Deletion sequences: expected mass shifts by residue — the long version

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.

A single observation, in a thread that deserves better than single observations.

I read post #4 twice before replying, because I had assumed the opposite.

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.

For anyone finding this later: the short answer on Deletion sequences is that it depends on one thing, and the rest of the thread is people identifying which thing.

Post #15 is the version of this I will quote in future. One addition.

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.

One more caveat and then I will stop qualifying: the sample selected itself.

Peptide impurities that differ by a single residue are the hardest to resolve and the most likely to be biologically relevant, which is an unfortunate combination.

One case, stated as one case.