Is 97.6% a realistic purity ceiling for cagrilintide given its chain length?

Stated plainly: 97.6% · cagrilintide.

The units are where I keep going wrong, so please be explicit about them.

I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.

How many significant figures are actually justified here?

97.6 per cent is at the top of what stepwise synthesis delivers on a chain this long, and it is reachable rather than fictional. Every coupling is high-yielding and none is quantitative, so the deletion and truncation sequences that survive purification are what occupies the remaining 2.4 per cent. Above roughly 98 per cent you are fighting the purification rather than the synthesis, which is why a 97.6 per cent figure on cagrilintide deserves a method question — column, gradient, wavelength — rather than either belief or dismissal.

Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

Compare purity within a single laboratory on the same method, never across laboratories.

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

Integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.

The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

Gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.

Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.

Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.

If you are ranking vendors, specify a method and have all samples tested at the same place.