Where Peptide Impurities Come From: Deletion, Truncation, Oxidation and Aggregation

A purity figure tells you how much of a sample is the target peptide. It says nothing about what the remainder is, and that omission matters, because the remainder is not random. Synthetic peptide impurities come from a small number of well-understood processes, each of which leaves a recognisable signature. Knowing which process produced a peak tells you whether it is a cosmetic blemish or a reason to reject a lot.

This note describes those processes and what each looks like on an analytical report. For how to read the report itself, see reading a certificate of analysis and reading an HPLC chromatogram.

How the chain is built, and where it goes wrong

Solid-phase peptide synthesis builds a chain one residue at a time on a resin support. Each cycle removes a protecting group from the growing chain’s free end, couples the next protected amino acid, and washes away what did not react. A 15-residue peptide is therefore the product of roughly fifteen deprotection and coupling cycles, followed by cleavage from the resin and removal of the side-chain protecting groups.

Every impurity class below is a specific failure of one of those steps. This is why impurity profiles are reproducible for a given sequence and a given process: the same sequence tends to fail in the same place.

Deletion sequences

If a coupling cycle does not go to completion, some fraction of the chains on the resin simply do not receive that residue. Synthesis continues on them regardless, and the result is a peptide missing one internal amino acid — a deletion sequence.

On a mass spectrum a deletion is unmistakable once you know to look: the mass is lower than the target by exactly the residue mass of the missing amino acid. Glycine deletion is 57 lower, alanine 71, valine 99, proline 97, leucine and isoleucine 113 each, phenylalanine 147, tryptophan 186. A peak 97 below the target in a proline-rich sequence is a proline deletion, and no other explanation fits as neatly.

Deletions matter because a peptide missing one internal residue can be chemically similar enough to co-elute with the target and biologically different enough to change a result. They are the impurity class that a purity percentage most easily hides.

Truncation

Truncation is loss from an end of the chain rather than from the middle. It arises when synthesis stalls entirely on some fraction of chains, or during cleavage and deprotection when a terminal residue is lost.

Truncated fragments are usually easier to resolve chromatographically than deletions, because losing a terminal residue — particularly a hydrophobic one — changes retention noticeably. On the mass spectrum they appear as a series rather than a single peak, each member lower than the last by one residue mass, which is a useful tell: a ladder of masses is a truncation series, a single low mass is more likely a deletion.

Incomplete deprotection

Side-chain protecting groups have to come off at the end. When one does not, the result is a peptide of the correct sequence carrying an extra chemical group, and the mass is higher than the target rather than lower.

The common ones have characteristic mass additions, and a peak above the target mass by a value that matches a protecting group is a deprotection failure rather than a contaminant. These species are often noticeably more hydrophobic than the target and elute later, so a late shoulder on a reverse-phase trace is worth checking against this explanation.

Oxidation

Oxidation is the impurity class that can appear after synthesis, which makes it the one most relevant to storage and handling rather than to the manufacturer.

Methionine is the most readily oxidised residue: the sulfur takes on an oxygen to give the sulfoxide, and the mass increases by 16. Cysteine oxidises to form disulfide bridges, which can be intramolecular or link two chains together. Tryptophan oxidises less readily but does so, with several possible products.

A sequence containing methionine, cysteine or tryptophan therefore has an oxidation route available to it, and a lot analysed as clean at manufacture can develop an oxidation peak in storage if it has been exposed to air, light or warmth. A mass 16 above the target in a methionine-containing peptide is the single most common example. Our note on how peptides degrade covers the conditions that drive it.

Deamidation and hydrolysis

Asparagine and glutamine residues can lose their side-chain amide group and become aspartate and glutamate, with a mass increase of 1 and a change in charge. The rate depends strongly on the neighbouring residue and on pH, and it is faster in solution than in the dry state — which is one reason lyophilised material is the shipped and stored form.

A mass difference of 1 is at the edge of what a low-resolution instrument resolves, so deamidation is a class that a chromatogram often shows more clearly than a mass spectrum: the deamidated species carries an additional negative charge and its retention shifts accordingly.

Aggregation

Aggregation is not a covalent impurity. The molecules are the right molecules; they have associated into dimers, higher oligomers or, in the worst case, insoluble particulates. Sequences that are hydrophobic or beta-sheet prone are the ones prone to it.

Aggregation is the impurity class that a reverse-phase purity figure is least equipped to detect, because the organic solvent and acidic conditions of a typical run tend to break aggregates apart. A peptide can therefore run at high purity and still be substantially aggregated in aqueous buffer, which is where the assay happens. Cloudiness or visible particulates in a solution that should be clear is the practical signal, and it is a solubility observation rather than an analytical one.

What this means when reading a report

• A single impurity peak at 2% is a different situation from twenty peaks at 0.1%. The first is one identifiable species; the second is process noise.
• A mass difference matching a residue mass points to deletion. A ladder of masses points to truncation. A mass above target points to deprotection or oxidation.
• Oxidation-capable sequences — anything with methionine, cysteine or tryptophan — deserve attention to storage rather than only to the certificate.
• Aggregation will not show up as an impurity peak. It shows up in the vial.
• Purity at 214 nm and purity at 280 nm are not the same measurement, and impurities without aromatic residues are under-reported at the longer wavelength. See why two certificates can disagree.

None of this makes an impurity profile alarming by itself. Synthetic peptides are not pure substances and no supplier’s are; the question is whether the profile is understood and whether it is consistent from lot to lot. A supplier publishing the trace rather than only the figure is one you can ask that question of. See third-party vs in-house testing and how HPLC and mass spectrometry verify purity.

ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use. Certificates of analysis are published on this site.