Common peptide impurities and where they come from

A deletion sequence is the most common SPPS impurity. It arises when a single coupling cycle in solid-phase synthesis fails completely — the activated amino acid does not bond to the resin-anchored chain — and subsequent cycles continue as normal. The result is a peptide identical to the target except missing one residue.

LC-MS signature. A satellite peak at the target mass minus the residue mass:

• −57.02 Da for Gly
• −71.04 Da for Ala
• −87.03 Da for Ser
• −99.07 Da for Val
• −113.08 Da for Leu or Ile
• −128.09 Da for Lys or Gln

HPLC behavior. Deletions of hydrophobic residues (Leu, Ile, Val) shift retention time noticeably and are usually separable. Deletions of small or polar residues (Gly, Ala, Ser) often co-elute and require LC-MS to detect. This is why HPLC purity alone can mask a real deletion problem.

Implications. A deletion impurity is a different molecule with potentially different biological activity. A receptor-binding peptide missing a critical residue may be inactive, partially active, or in some cases an antagonist of the original.

A truncation occurs when the growing chain is permanently capped — for example, by acetylation of a free amine during a coupling failure — and cannot continue. The result is a shorter peptide consisting only of residues already added.

LC-MS signature. A peak with mass much lower than the target, equal to the partial sequence's theoretical mass.

HPLC behavior. Truncations are typically less hydrophobic than the target and elute earlier in a reverse-phase gradient. They are usually well-separated and removed by purification — but residual truncation in a "purified" peptide is a sign of insufficient preparative chromatography.

Racemization is the loss of stereochemistry at the alpha-carbon of an amino acid, converting an L-residue to its D-isomer. It happens during the activation step of coupling and is most common at certain residues:

• Cysteine — high racemization risk; often coupled with specialized reagents (DIC + Oxyma, low temperature) to suppress it.
• Histidine — the imidazole side chain promotes racemization; protected with bulky Trt or Mtt groups to mitigate.
• Serine and threonine — moderate risk; sensitive to base concentration during coupling.

Detection. The D-isomer has identical mass to the L-form, so it is invisible to LC-MS. It often co-elutes by reverse-phase HPLC. Detecting it requires either chiral chromatography, GC-MS of derivatized hydrolysate, or specific D/L analysis after acid hydrolysis.

Implications. Most peptide receptors are stereospecific. A few percent D-isomer at a critical residue can substantially reduce biological activity. This is why a "99% pure by HPLC" peptide can still underperform if racemization went undetected.

Three residues are particularly prone to oxidation:

• Methionine oxidizes to methionine sulfoxide (+16 Da, partially reversible) and further to methionine sulfone (+32 Da, irreversible). Even ambient air during sample prep can introduce sulfoxide.
• Cysteine oxidizes to cystine (intramolecular or intermolecular disulfide, −2 Da per pair). For non-disulfide peptides this is an undesired modification; for disulfide peptides it is the target.
• Tryptophan oxidizes to oxindolylalanine (+16 Da) and further oxidation products. Trp is also damaged by reactive cations during cleavage if scavengers are insufficient.

Mitigation. Oxidation is reduced by lyophilizing under inert atmosphere, storing under nitrogen or argon, protecting from light, and avoiding repeated freeze-thaw cycles in solution.

Deamidation is the spontaneous conversion of asparagine (Asn) to aspartate (Asp) and of glutamine (Gln) to glutamate (Glu). It is a +1 Da satellite (NH₂ → OH). Deamidation rate depends on the neighboring residue (Asn-Gly is the fastest), pH, and temperature. It is one of the dominant degradation pathways during long-term storage in solution.

Aspartimide is a closely related side reaction: an Asp side chain cyclizes onto the backbone amide of the next residue, forming a 5-membered succinimide ring (−18 Da). The succinimide hydrolyzes back to a roughly 3:1 mixture of beta-aspartate and alpha-aspartate — the beta-form is a real, biologically distinct impurity that often co-elutes with the parent. Asp-Gly sequences are particularly prone, and aspartimide is one of the classic difficult-sequence challenges in SPPS.

Cysteine-containing peptides can dimerize through intermolecular disulfide bond formation. A dimer appears in LC-MS at exactly twice the monomer mass minus 2 Da (loss of 2H from the disulfide formation). Dimers usually elute later than the monomer due to increased hydrophobicity and are visible as distinct peaks on HPLC.

Beyond cysteine, peptides with hydrophobic stretches can aggregate non-covalently — producing soluble or insoluble higher-order assemblies. Aggregation is concentration- and pH-dependent and is a particular concern for amyloidogenic sequences. Aggregates are often detected by size-exclusion chromatography (SEC) rather than reverse-phase HPLC.

For recombinant proteins like somatropin, aggregate content is the dominant immunogenicity risk on an injectable label and SEC-HPLC is the required stability-indicating method — see the HGH testing methodology guide for how SEC dimer/aggregate limits and HCP ELISA fit together on a recombinant-protein release.

Most "impurities" by mass in a finished peptide vial are not peptide-related at all. They are:

• Counter-ion — TFA, acetate, HCl, or other — typically 5 to 20% of total dry mass.
• Water — lyophilized peptides typically retain 5 to 15% bound water.
• Residual organic solvents — acetonitrile from RP-HPLC, DMF from synthesis, diethyl ether from precipitation. Pharmacopoeia specifies limits for each (USP <467>).

None of these are caught by HPLC purity at 214 nm. They are caught by ion chromatography (counter-ion), Karl Fischer titration (water), and headspace GC (residual solvents) — which is why a complete COA reports each separately.

Most of the impurity categories above are invisible to a single-method test. A "99% HPLC" stamp on a vendor COA tells you nothing about racemization, deamidation, residual solvents, water, or counter-ion content. An honest characterization uses orthogonal methods that each see what the others cannot, and reports the gaps when they exist.