Peptide Stability, Degradation and Storage

The amide backbone hydrolyses, and the rate rises with temperature and with pH departure from neutrality. Aspartic acid residues are a particular weak point: Asp-Pro and Asp-Gly bonds are notably labile, and a sequence containing them is intrinsically less stable than one that does not.

This is the first reason lyophilised material stores better than solution. Removing water removes the reagent.

Methionine oxidises readily to the sulfoxide, adding sixteen daltons — a change plainly visible by mass spectrometry and a common finding in aged material. Cysteine, tryptophan and histidine are also susceptible.

Oxidation is promoted by dissolved oxygen, light, and trace metal ions. Amber vials, headspace displacement with inert gas and avoiding metal contamination are therefore not fussiness but direct countermeasures.

Asparagine and, more slowly, glutamine deamidate to aspartate and glutamate, a change of roughly one dalton that is easy to miss on a low-resolution instrument. Asn-Gly sequences deamidate fastest because the reaction proceeds through a cyclic succinimide intermediate that this pairing forms readily.

The resulting isoaspartate alters the backbone and can change activity while barely changing mass — one of the better arguments for chromatographic as well as mass-based monitoring.

Some sequences self-associate into oligomers and fibrils. Human amylin is the standard example, and the reason pramlintide exists: three proline substitutions borrowed from the rat sequence disrupt the beta-sheet stacking that drives fibril formation.

Aggregation is concentration-dependent, temperature-dependent and frequently irreversible. Visible haze or particulates in a solution that was clear is a meaningful observation and not a cosmetic one.

Because every route above runs continuously, a purity figure describes the material at the moment it was measured. Material tested at manufacture and stored for a year is not the material the certificate describes, and the gap widens with poor storage.

This is what a retest date is for: a statement of how long the supplier expects the specification to hold under the stated conditions. It is not an expiry in the pharmaceutical sense, and it is only meaningful if the stated storage conditions were actually maintained — which is why storage history belongs in the laboratory record alongside the certificate.

Each practice below follows from a specific degradation route rather than from custom.