Research Peptide Quality: Impurities, Content and Stability

Solid phase peptide synthesis builds a chain one residue at a time, and each step can leave a small fraction of chains incomplete. The resulting related impurities include deletion sequences missing a residue, truncated chains, incompletely deprotected forms, oxidized methionine or tryptophan, deamidated asparagine or glutamine and racemized residues. Published reviews of peptide medicines catalogue these families and note that some sit very close to the main peak in a chromatogram.

A lyophilized peptide powder contains the peptide plus water and counterions, commonly trifluoroacetate or acetate left from purification. A sample can be very pure by HPLC and still be well below 100 percent peptide by weight. Net peptide content, measured by amino acid analysis or elemental nitrogen, is what tells you how much peptide a given mass of powder actually holds.

Peptides can aggregate, form fibrils or adsorb to surfaces, and those changes are not always visible. Research on peptide stability shows that temperature, pH, concentration, ionic strength and agitation all influence aggregation, and that lyophilized products can develop particulates over time. Storage conditions are part of quality, not separate from it.

• Identity by mass spectrometry and purity by HPLC, both for the same lot.
• Method conditions printed with the chromatogram.
• Water content and, where available, net peptide content.
• A named testing laboratory and analysis date.
• Clear storage conditions on the label.