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Why is peptide purity never 100%?

In brief

Because that's how synthesis works. The chain is built through dozens of sequential reactions, and none runs at exactly 100% yield: chains with skipped links and truncated fragments inevitably remain in the material. Purification shrinks their share — never to zero. And "99% purity" doesn't mean "99% peptide by weight": water and counterions legitimately sit in the powder's mass. Two different numbers — and confusing them only profits the seller.

Illustration for the research article

Arithmetic versus the ideal

Solid-phase synthesis is an assembly line of dozens of cycles, each running at slightly under unity. Even at an excellent 99% per cycle, a thirty-link chain comes out fully correct only about three-quarters of the time. The rest are deletion sequences missing a link, and truncated stubs.

The treachery is that these impurities are near-twins of the target molecule, and chromatographically separating near-identical chains is delicate work. The composition of such mixtures — and how mass spectrometry cracks it open — is mapped in Metzger's classic paper.

What "purity" actually measures

HPLC purity is the target peak's share of total peak area on the chromatogram. The methodology and its limits are covered in the standard references. Two limits matter: the detector doesn't see everything (water and inorganics don't light up in UV), and area percent is not mass percent.

Hence a second, separate number — peptide content: how much actual peptide is in a milligram of powder. Lyophilisate routinely contains bound water and counterions — most often residues of trifluoroacetic acid, standard synthesis's travelling companion. Material at 99% purity can hold, say, 80% peptide by mass — and that's not fraud but the norm, provided both numbers are honestly stated. More in our "purity vs quantity" article.

Reading a certificate like an adult

Look at a trio, not one number: HPLC purity with a visible chromatogram, mass confirmation by MS, peptide content. "100%" in any field is cause for questions, not celebration: instruments don't work at that precision, and an honest laboratory doesn't write it.

Our position is simple: every number in documentation must be explainable by the method that produced it. For what that means in practice, see the testing and lab-document sections.

References

  1. Electrospray Mass Spectrometry and Tandem Mass Spectrometry of Synthetic Multicomponent Peptide Mixtures: Determination of Composition and Purity — Analytical Biochemistry, 219(2), 261–277 (PMID 8080083; DOI 10.1006/abio.1994.1266), 1994.
  2. Reversed-Phase High-Performance Liquid Chromatography in Peptide and Protein Analysis — Encyclopedia of Analytical Chemistry (DOI 10.1002/9780470027318.a1631), 2000.