Peptide Deamidation in the Lab: Asparagine, Glutamine and Isoaspartate

What deamidation is and why isoaspartate appears

Deamidation removes the side-chain amide nitrogen of asparagine (Asn) or glutamine (Gln) as ammonia and leaves a carboxylic acid. The mass rises by 0.984 Da and each event adds a negative charge at neutral pH.

At neutral and basic pH, asparagine usually reacts through a ring. The backbone nitrogen of the next residue attacks the side chain, forming a five-membered succinimide (aspartimide). In a model hexapeptide containing Asn-Gly, this happened with a half-life of only 1.4 days at 37 °C and pH 7.4; the succinimide then hydrolysed (half-time 2.3 h) and racemized (half-time 19.5 h), giving a mixture of L- and D-aspartyl and isoaspartyl peptides. Aspartic acid itself formed the same ring 34 times more slowly, so Asp residues can isomerize without any deamidation.^[1]

Isoaspartate (isoAsp) is usually the main product: in small peptides at neutral and basic pH the isoAsp:Asp product ratio was about 3:1.^[2] IsoAsp inserts an extra methylene into the backbone, and D-forms from racemization add further isomers.

What sets the rate: sequence, pH and buffer

The residue after the asparagine dominates. In pentapeptides of the form VSNXV and VXNSV, the residue before Asn had little or no effect at neutral and alkaline pH, while increasing size and branching after Asn slowed deamidation by as much as 70-fold compared with glycine. Peptides with Ser after Asn also underwent backbone cleavage at about 10% of the deamidation rate. The rate was not affected by ionic strength, but small bases such as ammonia acted as general base catalysts.^[2] Ammonium bicarbonate is therefore a poor solvent for Asn-containing sequences.

A larger data set combined rates for 52 glutamine and 52 asparagine pentapeptides (pH 7.4, 37 °C, 0.15 M Tris) with 306 earlier asparagine rates into a structure-dependent model covering all 648 sequences GlyXxxAsnYyyGly and GlyXxxGlnYyyGly, with half-times out to 2 to 50 years.^[3]

pH changes the mechanism, not only the speed. In acidic solution, the rate no longer depended on sequence and aspartate was the only product.^[2] See the buffers and pH guide for choosing a buffer.

Worked example: how much deamidates, and how fast

Deamidation at one site usually follows first-order kinetics, so the fraction remaining after time t is 2^(-t / t½). The numbers are illustrative.

For the Asn-Gly hexapeptide above (half-life 1.4 days at 37 °C, pH 7.4), the fraction deamidated after 1 day is 1 - 2^(-1/1.4) = 39%, and after 7 days 1 - 2^(-5) = 97%. A site 50 times slower (t½ 70 days) under the same conditions is only 1.0% deamidated after 1 day and 6.7% after 7 days. Using the 3:1 isoAsp:Asp ratio, that 6.7% splits into roughly 5.0% isoAsp and 1.7% Asp.

In phosphate buffer at pH 7.4 and 37 °C, a growth hormone-releasing factor analog with Asn8 disappeared with a half-life of 202 h, mainly by deamidation to beta- and alpha-Asp8 forms, while the Ser8 analog had a half-life of 1,550 h; after 14 days only the Asn8 analogs had lost activity in cultured pituitary cells.^[4] After 72 h the two half-lives predict 2^(-72/202) = 78% versus 2^(-72/1550) = 97% remaining. Sermorelin, the 1-29 fragment, also carries Asn8-Ser9.

Reading deamidation in mass spectra

The 0.984 Da shift almost coincides with the carbon-13 isotope peak, which sits 1.003 Da above the monoisotopic peak. Take Kisspeptin-10 (Tyr-Asn-Trp-Asn-Ser-Phe-Gly-Leu-Arg-Phe-NH2, C63H83N17O14). Its calculated monoisotopic mass is 1,301.63 Da and the [M+2H]²⁺ ion appears at m/z 651.82. A singly deamidated form gives m/z 652.315, while the unmodified peptide's carbon-13 peak is at m/z 652.324, only 0.0097 m/z away at that charge state. Resolving them needs a resolving power of about 652.3 / 0.0097 ≈ 67,000.

At lower resolution the signals merge, and deamidation appears only as a distorted isotope pattern. For this formula the second isotope peak is about 76% of the first. If 10% of the peptide is deamidated, the first peak falls to 0.90 and the second becomes 0.90 × 0.76 + 0.10 = 0.78, a ratio of about 87% instead of 76%. The peptide mass calculator gives the starting mass; HPLC separation is the usual confirmation. Its candidate sites are Asn2 (before bulky Trp) and Asn4 (before Ser).

Oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2) shows that the C-terminal amide can also deamidate. In accelerated studies, deamidation of Gln4, Asn5 and Gly9-NH2, singly and in combination, was found at pH 2.0 and mono-deamidation at pH 9.0, while oligosulfide and dimer products were found at pH 4.5 and 7.0; degradation was slowest at pH 4.5 and followed the Arrhenius equation.^[5] See reading mass spectrometry results.

Telling isoaspartate from aspartate

IsoAsp and Asp forms have the same mass, and ordinary fragmentation needs reference standards. Electron capture dissociation solves this: in model peptides, fragment ions at c + 58 and z - 57 appeared only when isoaspartate was present, because the extra backbone carbon is split between the two fragments, while aspartate gave a characteristic neutral loss of its side chain.^[6] Electron transfer dissociation works similarly.

In reversed-phase HPLC, isoAsp, Asp and succinimide forms often elute close to the parent and can co-elute, so a single symmetric peak does not rule them out. See reading an HPLC chromatogram.

Deamidation created during analysis

Digestion at basic pH and 37 °C can itself add deamidation. Digesting in water enriched in oxygen-18 labels any deamidation that happens during digestion with a +3 Da shift, versus +1 Da for deamidation already present; with this method, a fast (about 4 h) trypsin digestion created no additional detectable deamidation in three rapidly deamidating peptides.^[7]

Aspartimide formation during synthesis

The same five-membered ring forms from protected aspartic acid during Fmoc solid-phase synthesis, because each base deprotection step can drive ring closure, especially at Asp-Gly. Ring opening then gives isoAsp and D-Asp peptides, and reaction with piperidine gives piperidides. Mass signatures are -18.011 Da (aspartimide) and +67.08 Da (piperidide). In a model hexapeptide (VKDGYI), side-chain protecting groups and deprotection protocols changed the amount of these by-products measured by RP-HPLC, and a tert-butyl side-chain ester combined with backbone protection on the adjacent glycine suppressed them completely.^[8]

Practical handling and a record checklist

Practical points: keep Asn-containing peptides lyophilized and cold until needed; prepare solutions shortly before use; prefer mildly acidic conditions where solubility allows; avoid prolonged storage at neutral-to-basic pH, warm temperatures and ammonium buffers; and divide stocks into single-use aliquots. For results to be interpretable, record:

• Sequence. Every Asn, Gln and Asp, the residue after each, and any Asn-Gly, Asn-Ser or Asp-Gly motifs.
• Solution conditions. Buffer species and concentration, pH, temperature and time in solution before analysis.
• Mass data. Resolving power, observed isotope pattern compared with theory, and any +0.984 Da or -17/-18 Da species.
• Isomer analysis. HPLC method, isoAsp-specific fragmentation if used, and a fresh reference sample analysed the same day.

This guide describes general laboratory chemistry and published literature only. It is not a statement about the content or behaviour of any particular material. Related degradation routes are covered in peptide storage and degradation, terms are defined in the glossary, and questions can be sent to support.