An Ac- prefix often indicates an acetyl group attached at the peptide's N-terminus. That small notation change describes a different covalent structure, not simply a different way of writing the same sequence. Reading the attachment site is essential because acetylation can occur at other positions, and acetate in a material description means something different again.
Locate the acetyl group in the structure
N-terminal acetylation converts the terminal amino functionality into an acetylated amide. It changes the chemistry of that end of the chain while leaving the order of the amino-acid residues intact. Side-chain acetylation, such as modification of a lysine amino group, identifies a different attachment site.Maltsev, Ying and Bax — N-terminal acetylation of alpha-synuclein (opens in a new tab)Unimod 1 — Acetylation (opens in a new tab)
| Description | What it specifies |
|---|---|
| N-acetylated peptide | A covalently modified N-terminal amino group |
| Lysine side-chain acetylation | A modification at the named lysine site |
| Peptide acetate | An acetate-associated material form, not proof of N-acetylation |
For an illustrative sequence, Ac-Ala-Gly-OH specifies an acetylated N-terminus and a free C-terminal acid. It is not equivalent to H-Ala-Gly-OH. The example is a notation exercise and does not describe any product or measured sample.
Read the mass change relative to the unmodified structure
Unimod records the net elemental change for acetylation as C2H2O, with a monoisotopic mass difference of +42.010565 Da. The net change accounts for the attachment chemistry; it is not the mass of a freely floating acetate ion added to the spectrum.Unimod 1 — Acetylation (opens in a new tab)
If an invented neutral peptide has monoisotopic mass 1,000.000000 Da, adding one specified acetyl modification gives 1,042.010565 Da in that calculation. This is a neutral-mass bookkeeping example, not a prediction of a particular mass-to-charge peak.
The same elemental change can be associated with more than one possible site. A compatible intact mass therefore supports a composition question without necessarily answering the localisation question. The sequence annotation and evidence for the site should be checked separately.
A terminal modification can alter local behaviour
Maltsev and colleagues compared acetylated and nonacetylated alpha-synuclein, including synthetic 15-residue N-terminal fragments. The fragment analysis found local helical features in the acetylated construct that were not observed in the same way for the nonacetylated comparison.Maltsev, Ying and Bax — N-terminal acetylation of alpha-synuclein (opens in a new tab)
This is an example of a measured structural consequence for defined constructs. It does not establish that N-acetylation makes every peptide helical, more stable or more effective in an assay. The starting sequence and measurement conditions remain part of the comparison.
Check that two sources describe the same modification
When one source writes Ac- and another writes acetylated, confirm that both mean the N-terminus. A peptide containing lysine can otherwise be assigned the wrong modified structure while retaining the same residue string and total modification count.
A useful comparison record includes the sequence, the modified atom or group, the number of acetyl groups and all other terminal annotations. It should also distinguish a designed structure from an experimentally verified site assignment.
- Read the prefix together with the complete chemical notation.
- Separate N-terminal and side-chain acetylation.
- Keep acetate counterions separate from covalent acetyl groups.
- Do not turn a mass match into an unsupported site assignment.
Sources and further detail
- Unimod 1 — Acetylation (opens in a new tab)
Elemental change C2H2O and monoisotopic mass difference +42.010565 Da. The record includes several possible sites; mass alone does not localise a modification.
- Maltsev, Ying and Bax — N-terminal acetylation of alpha-synuclein (opens in a new tab)
Biochemistry 51, 5004–5013 (2012), DOI 10.1021/bi300642h. Includes a defined 15-residue fragment comparison; its structural observations are not universal effects for all acetylated peptides.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.