Peptide Stability and Degradation Pathways

Hydrolysis

Water attacks the peptide bond itself, cleaving the chain into fragments. The rate rises with temperature and at extremes of pH, and aspartyl residues are particularly vulnerable.

This single pathway explains why the industry ships powder rather than solution: no water, no hydrolysis.

Oxidation

Methionine, cysteine and tryptophan react with dissolved oxygen and with trace metal contaminants. Methionine oxidation adds sixteen daltons and is visible on a mass spectrum, which makes it a useful early indicator of a degrading lot.

Minimising headspace, avoiding metal contamination and keeping material dark all slow this pathway.

Deamidation

Asparagine and glutamine side chains lose their amide group over time, converting to aspartate and glutamate. This changes the charge of the molecule and shifts its retention time on HPLC, and it accelerates at higher pH.

Aggregation

Chains associate into dimers and larger assemblies, sometimes irreversibly. Concentration, agitation, freeze–thaw cycling and air–liquid interfaces all promote it. Visible haze or particulates in a previously clear solution is the late-stage signal.

Practically, the countermeasures are the same across all four pathways: cold, dark, dry, sealed, undisturbed.