Where the impurities in solid-phase peptide synthesis reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
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.
Impurities in solid-phase peptide synthesis, and specifically the version of it that the documentation does not cover. The maintained page handles the general case well and stops exactly where my question starts.
Setting out the gap in case it is a gap in the page rather than a gap in what is known.
Comparing impurity profiles across suppliers is much more informative than comparing purity figures, and almost nobody has the documents to do it.
I would want to see it done twice before believing it once.
The opening post answers the question as asked. The question underneath it is different.
Aggregates may be a multiple of the monomer mass and may not elute at all under a standard method. What does not come off the column does not appear in the area percentage.
Reading it back, the second half matters more than the first.
Worth separating two things that the opening post runs together.
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.
I would hold that lightly until someone with a larger sample weighs in.
Where the impurities in solid-phase peptide synthesis reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
Incomplete deprotection: mass higher by the protecting group mass. Usually markedly later eluting. A synthesis artifact from incomplete removal of protecting groups.
Adding it in case it saves somebody the afternoon it cost me.
Aggregates: multiples of the monomer mass. May not elute at all under a standard reversed-phase method. A species that does not come off the column does not appear in the area percentage.
Noting that I have skin in this question and have tried to discount for it.
Confirming post #11 from a second method, which matters more than confirming it from a second person.
Relative response factors mean impurities are not detected in proportion to how much of them is present. A one per cent peak is not one per cent by mass unless the response factors happen to match.
On impurities in solid-phase peptide synthesis I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
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.
I am aware this is the third time this month I have made this point.
The honest answer on impurities in solid-phase peptide synthesis is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
The related-substances total is only as informative as the method that generated it. Two per cent by a method that resolves everything is a different statement from two per cent by a method that resolves little.
Reporting the observation and leaving the explanation open deliberately.
The arithmetic in post #18 is right; the assumption feeding it is the part to check.
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.
Answering the question post #16 raises rather than the one it answers.
Adding a reference point for impurities in solid-phase peptide synthesis. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.
Practical answer on impurities in solid-phase peptide synthesis, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
Oxidation at methionine or tryptophan adds sixteen per oxygen and typically elutes earlier. In aged material it is the modification that grows.
Everything in post #21 holds. The case it does not cover is the one I have.
Truncation products: fragments from incomplete synthesis or from degradation. They elute quite differently from the intact peptide because they are much smaller and have different hydrophobicity. They are usually well separated.
Somebody will have a better source than mine, and I hope they post it.
Fine by me. I had wanted a stronger conclusion and there is not one available.
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.
Post #25 answers the question as asked. The question underneath it is different.
I would call the community position on impurities in solid-phase peptide synthesis likely rather than established, and I would be comfortable defending that hedge.
I had written a reply contradicting post #25 and deleted it. Here is what survived.
Where an impurity is identified rather than merely counted, the certificate is telling you the manufacturer has characterised its own process. That is a meaningful difference in documentation quality.
It reads as pedantry until the day it does not.
Confirming post #29 from a second method, which matters more than confirming it from a second person.
Deletion sequences arise from incomplete coupling and are lower by one residue mass. Where they elute depends on the hydrophobicity of the residue that is missing, so they can appear on either side of the main peak.
Posting it because the silence on this was starting to look like agreement.