What is Deamidation?

Deamidation is the spontaneous conversion of an asparagine (Asn) side chain, and much more slowly a glutamine (Gln), from an amide to a carboxylic acid. No enzyme is needed: in neutral solution at body temperature the backbone nitrogen of the next residue attacks the Asn side chain, forming a five-membered succinimide ring that then hydrolyses. The products are a mixture of normal aspartyl and isoaspartyl (β-linked) peptides, in both L and D forms, because the succinimide also racemises ^[1].

How fast, and where

Sequence decides the rate. The model peptide Val-Tyr-Pro-Asn-Gly-Ala deamidated with a half-life of 1.4 days at 37 °C and pH 7.4; its succinimide intermediate hydrolysed with a half-time of 2.3 hours and racemised with a half-time of 19.5 hours ^[1]. Replacing the Gly after Asn with bulky Leu or Pro slowed degradation 33- to 50-fold, and replacing Asn with Asp slowed succinimide formation 34-fold ^[1]. Asn-Gly is the classic hot spot; Asp residues can follow the same succinimide route to isoaspartate, more slowly ^[1].

Worked example: a culture well is a deamidation experiment

In a growth hormone-releasing factor analogue, [Leu27]hGRF(1-32)-NH2, incubation at pH 7.4 and 37 °C produced β-Asp8 and α-Asp8 forms through deamidation of Asn8. In cultured bovine pituitary cells, the β-Asp8 product was estimated to be 400–500 times less potent than the parent and the α-Asp8 product 25 times less potent ^[2]. Reported half-lives were 202 hours for the Asn8 peptide and 1,550 hours when Asn8 was replaced by Ser ^[2]. Assuming first-order loss, the fraction of intact peptide over a typical incubation works out as:

These are exactly the conditions of a cell-culture incubator. A week-long assay with the Asn8 peptide would end with roughly 44% of the parent gone, mainly to deamidated forms of much lower potency ^[2], so a response that fades over time may reflect chemistry rather than biology. The 29-residue sequence of CJC-1295 carries Gln at position 8, where native GHRH, and therefore tesamorelin, carries Asn; the CJC-1295 DAC versus no-DAC guide covers that sequence. The half-life guide explains how chemical and enzymatic loss combine.

How it shows up analytically

• Mass. Replacing –NH₂ with –OH adds 0.984 Da. That sits 0.019 Da from the ¹³C isotope spacing (1.003 Da), so a low-resolution mass spectrum can hide a deamidated form inside the parent's isotope envelope.
• Isomers. Asp and isoAsp products have identical mass; telling them apart needs chromatographic separation or specialised fragmentation.
• Charge. The new carboxylate lowers the isoelectric point, so charge-based methods such as ion exchange or isoelectric focusing resolve deamidated forms well.
• Reversed-phase HPLC. Deamidated species can elute close to the main peak and shelter under it, flattering the purity figure.

Common misunderstandings

• "Lyophilised peptides can't deamidate." Deamidation is one of the main reactions that continue in the solid state, with temperature and moisture content among its controlling factors ^[3]. Dry, cold storage slows it.
• "Gln is as risky as Asn." Glutamine has to form a six-membered ring rather than a five-membered succinimide, which it does far less readily. A Gln in place of Asn changes the stability picture, though it is not inert.
• "A +1 Da peak is just an isotope." Sometimes it is a degradation product; resolution decides.

Storage conditions that slow deamidation, alongside oxidation, are set out in the peptide storage guide.