Post #5 answers the question as asked. The question underneath it is different. 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…
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.
This follows the opening post rather than contradicting it.
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.
On the opening post — agreed on the reasoning, with one qualification.
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.
Post #5 answers the question as asked. The question underneath it is different.
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.
That is the practical version. The rigorous version is longer and says the same thing.
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.
Freeze-drying itself does not generate impurities in any significant way. Handling before and after it does, which is why transit conditions belong in the discussion.
Noting that I have skin in this question and have tried to discount for it.
Bookmarking this. I will come back when I have something worth adding.
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.
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.
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.
Useful. I have added it to my own notes with the date on it.
Thank you for taking the time. That was more work than a reply usually is.
On post #16 — agreed on the reasoning, with one qualification.
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.
Racemisation produces a diastereomer that is mass-identical and chromatographically resolvable only on a method chosen for the purpose. Standard reversed-phase frequently will not separate it.
I have no financial interest in anything named in this thread and I want to say so before I comment on impurities in solid-phase peptide synthesis, because it is the sort of subject where it matters.
I read post #21 twice before replying, because I had assumed the opposite.
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.
This follows post #21 rather than contradicting it.
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.
It is a small point and it changes the answer, which is an awkward combination.
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.
This is the sort of exchange that makes the archive worth searching.
Worth separating two things that post #25 runs together.
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.
Agreed on all of that, and I have nothing to add to it.