A peptide's HPLC purity of "98%" looks clean on paper, but it does not tell the whole story. That remaining 2% could be deletion sequences (peptides missing a single amino acid), truncated chains (incomplete synthesis), or other byproducts. These impurities are structurally similar to the target and often impossible to fully remove, yet they can significantly affect research results. This guide explains what these impurities are, how they form, and why they matter.
Truncated peptides and deletion sequences are the two most common synthesis impurities, and they have different origins:
Truncated sequences are incomplete chains — the synthesis was terminated before all amino acids were added. For example, if you are synthesizing a 20-amino-acid peptide and the reaction stops after 15 additions, you end up with a 15-amino-acid truncated version. In solid-phase peptide synthesis (SPPS), truncation occurs when a coupling reaction fails or an amino acid-activation step is incomplete, and the growing chain is prematurely cleaved from the resin.
Characteristics: Truncated peptides elute earlier (shorter retention time) on a reverse-phase HPLC column because they are smaller and less hydrophobic.
Deletion sequences are full-length chains that are missing one or more internal amino acids. This occurs when a coupling step fails at a specific position during synthesis, and the resin "skips" that step, continuing with the next amino acid. A deletion peptide is thus only a few mass units lighter than the target, making it harder to distinguish.
Characteristics: Deletion peptides elute very close to the target on HPLC, sometimes overlapping, making them difficult to fully resolve and quantify. They retain roughly the same hydrophobicity as the full-length peptide, so they are not effectively removed by typical purification methods.
Biological activity: Truncated and deletion peptides are structurally different enough that they often lose the target's bioactivity. A peptide that works by precise receptor binding or enzyme recognition will not function if it is missing amino acids. Contaminating a research sample with 2–5% inactive impurities can reduce the apparent potency of your experimental peptide.
Immunogenicity: Impurities may trigger immune responses that the pure target does not. If your deletion-peptide impurity is recognized as foreign (epitope), it can bias results in any cell-based or immunological assay.
Aggregation: Misfolded or incomplete peptides are more prone to aggregation. If impurities aggregate, they can seed aggregation of the main peptide, reducing its shelf life and potency.
Reproducibility: Different batches or suppliers may have different impurity profiles. If one source is 98% pure but with different impurities than another source, your results between batches may diverge due to the contamination, not biological differences.
When a Certificate of Analysis states "98% purity by HPLC," this usually means the main peak (your target peptide) represents 98% of the total area under the chromatogram. The remaining 2% is the sum of all other peaks.
Important caveat: HPLC purity measures chromatographic separation, not chemical identity. Two peptides with similar hydrophobicity may co-elute and appear as a single peak, artificially inflating purity. To truly confirm identity, you need complementary methods:
A high-quality Certificate of Analysis should include HPLC chromatography AND mass spectrometry, which together provide strong evidence of both purity and identity.
When ordering research peptides, you can request: