Peptide Impurities Explained: Deletions, Truncations and Oxidation
A purity percentage answers only one question: how much of the sample is the intended sequence. It says nothing about what makes up the rest. Yet peptide impurities are rarely random. Almost all of them trace back to a handful of known failure points in synthesis, purification and storage, and each leaves a characteristic fingerprint on a mass spectrum or chromatogram. For QC staff and lab managers comparing lots across a large order, being able to name an impurity from its signature turns a list of small peaks into useful information: is this a routine by-product, or a reason to hold a lot?
Why synthetic peptides always carry by-products
Solid-phase peptide synthesis assembles a chain one amino acid at a time on an insoluble resin. In every cycle, the growing end is unmasked by stripping a temporary protecting group, a fresh protected building block is attached, and surplus reagents are rinsed off. At the end, the chain is cut from the resin and the permanent side-chain protecting groups are removed, usually in strong acid with scavengers.
Even when each coupling step runs to near completion, small losses compound over many cycles. A 20-residue peptide goes through roughly 20 couplings and deprotections before cleavage, so a minor inefficiency repeated at every step becomes a visible population of related species. Purification removes most of them. What remains is the impurity profile, and for a given sequence and process it tends to be reproducible, because the same sequence tends to struggle at the same positions.
A field guide to peptide impurities
| Impurity class | Where it comes from | Mass signature relative to target |
|---|---|---|
| Deletion sequence | A coupling step fails on some chains, which then keep growing | Lower by one residue mass (e.g. Gly 57, Ala 71, Pro 97, Val 99, Leu/Ile 113) |
| Truncated sequence | Chain growth stops early, or a terminal residue is lost | A series of lower masses stepping down residue by residue |
| Capped truncation | Unreacted chains deliberately acetylated to stop growth | A truncated mass plus 42 Da for the acetyl group |
| Incomplete deprotection | A side-chain protecting group survives cleavage | Higher, e.g. +56 (tert-butyl), +242 (trityl), +252 (Pbf) |
| Oxidation | Exposure of Met, Trp or Cys to oxygen, light or oxidants | +16 for methionine sulfoxide; +32 for a second oxidation |
| Deamidation | Asn or Gln side-chain amide converts to an acid | About +0.98 |
| Disulfide changes | Cysteines pair up within or between chains | -2 for an internal bridge; roughly double mass for a dimer |
| Aggregates | Non-covalent association of correct molecules | No change in monomer mass |
Deletions and truncations: the synthesis signatures
A deletion is missing one internal residue. It weighs precisely one residue mass less than the target, which makes it one of the easiest impurities to assign once a spectrum is available. A peak 113 Da light in a leucine-rich peptide almost certainly marks a missing leucine or isoleucine.
Deletions are also among the hardest impurities to remove, because losing one residue from the middle of a chain often barely changes hydrophobicity. They can sit close to, or inside, the main HPLC peak. That makes them the class most likely to hide behind a good-looking purity figure.
Truncations are missing residues from one end. On the mass spectrum they tend to appear as a ladder, each step one residue lighter. Many synthesis routines cap unreacted chains with acetic anhydride so they cannot grow further, and those capped fragments carry an extra 42 Da. Truncations usually differ enough in retention to separate cleanly during purification.
Leftover protecting groups
If a side-chain protecting group is not fully removed at the end, the product has the correct sequence plus an extra chemical group. The mass is higher than the target by the mass of that group, which is a distinctive clue. These species are usually more hydrophobic than the parent and elute later on reversed-phase HPLC, so a late shoulder or small trailing peak is worth checking against the list of protecting groups used for that sequence.
Oxidation and other storage-driven changes
Unlike synthesis impurities, oxidation can appear or grow after the lot has been made. That makes it the class most directly affected by how a lab receives and stores its stock.
- Methionine oxidises most readily, gaining one oxygen (+16 Da) to form a sulfoxide. A peptide with two methionines can show both +16 and +32 species.
- Tryptophan oxidises more slowly and can give several products of different masses.
- Cysteine tends to form disulfide bonds, either within a molecule or linking two chains.
- Asparagine and glutamine can deamidate, especially in solution, adding under one dalton and shifting charge.
Keeping vials sealed, cold, dry and protected from light slows all of these. For bulk stock, logging storage conditions against each lot makes it easier to tell a manufacturing impurity from one that developed on the shelf.
Aggregation: the impurity HPLC often cannot see
Aggregates are correctly made molecules that have clumped together. Reversed-phase HPLC uses organic solvent and acid, which often break such clusters apart, so an aggregated sample can still show high chromatographic purity. The practical signs appear when material is dissolved for analytical or in-vitro work: haziness, visible particles or unexpectedly low recovery. Size exclusion chromatography and light-scattering techniques are the usual ways to measure it directly.
Reading an impurity profile across lots
For a lab buying the same compound repeatedly, the pattern matters as much as the total:
- One impurity at 2% is a single, identifiable species. Twenty at 0.1% each is background process variation. The two can give the same purity figure but tell different stories.
- Match any assigned masses to the classes above before drawing conclusions.
- Compare profiles between lots, not just totals. A new peak in a later lot deserves a question even if overall purity is unchanged.
- For sequences containing Met, Trp or Cys, pay particular attention to storage and to the age of the stock at the time of use.
Bulk Peptides has its products tested by an outside laboratory for HPLC purity. Some products have certificates available to view, and cap and crimp colour on the vial lets you match each one to its certificate when logging a shipment.
No synthetic peptide is free of related substances. What matters is whether the profile is understood and consistent from one lot to the next.
Bulk Peptides material is provided for research and analytical use in the laboratory only. It is not intended for people or animals.
