A purchasing analyst is filling in a supplier comparison sheet. Two catalogs list what looks like the same peptide under the same name. One gives the sequence ending in “-NH2” with a molecular weight a little under a round number; the other gives the bare sequence and a weight about one unit higher. Is that a typo, a rounding difference, or two different molecules? More often than not it is two different molecules, and the difference lies at the very ends of the chain. Terminal modifications are among the smallest structural changes a peptide can carry, and among the easiest to overlook.
Reading the ends of a sequence
By convention a peptide sequence is written from the N-terminus on the left to the C-terminus on the right. If nothing else is shown, the reader is supposed to assume a free amine (-NH2 on the alpha carbon of the first residue, protonated at neutral pH) and a free carboxylic acid (-COOH on the last residue). Two common additions change that:
- “Ac-” at the start means the N-terminal amine carries an acetyl group.
- “-NH2” at the end means the C-terminal carboxyl has been replaced by a carboxamide.
The weakness is that not everyone writing a sequence follows the convention. Some catalogs drop the notation because it looks untidy, and trade names say nothing about the termini at all. A bare sequence may mean “free acid and free amine” or may simply mean nobody recorded the caps. The more reliable clue is the calculated molecular mass, because it changes with each modification.
The two caps side by side
| Feature | N-terminal acetylation | C-terminal amidation |
|---|---|---|
| Notation | Ac-XXXX | XXXX-NH2 |
| Chemical change | Acetyl group (CH3CO-) added to the terminal amine | Terminal -OH of the carboxyl replaced by -NH2 |
| Monoisotopic mass change | +42.011 Da | -0.984 Da |
| Charge removed | The positive charge of the free amine | The negative charge of the carboxylate |
| Typical synthetic route | Acetic anhydride on the resin after the final deprotection | Synthesis on an amide-generating resin, such as Rink amide |
Both modifications occur naturally. Acetylation of the N-terminus is very common among eukaryotic proteins, and C-terminal amidation is characteristic of many short signaling peptides. In synthesis they are reproduced deliberately whenever the target structure calls for them.
Why a single dalton makes it a different compound
It is tempting to regard an amidated peptide and its free-acid form as the same thing with a minor tag. Analytically they are not. Removing a charged terminus changes the net charge of the molecule at a given pH, which in turn moves its isoelectric point and the pH range in which it is least soluble. Retention in reverse-phase HPLC often shifts as well, so the two forms can appear as separate peaks, and a free-acid sequence can show up as an impurity in an amidated product or the other way round. Each form has its own molecular formula and, where one has been assigned, its own registry entry.
There is also a structural reason these caps appear so often in designed analogs. Exopeptidases need a free terminus to begin removing residues: aminopeptidases act at a free N-terminus, carboxypeptidases at a free C-terminal carboxyl. A capped end is a poor substrate for them. That is a statement about enzyme chemistry in general, not about how any particular material behaves outside the laboratory.
Two examples from the catalog
Sermorelin corresponds to the first 29 residues of growth hormone-releasing hormone and carries a C-terminal amide; the free-acid version of that sequence is a distinct compound (see what sermorelin is). Thymosin alpha 1 is a 28-residue sequence with an acetylated N-terminus, so its formula includes the extra acetyl group (see what thymosin alpha 1 is). In both cases the full name alone does not tell you about the cap; the sequence notation or the formula does.
Where terminal changes show up as impurities
Terminal chemistry is also a source of related substances. Three situations are worth knowing:
- Capped truncations. Many solid-phase protocols include a capping step with acetic anhydride to block chains that failed to couple. Those shortened chains then carry an acetyl group and appear as acetylated truncated species. More on synthesis by-products is in peptide synthesis impurities.
- Amide hydrolysis. A C-terminal amide can hydrolyze back to the free acid, adding 0.984 Da. That is the same mass change as deamidation of an internal asparagine or glutamine, so mass alone cannot tell the two apart; fragmentation data or retention behavior is needed.
- Wrong resin, right sequence. A synthesis run on an acid-generating resin when an amide was intended gives a product that is correct in every residue and still the wrong compound.
Confirming the termini analytically
An acetyl group is easy to spot by mass: 42 Da is well beyond the resolution of any routine mass spectrometer. The one-dalton amide question is more demanding. The observed and calculated masses have to be compared like with like, monoisotopic against monoisotopic or average against average. Mixing the two can create or hide a one-dalton discrepancy, especially for larger peptides where the monoisotopic peak is small. The instrument also needs enough resolution to assign the isotope pattern correctly. For more on how molecular mass supports identity, see mass spectrometry and peptide identity.
A short checklist for anyone comparing listings:
- Check the sequence notation at both ends for Ac- or -NH2.
- Compare the stated molecular formula or mass across sources.
- If masses differ by about 1 Da, suspect amide versus acid before assuming an error.
- If masses differ by about 42 Da, suspect the presence or absence of an N-terminal acetyl.
- Note whether each figure is monoisotopic or average.
Questions
Is an amidated peptide just a salt form of the free acid?
No. A salt differs only in its counter-ion. Amidation changes the covalent structure and the molecular formula, so it is a different compound.
Why is the acetyl mass change quoted as 42 rather than 43?
The acetyl group adds C2H3O but replaces one hydrogen on the amine, so the net addition is C2H2O, about 42.011 Da monoisotopic.
Can HPLC purity alone confirm the right terminus?
Not reliably. The two forms may separate, but without a reference or mass data a single main peak does not prove which terminus is present.
Does missing notation mean the peptide is unmodified?
Not necessarily. Check the stated formula or mass; that is more dependable than the way the name is written.
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