Deamidation

Deamidation is the conversion of an asparagine or glutamine side-chain amide to a carboxylic acid, with loss of ammonia. It is among the most common chemical degradation routes in peptides and proteins, and it proceeds spontaneously in aqueous solution without any external agent.^[1]

The mass change is +0.984 Da, which is small enough that unit-resolution mass spectrometry cannot distinguish a deamidated peptide from its parent. The charge change is more consequential: an uncharged amide becomes a negatively charged carboxylate, which alters chromatographic behaviour and, in a receptor-binding peptide, may alter activity.^[2]

Rate depends strongly on sequence context. Asparagine followed by glycine deamidates orders of magnitude faster than most other contexts, because the small following residue permits formation of the cyclic succinimide intermediate through which the reaction proceeds.^[1]

At neutral and alkaline pH the dominant pathway is intramolecular: the backbone nitrogen of the following residue attacks the asparagine side-chain carbonyl, forming a five-membered succinimide with loss of ammonia. The succinimide then hydrolyses to give aspartate or isoaspartate, typically in roughly a one-to-three ratio.^[1]

Isoaspartate formation is the more damaging outcome, because it inserts an extra methylene into the backbone and changes the local conformation. It is isobaric with aspartate and is not distinguishable by intact mass; detection requires a specific enzymatic or chromatographic method.

Rate rises with pH above about 6, with temperature, and with the flexibility of the local backbone. The Asn-Gly context is the fastest by a wide margin; Asn-Ser and Asn-His are also comparatively fast. Glutamine deamidates by the same mechanism but far more slowly, because the corresponding intermediate is a six-membered ring.^[3]

Chromatographically, deamidated species usually elute slightly earlier than the parent on reverse phase and are better resolved by ion-exchange, which separates on the charge difference that deamidation creates.^[2]

By mass spectrometry, the +0.984 Da shift requires high-resolution instrumentation to see on a peptide of a few kilodaltons. On a quadrupole instrument a deamidated peptide is indistinguishable from its parent, and a certificate reporting "observed mass matches calculated" from such an instrument has not excluded it.^[3]

Because deamidation is slow at low temperature and in the solid state, it is largely a solution-phase problem. A lyophilised peptide stored cold deamidates very slowly; the same peptide in solution at ambient temperature can accumulate a measurable fraction over weeks. This is a principal reason for the beyond-use dating of reconstituted material.^[4]

Several therapeutic peptides in this field contain asparagine or glutamine residues, so deamidation is a specified degradation product in their stability programmes. Whether a given deamidated species retains activity is sequence-specific and is established experimentally rather than predicted.^[2]

For material held outside a controlled cold chain the practical significance is that a purity figure determined at release does not describe a vial that has since spent time warm and in solution. See Temperature excursion and Chain of custody — a low purity figure on community-tested material may be characterising storage rather than manufacture.

A method described as stability-indicating must resolve deamidated species from the parent; that is one of the things forced degradation demonstrates. A certificate making the claim without the supporting work is asserting rather than evidencing it. See Analytical method validation.^[4]

• Methionine oxidation
• Peptide aggregation
• Mass spectrometry
• Temperature excursion
• Beyond-use date
• Analytical method validation