Second pass at: Where impurities in solid-phase peptide synthesis come from

Second pass at: Where impurities in solid-phase peptide synthesis come from Writing it up because I had to work it out twice and would rather nobody else did.

A documentation question rather than an analytical one.

I have a certificate in front of me that reports a purity figure, names a technique, gives a wavelength, and stops. No gradient, no column, no injection volume, no chromatogram.

What can I legitimately conclude from that document? My instinct is "almost nothing, but not literally nothing", and I would like to know where the people who read these professionally draw the line.

Dimer and higher-order multimers: two or more peptide molecules bonded together. They appear at double the mass and higher. They may or may not separate from the monomer on HPLC depending on the method.

Deletion sequences (incomplete coupling during synthesis): lower in mass by one residue. Chromatographically they usually elute earlier or later depending on the residue's hydrophobicity. They are the most common impurity in solid-phase synthesis.

This follows post #2 rather than contradicting it.

Off-target structures: if the sequence synthesis goes wrong, a completely different amino acid can be incorporated. The resulting off-target peptide is a structural isomer with the same mass but a different sequence. No chromatographic purity method detects this without a reference standard.

On the opening post — agreed on the reasoning, with one qualification.

Acetate content: counter-ion content. Trifluoroacetate or acetate from the salt form of the peptide. Affects mass calculations and should be stated on a complete certificate.

post #5 answers the question as asked. The question underneath it is different.

Residual solvents: traces of solvents used in purification. These are usually tested by gas chromatography, not by HPLC. A specification for residual solvents should be stated separately from the purity.

Disulfide formation: if a peptide contains cysteine, it can form disulfide bonds with itself or with other molecules. Under oxidising conditions multiple species appear. Reducing conditions (like DTT) convert them back.

I disagree with the reply above, and I think the disagreement is substantive rather than terminological.

The distinction being drawn does not survive when you look at the published data for this specific question. I would be glad to be shown wrong on this, because the version I am arguing against is more convenient.

Related substances: compounds chemically related to the target peptide but not the target peptide itself. The standard method separates them and reports them as area percent. How related they can be before they exceed specification is a regulatory question.