What is Peptide purity?
Also called: HPLC purity, Chromatographic purity
The proportion of peptide-related material in a sample that is the intended sequence, normally reported as the main peak's share of total HPLC peak area.
By the APL Research Team · Updated
Peptide purity answers one question: of the peptide-like material in a sample, how much is the target sequence? It is measured by reversed-phase HPLC with UV detection near 214 nm and reported as the main peak's percentage of total integrated peak area. A result of 99% means the impurities visible to that method add up to about 1% of the signal.
Where the other 1% comes from
Most synthetic peptides are made by solid-phase peptide synthesis, and a review of peptide medicines sorted their related impurities into those created during synthesis and those formed by later degradation [1]:
| Impurity | Origin | Usually detected by |
|---|---|---|
| Deletion sequence | Incomplete Fmoc removal or coupling | HPLC and MS (one residue missing) |
| Insertion sequence | Excess amino acid reagent | HPLC and MS (one residue extra) |
| Diastereomer | Racemisation during Fmoc removal | HPLC only; the mass is unchanged |
| Protecting-group adduct | Incomplete side-chain deprotection | HPLC and MS |
| Oxidised peptide | Reaction at susceptible side chains | HPLC and MS (+16 Da per oxygen) |
| Succinimide, pyroglutamate, diketopiperazine | Degradation of susceptible sequences | HPLC and MS |
| Dimers and oligomers | Covalent or non-covalent association | HPLC; size-exclusion for non-covalent forms |
| Unrelated peptides | Cross-contamination during manufacture | HPLC and MS |
The same review warns that such impurities can influence early functional studies enough to produce erroneous conclusions [1]. A hypothetical case shows why: a deletion analogue making up 1% of the material but ten times more potent at the receptor would contribute roughly a tenth of the measured activity.
Worked example: 98% versus 99%
Purity grades are easier to compare in milligrams. Take a 10 mg vial whose powder is 80% peptide by weight, i.e. 80% net peptide content:
| HPLC purity | Peptide material | Target sequence | Related impurities |
|---|---|---|---|
| 95% | 8.0 mg | 7.60 mg | 0.40 mg |
| 98% | 8.0 mg | 7.84 mg | 0.16 mg |
| 99% | 8.0 mg | 7.92 mg | 0.08 mg |
Going from 98% to 99% halves the impurity load, which matters when impurities could be active, but changes the amount of target peptide by about 1%. By contrast, switching the same peptide between trifluoroacetate and acetate salt forms shifts its peptide fraction by 6–12 percentage points for the sequences worked through under counter-ion. For the accuracy of a stock concentration, net content is the bigger lever; for confidence that an effect belongs to the target, purity is.
Common misunderstandings
- "99% pure" is not "99% peptide". Water and counter-ions do not appear in the chromatogram, so they are not in the denominator.
- Purity is method-specific. A different gradient, column or wavelength can give a different number for the same batch, and an impurity hidden under the main peak counts as product.
- Purity is not identity. A clean single peak of the wrong peptide would read 100%; mass spectrometry confirms the molecule.
- Purity is a snapshot. Oxidation, deamidation and aggregation continue after testing, faster in solution than in the dry solid.
The chromatogram entry shows the percentage being calculated from a peak table, and the peptide purity guide compares grades and what each suits.
References
- 1.D'Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014. PubMed 25044089