Why Peptides Degrade: The Chemical and Physical Routes to Instability

Peptide materials are not chemically inert. The scientific literature on protein and peptide stability distinguishes two broad categories of change, both of which can alter what a laboratory is actually working with.

Chemical instability

Chemical instability involves the formation or breaking of covalent bonds. The review Stability of Protein Pharmaceuticals: An Update by Manning and colleagues (Pharmaceutical Research, 2010) discusses the principal reactions, including:

• Deamidation of asparagine and glutamine residues, which changes charge and can generate isomeric products.
• Oxidation, particularly of methionine, cysteine, tryptophan and histidine residues.
• Hydrolysis of peptide bonds, with some sequences being especially susceptible.
• Disulfide exchange and other cysteine-related reactions.

Many of these reactions depend on pH, temperature, moisture and exposure to oxygen or light.

Physical instability

Physical instability changes higher-order structure or state without altering covalent structure. The same review covers aggregation, adsorption to surfaces and precipitation. Aggregates can be difficult to detect by a single chromatographic method and may require orthogonal techniques.

How stability is studied

The ICH Q1A(R2) guideline on stability testing of new drug substances and products describes stress testing as a way to help identify likely degradation products and degradation pathways, and to support the suitability of the analytical procedures used. It also defines long-term and accelerated storage studies.

What this means in the laboratory

For research use, the practical lessons are consistent with this literature: keep lyophilised material sealed, dry and protected from light at the storage temperature stated on the product documentation; minimise repeated temperature cycling; and record the lot number so that any analytical data can be tied to the material actually used.