation happen: synthesis, storage or analysis?

All three. The Erckes study’s title makes the point: it looked at deamidation and isoaspartate formation during peptide analysis, purification and storage.

• Synthesis and manufacture. Aspartate residues can close into the same kind of ring, known in peptide synthesis as an aspartimide, and ring opening again gives aspartate or β-aspartate (isoaspartate). The European Medicines Agency (EMA) guideline on synthetic peptides lists cyclic imide (aspartimide) formation, deamidation and β-Asp-containing sequences among the pathways to consider.
• Storage. In the Erckes study, isoaspartate formed over time in peptides held in phosphate-buffered saline, while amidated peptides stayed stable in neutral water–organic mixtures or at lower temperatures.
• Analysis. The Erckes authors point out that these reactions occur in solvents and additives routinely used for peptide analysis, purification and storage, such as aqueous trifluoroacetic acid and phosphate-buffered saline. Sample handling can therefore create the very species being measured, and a result needs controls that separate pre-existing deamidation from deamidation formed during the test.

A 2025 study in Separations by Bosc-Bierne and Weller, which examined a pool of 32 synthetic peptides by UHPLC with high-resolution MS, found aspartimide formation and deamidation among the impurities, alongside methionine oxidation and small traces of deletion peptides. A 2014 review of impurities in peptide medicines by D’Hondt and colleagues lists succinimide formation among the degradation routes that produce peptide-related impurities.

What the research covers

The evidence on deamidation is analytical and in vitro: synthetic model peptides (Geiger and Clarke; Robinson and Robinson; Cournoyer; Erckes), pharmaceutical impurity standards (Mishra) and a pool of synthetic peptides (Bosc-Bierne and Weller). Guidance documents set the regulatory context.

• Regulatory guidance. The EMA guideline, legally effective from 1 June 2026, asks for potential degradation pathways to be discussed in the light of a peptide’s composition and sequence, naming deamidation and β-Asp-containing sequences. ICH Q6B, an International Council for Harmonisation (ICH) guideline written for proteins made in cell-culture systems and excluding synthetic peptides, lists deamidated and isomerised forms among its examples of product-related molecular variants.
• What it does not show. These studies show that deamidation and isoaspartate can form and how they can be measured. They do not tell you how much is present in any particular batch, and an analytical finding is not a biological or clinical finding.

How to check a batch for deamidation

• Look at the sequence. Asparagine, especially when followed by glycine, is the main risk. Aspartate residues and a C-terminal amide are secondary ones.
• Check the MS resolution. Ask whether the method can separate a +0.984 Da species from the carbon-13 isotope peak, or whether deamidated forms were separated by chromatography first.
• Ask what was looked for. A standard identity test confirms the main peak’s mass. It does not usually report deamidation or isoaspartate unless the method was designed to.
• Keep handling consistent. Peptides held in near-neutral buffers such as phosphate-buffered saline can form isoaspartate, so record how long solutions were kept and at what pH and temperature. See our peptide storage guide.
• Read the report in context. See How to read a peptide COA. Thistle shows a batch laboratory report on each product page, and you can look up a batch on our batch report lookup.

Deamidation often sits beside other changes that also need structural context, such as methionine oxidation and sequence impurities.

Peptide deamidation FAQs

What is the mass shift for deamidation?

Deamidation adds 0.984 Da (monoisotopic). That is only about 0.019 Da away from the first carbon-13 isotope peak, so it needs high resolution or prior separation to see clearly.

What is isoaspartate?

Isoaspartate is an isomer of aspartate in which the peptide backbone runs through the side-chain carbon, lengthening the backbone by one CH2 unit. It has the same mass as aspartate but a different structure.

Can mass spectrometry detect isoaspartate?

Not from intact mass alone. Electron-based fragmentation (ETD or ECD), enzymatic digestion with Asp-N, or comparison with synthetic isoaspartate standards can distinguish it.

Which amino acids deamidate?

Asparagine and glutamine. Aspartate can also convert to isoaspartate through the same succinimide intermediate, but with no change in mass.

Can deamidation happen during testing?

Yes. A 2026 study found that deamidation can occur in solvents and additives routinely used for peptide analysis, purification and storage, so a reliable result depends on controls that separate pre-existing deamidation from deamidation formed during the test.

Sources

• Erckes V, Chamera Rendueles L, Misiek A, Steuer C. Revealing deamidation and isoaspartate formation during peptide analysis, purification and storage by tandem mass spectrometry. RSC Medicinal Chemistry. 2026;17(2):1144–1154. Model peptides.
• Geiger T, Clarke S. Deamidation, isomerization, and racemization at asparaginyl and aspartyl residues in peptides. Succinimide-linked reactions that contribute to protein degradation. Journal of Biological Chemistry. 1987;262(2):785–794. Model peptides.
• Robinson NE, Robinson AB. Molecular clocks. Proceedings of the National Academy of Sciences of the USA. 2001;98(3):944–949. Model peptides and a protein.
• Cournoyer JJ, Pittman JL, Ivleva VB, et al. Deamidation: differentiation of aspartyl from isoaspartyl products in peptides by electron capture dissociation. Protein Science. 2005;14(2):452–463. Model peptides.
• Ni W, Dai S, Karger BL, Zhou ZS. Analysis of isoaspartic acid by selective proteolysis with Asp-N and electron transfer dissociation mass spectrometry. Analytical Chemistry. 2010;82(17):7485–7491. Analytical method.
• Mishra SH, Kaur A, Vu D, Bhavaraju S. Comprehensive solution NMR characterization of bivalirudin, a direct thrombin inhibitor peptide, and its process impurities by heteronuclear spectral fingerprinting at natural isotopic abundance. Journal of Pharmaceutical and Biomedical Analysis. 2026;281:117684. Pharmaceutical impurity standards.
• Bosc-Bierne G, Weller MG. Investigation of impurities in peptide pools. Separations. 2025;12(2):36.
• D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis. 2014;101:2–30. Review.
• European Medicines Agency. Guideline on the development and manufacture of synthetic peptides. EMA/CHMP/CVMP/QWP/367182/2025. Legally effective 1 June 2026. Medicinal-product guidance.
• International Council for Harmonisation. ICH Q6B Specifications: test procedures and acceptance criteria for biotechnological/biological products. Step 4, 10 March 1999. Guideline; excludes synthetic peptides.

Published by Thistle BioLabs, a UK supplier of research materials. This article summarises external research, not studies conducted by Thistle. Educational information only; no medical advice or recommendation for personal use.