Common Peptide Impurities: Deletions, Truncations and Adducts

Every synthetic peptide carries a fingerprint of the chemistry that made it. The main peak on a chromatogram is the target sequence, but the small peaks around it are rarely random: they are common peptide impurities with predictable structures, predictable mass shifts and predictable origins in solid-phase synthesis. This article catalogues the main classes (deletions, truncations, insertions, protecting-group adducts, side-reaction products and diastereomers), gives the mass differences that identify them, and walks through how a real GHRH analog sequence maps onto those risks.

Where common peptide impurities come from

Most research peptides are assembled by Fmoc solid-phase peptide synthesis (SPPS), a cycle of deprotection, washing and coupling repeated once per residue [2,3]. Each step is highly efficient but not perfect, and each reagent can react where it should not. A systematic review of impurities in peptide medicines grouped them into synthesis-related species (deletions, insertions, diastereomers, protection adducts) and degradation products (oxidation, deamidation, aggregation), and noted that such impurities can distort early functional studies if they are not controlled [1]. The same logic applies to any peptide used in an assay: knowing what the minor peaks are tells you whether they matter.

For background on the chemistry itself, see our overview of solid-phase peptide synthesis.

Deletion sequences

A deletion sequence (often written "des-Xaa" or "n-1") lacks one internal residue. It arises when a coupling does not go to completion, or when Fmoc removal is incomplete so that one cycle is skipped for part of the resin population; that chain then continues growing normally in the next cycle [1].

Deletions are the single most common impurity class in crude synthetic peptides because inefficiency compounds. At 99% per-step efficiency, a 29-residue peptide requires 28 couplings, and only about 0.99^28, or roughly 75%, of chains are full length before any other loss.

Deletions are identified by a mass deficit equal to one residue:

Missing residue	Monoisotopic mass shift (Da)
Gly	-57.02
Ala	-71.04
Ser	-87.03
Leu / Ile	-113.08
Gln	-128.06
Lys	-128.09
Arg	-156.10

Two cautions apply. Leu and Ile are isobaric, so a "-113" peak does not say which one is missing. And Gln and Lys differ by only 0.036 Da, which a low-resolution instrument cannot separate. Locating a deletion within the sequence requires MS/MS fragmentation, discussed in mass spectrometry and identity confirmation.

Truncations and capped sequences

A truncated sequence is a chain that stopped growing. Many synthesis protocols deliberately cap unreacted amines with acetic anhydride after a difficult coupling, converting would-be deletion sequences into acetylated truncations [3]. The logic is practical: a short, capped fragment is usually much easier to separate by preparative HPLC than an n-1 deletion that differs from the target by one residue.

Capped truncations carry an N-terminal acetyl group (+42.01 Da relative to the free-amine fragment) and are missing the entire N-terminal portion of the sequence, so their masses fall well below the target. Truncations can also arise from chain loss during synthesis, for example diketopiperazine formation at the dipeptide stage, which cleaves the first two residues from the resin [2].

Insertions

Insertion impurities contain an extra copy of a residue (+ one residue mass). They are less common than deletions and arise from excess activated amino acid or from premature Fmoc loss, which lets two residues couple in a single cycle [1]. An "n+1" peak with the mass of the preceding residue is the tell-tale sign.

Protecting-group and reagent adducts

Side-chain protecting groups are designed to come off during the final trifluoroacetic acid (TFA) cleavage, but not all do so cleanly, and the carbocations they release can re-attach elsewhere [1,2]. Useful mass shifts to recognise:

• tert-butyl (tBu) adduct: +56.06 Da, from incomplete deprotection of Ser, Thr, Tyr, Asp or Glu, or from alkylation of Trp, Met or Cys by released tBu cations.
• Pbf retained on Arg: +252.08 Da, typical of Arg-rich sequences with short cleavage times.
• Trityl retained: +242.11 Da, seen with Cys, His, Asn or Gln.
• Trifluoroacetylation: +95.98 Da on a free amine.
• Acetyl capping: +42.01 Da.

These are covalent species and represent real impurities. By contrast, sodium (+21.98 Da relative to [M+H]+) and potassium (+37.96 Da) adduct ions usually form in the electrospray or MALDI source; they are artefacts of ionisation, not separate molecules in the vial. How the ionisation method changes what you see is covered in LC-MS vs MALDI-TOF for peptide analysis.

Side-reaction products: aspartimide, oxidation and deamidation

Some impurities come from sequence-specific chemistry rather than incomplete steps.

Aspartimide formation. Under the basic conditions of Fmoc removal, an aspartate side chain can cyclise onto the backbone amide to form a succinimide (-18.01 Da). The ring then opens to a mixture of alpha- and beta-aspartyl peptides (same mass as the target, different retention) or reacts with piperidine to give piperidides (+67.07 Da). A systematic study of the model series H-Val-Lys-Asp-Xaa-Tyr-Ile-OH showed that the extent depends strongly on the residue following Asp, with considerable by-product levels for several Xaa residues including Asn, Arg and Asp itself [4].

Oxidation. Methionine oxidises to the sulfoxide (+15.99 Da); tryptophan and free cysteine are also susceptible.

Deamidation. Asparagine, and more slowly glutamine, lose ammonia via a cyclic intermediate to give Asp or isoAsp (+0.98 Da). Degradation pathways of this kind are the subject of our article on deamidation and peptide degradation.

Diastereomers: the impurities mass spectrometry cannot see

Racemization during amino acid activation, most notably at Cys and His, produces peptides with one inverted stereocentre [1]. These diastereomers have exactly the same mass and composition as the target. They are visible only as separate or shouldering peaks in reversed-phase HPLC, which is one reason identity (MS) and purity (HPLC) must be read together. See HPLC purity testing for research peptides for how resolution and gradient slope affect whether a diastereomer is separated from the main peak.

Worked example: a modified GHRH(1-29) sequence

CJC-1295 without DAC, also known as modified GRF(1-29), is a 29-residue amidated analog of the N-terminal fragment of growth hormone-releasing hormone. Relative to native hGRF(1-29) it carries four substitutions: D-Ala at position 2, Gln at 8, Ala at 15 and Leu at 27. Several of these read almost like an impurity-control checklist:

• Asn8 to Gln. The native Asn8-Ser9 motif is a deamidation hot spot. In a study of a related hGRF analog incubated at pH 7.4 and 37 C, the major degradants were alpha- and beta-Asp8 products of Asn8 deamidation, and replacing Asn8 with Ser markedly extended the half-life in buffer [5]. Gln deamidates far more slowly than Asn.
• Met27 to Leu. Removing methionine eliminates the +16 Da sulfoxide pathway.
• D-Ala2. Deliberately introduces a D-residue, which means the L-Ala2 epimer is a possible same-mass impurity that only chromatography can reveal.

Other features still predict specific impurities. The two Asp residues are followed by Ala and Ile rather than Gly, a lower-risk context for aspartimide formation [4], but not zero risk. Three Arg residues make retained Pbf (+252 Da) worth checking. And the repeated Leu-Leu and Lys motifs mean a -113 or -128 deletion peak cannot be assigned to a position without fragmentation data.

Per-lot chromatograms and mass spectra for this and other sequences are posted on our lab reports page, and reading them with this table in hand makes the minor peaks far less mysterious.

Key takeaways

• Deletions (-1 residue) are the most common SPPS impurity because per-step inefficiency compounds over the chain length.
• Truncations are chains that stopped growing, often acetyl-capped (+42 Da) by design to make them easier to remove.
• Covalent adducts (tBu +56, Pbf +252, Trt +242, TFA +96) are real impurities; Na+ and K+ adduct ions usually are not.
• Aspartimide (-18), oxidation (+16) and deamidation (+1) products are sequence-dependent and predictable.
• Diastereomers have identical mass, so HPLC purity and MS identity must be read together.

This article summarizes published research for informational purposes. All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption.

References

1. D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. PubMed
2. Behrendt R, White P, Offer J. Advances in Fmoc solid-phase peptide synthesis. J Pept Sci. 2016;22(1):4-27. PubMed
3. Coin I, Beyermann M, Bienert M. Solid-phase peptide synthesis: from standard procedures to the synthesis of difficult sequences. Nat Protoc. 2007;2(12):3247-3256. PubMed
4. Mergler M, Dick F, Sax B, et al. The aspartimide problem in Fmoc-based SPPS. Part II. J Pept Sci. 2003;9(8):518-526. PubMed
5. Friedman AR, Ichhpurani AK, Brown DM, et al. Degradation of growth hormone releasing factor analogs in neutral aqueous solution is related to deamidation of asparagine residues. Replacement of asparagine residues by serine stabilizes. Int J Pept Protein Res. 1991;37(1):14-20. PubMed