Why a "Pure" Peptide Can Still Be Missing an Amino Acid

Why a "Pure" Peptide Can Still Be Missing an Amino Acid

A clean purity number tells you how much of one HPLC peak is in the vial. It does not always tell you whether that peak is the complete chain, or a near-twin missing a single residue.

Imagine a lab ordering a short research peptide, 30 amino acids in a precise order. The certificate reports 98% purity by HPLC. That sounds clean enough to trust.

But what if one coupling step in the synthesis quietly failed, and the vial actually holds a mixture of the full 30-amino-acid chain and a 29-amino-acid twin that is missing one link in the middle?

That is not a rare edge case. It is the reason deletion sequences are among the most closely watched impurities in synthetic peptides.

A missed coupling, not a broken vial

Solid-phase peptide synthesis builds a chain one amino acid at a time on a solid bead. Each cycle attaches one residue to the growing end.

If a single coupling fails to attach, the chain simply does not grow that round. The next cycle adds the following amino acid anyway, and the finished product ends up one residue short. That is a deletion sequence.

Bachem, one of the world's largest peptide manufacturers, states it directly. After synthesis, "usually a few peptidic contaminants remain, mostly deletion sequences (peptides lacking one or more amino acids of the target sequence)."

Two quiet ways a link goes missing

Deletions form in at least two ways.

The obvious one is an incomplete coupling, where the activated amino acid fails to fully react and the chain skips a beat. The second is incomplete Fmoc deprotection. If the temporary protecting group on the chain's growing end is not removed cleanly, that spot cannot accept the next residue, and the peptide advances one step short. Bachem describes these as "point deletions caused by incomplete Fmoc-deprotection."

Chemists push back with double couplings and a step called capping. After each coupling, unreacted chain ends are capped with an acetyl group so they stop growing. That converts would-be deletions into shorter, capped truncations that are easier to separate out during purification.

Why the number still looks fine

Here is the part that trips people up.

A deletion sequence is chemically almost identical to the target chain. It has the same two ends, nearly the same overall shape, and nearly the same mass. The only difference is one missing amino acid.

That similarity is exactly what reversed-phase HPLC struggles with. HPLC sorts molecules by how long they stick to a column, and a peptide down by a single residue can sit right on top of, or immediately beside, the full-length chain. An area-percent purity number, which sums the light absorbed by each peak, can then blend the two into one tidy "98% pure" reading.

The arithmetic gets worse over longer chains. Even at 99.5% success per coupling step, a 30-residue chain has only about an 86% chance of getting all 30 residues right. The other 14% are chains missing something, before any purification or confirmation happens. That figure is arithmetic, not a lab measurement, but it shows why longer sequences accumulate more of these near-twins.

What "full length" actually requires

So how do you confirm the chain is complete? You reach for a second, independent method.

Mass spectrometry is the workhorse. A mass reading that matches the target's theoretical molecular weight confirms the right overall size, and a high-resolution instrument can catch the small mass shifts a missing residue leaves behind. Tandem mass spectrometry goes further, reading the sequence in fragments so a skipped position shows up as a gap.

Amino acid analysis is a third check. It breaks the peptide into its individual residues and counts them, and a missing amino acid shows up as a ratio that reads too low for one spot.

All three lead to the same point: a purity number alone is a snapshot, and it does not prove the chain is intact.

Peptides caught mid-stride in the lab

Metrology labs bump into this constantly when they assign reference values to peptide standards.

In one study of synthetic human C-peptide, researchers identified and quantified more than 65 structurally related impurities using liquid chromatography with high-resolution mass spectrometry. Their conclusion was direct: any purity assignment needs an explicit correction for these related impurities, or the number is biased.

The takeaway is not that synthetic peptides are failing. It is that purity and identity are two different tests, and a research COA that reports only the first leaves the second unanswered.

Across the East Bay and the wider Bay Area, this is also a local story. The analytical instruments and contract labs that run these confirmations are built and operated right here, from Bio-Rad's design and manufacturing campus in Hercules, a short drive from Walnut Creek, to the mass-spec cores at the region's research universities.

One question to ask on the next COA

The next time a research COA arrives with a single glowing percentage, ask what else came with it.

Two documents are worth requesting: the mass spectrum that confirms the molecular weight, and the chromatogram that shows whether that one big peak is really one clean peak.

For any research peptide, the more useful half of the story is not how pure it is. It is whether the chain the supplier claims is the chain they actually made.

For research use only. Not for human or veterinary consumption.

Sources

• Bachem, "FAQ - frequently asked questions" (March 2021). https://www.bachem.com/knowledge-center/faq-frequently-asked-questions/

• Bachem, "A New Turn in Peptide Purification - Peptide Easy Clean" (February 2021). https://www.bachem.com/knowledge-center/a-new-turn-in-peptide-purification-peptide-easy-clean/

• Coin, I., Beyermann, M., & Bienert, M. "Solid-phase peptide synthesis: from standard procedures to the synthesis of difficult sequences." Nature Protocols 2, 3247-3256 (2007). https://doi.org/10.1038/nprot.2007.454

• Li, M., et al. "Identification and accurate quantification of structurally related peptide impurities in synthetic human C-peptide by liquid chromatography-high resolution mass spectrometry" (2018). PMID 29862433. https://pubmed.ncbi.nlm.nih.gov/29862433/