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.
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 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.
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.
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.
Post #38 put the caveat in the right place and I want to underline it.
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.
For what it is worth, the same held on the two occasions I checked.
This follows post #50 rather than contradicting it.
Incomplete deprotection leaves a protecting group attached, raising the mass substantially and usually pushing retention much later. A late-eluting peak on a peptide chromatogram is worth asking about.
The part I am sure of is shorter than the part I have written.
Read the full topic (60 posts)
| Topic | Participants | Replies | Views | Activity |
|---|---|---|---|---|
|
Deamidation and the close-eluting pair it produces — the long version
Posting this under the heading it deserves: Deamidation and the close-eluting pair it produces — the long version Everything below is what sits behind that. A narrow question about Deamidation, deliberately…
|
3 | 347 | 24d | |
|
Follow-up: Trifluoroacetate content and its consequences
On the subject in the title: Trifluoroacetate content and its consequences Working notes rather than a conclusion. A follow-up question about Trifluoroacetate content that I did not know to ask the first…
|
+12 | 16 | 5.7k | 21mo |
|
Attributing a new peak: degradation product or excipient?
Asking directly, because I could not find a straight answer: Attributing a new peak: degradation product or excipient? A narrow question about attributing a new peak, deliberately narrow, because the broad…
|
2 | 15k | 18mo | |
|
Revisiting: Aggregates and why a purity assay may miss them entirely
Posting this under the heading it deserves: Revisiting: Aggregates and why a purity assay may miss them entirely Everything below is what sits behind that. I would like to disagree carefully with the settled…
|
+48 | 53 | 918 | 8mo |
|
[2026 update] A rising related-substance total over six months: degradation or method drift?
The question in the title: A rising related-substance total over six months: degradation or method drift? I will give what I have already checked below so nobody repeats it. Posting the method first, because…
|
+71 | 78 | 34k | 5mo |
| Topic | Participants | Replies | Views | Activity |
|---|---|---|---|---|
|
Carryover and the ghost peak from last week's standard — what changed since
Carryover and the ghost peak from last week's standard — what changed since Writing it up because I had to work it out twice and would rather nobody else did. What changes if the standard account of Carryover…
|
+98 | 114 | 37k | 22d |
|
Deletion sequences: expected mass shifts by residue — the long version
On the subject in the title: Deletion sequences: expected mass shifts by residue — the long version Working notes rather than a conclusion. Deletion sequences keeps being re-asked here and I think that is…
|
+21 | 25 | 6.9k | 10h |
|
Revisiting: Why I ask for the gradient before I discuss the number
Posting this under the heading it deserves: Revisiting: Why I ask for the gradient before I discuss the number Everything below is what sits behind that. I would like to understand what this number means…
|
+15 | 19 | 43k | 10mo |
|
Blinding yourself: practical methods and their limits
On the subject in the title: Blinding yourself: practical methods and their limits Working notes rather than a conclusion. A methods question rather than a substantive one, about blinding yourself. Everyone…
|
+40 | 44 | 33k | 5d |
|
Trifluoroacetate content and its consequences
Trifluoroacetate content and its consequences — setting out what I have, and where I think it stops being reliable. What changes if the standard account of trifluoroacetate content is wrong? I ask because I…
|
+123 | 134 | 31k | 5mo |