Stability

Peptide stability refers to its ability to preserve its structure, chemical composition, and biological activity over time and across different environmental conditions. It is a central concern for both research (reproducibility of results) and pharmaceutical development (shelf life, formulation stability, compatibility with administration routes).

Chemical degradation pathways are numerous. Hydrolysis of the peptide bond, although slow at neutral pH, accelerates at extreme pH; it preferentially affects Asp-X and X-Pro bonds. Asparagine deamidation (Asn → Asp + iso-Asp via a succinimide intermediate) and glutamine deamidation are among the most frequent degradations in aqueous solution; they modify charge and can alter activity. Oxidation of methionine (to methionine sulfoxide), cysteine (formation of unwanted intramolecular disulfides or scrambling), and tryptophan (to kynurenine and derivatives) is favored by dissolved oxygen, trace metals (iron, copper), and light. Racemization (L to D inversion), Maillard reactions with reducing sugars, and β-elimination of disulfide bridges complete the picture.

Physical degradation includes aggregation (formation of dimers, oligomers, amyloid fibrils) driven by high concentration, mechanical agitation, contact with surfaces (glass, plastic), or repeated freeze-thaw cycles. Simple precipitation, often observed at the isoelectric point, is an extreme case.

Influential factors to know in the laboratory are temperature (rule of thumb: +10 °C doubles degradation rate), pH (optimal stability zone often between 4 and 6 for neutral peptides), ionic strength, dissolved oxygen, light (UV particularly aggressive), trace metals, freeze-thaw cycles, and container material (borosilicate glass > polypropylene > polystyrene for long-term stability).

Laboratory storage good practices recommend lyophilization for long-term storage (stable powder for several years at -20 °C or -80 °C), working in aliquots to limit freeze-thaw cycles, reconstituting just before use, adding antioxidants (ascorbic acid, methionine in excess) or chelators (EDTA) for oxidation-prone peptides, and light protection (amber vials or opaque packaging).

Stabilization strategies in peptide design include substitution of labile amino acids (Asn → Ser, Met → Nle), cyclization (reduces exopeptidase susceptibility), PEGylation (masks sensitive sites), use of D-amino acids, hydrocarbon staples, N-methylation of the backbone, and conformational rigidification through α,α-disubstituted analogs (Aib).