C-terminal amide, N-terminal acetyl and other modifications: checking a listing names the right molecule

Why a sequence string is not a structure

The EMA guideline on synthetic peptides says the N- and C-terminus of a peptide chain must be clearly indicated, that modifications on the termini need to be shown in the structure, that cyclisation positions should be described, and that non-peptidic side chains and linkers need a full chemical structure. [2] Those are rules for medicines, but they describe what any buyer needs to know to be sure two documents describe the same compound.

Natural peptides are often modified. UniProt records oxytocin as residues 20 to 28 of a larger precursor, with a glycine amide at residue 28 and a disulfide between residues 20 and 25. [9] A catalog entry that says only "CYIQNCPLG" leaves out both features. The amide and the free acid are separate compounds, and so are the bridged and unbridged forms.

Glossary of common modifications

Notation follows IUPAC-IUB conventions where they exist: Ac- for acetyl, -NH2 for a C-terminal amide, Glp for pyroglutamyl, cyclo( ) for homodetic cyclic peptides and a D- prefix for D-residues. [1] Bachem writes pyroglutamyl as Pyr. [11] Mass changes are monoisotopic deltas from Unimod and ExPASy. [3] [4] [5] [6] [7]

Worked example: oxytocin and three near neighbours

Using ExPASy residue masses, the nine residues of oxytocin sum to 991.4256 Da. [7] Add water for the termini, apply the amide and the disulfide, and the neutral monoisotopic mass is 1006.4365, matching PubChem. [7] [10] Now change one feature at a time.

The free-acid form, with the disulfide, is 1007.4205: 0.984 heavier. The amide without the disulfide (reduced) is 1008.4521: 2.016 heavier. A deamidated amide form, with Asn or Gln converted to Asp or Glu, adds the same +0.984016 as the free acid, so it has an identical composition to the free acid and cannot be told apart by intact mass. [3] [8] Unimod lists amidation as -0.984016 and deamidation as +0.984016: the same atoms moving in opposite directions. [3] [8]

When the modification is the identity

Bachem explains that nature uses N-terminal acetylation, pyroglutamate formation and C-terminal amidation to stabilise peptides, because these blocked peptides are less rapidly broken down by enzymes, that disulfide bridges stabilise three-dimensional structure and that correct cysteine pairing matters when several bridges exist, and that side-chain cyclisation requires an additional level of protecting-group orthogonality. [11] For a buyer, that means a supplier who omits a modification may have made a different product, not merely written a shorter label.

Multiple disulfides raise a further issue: the same atoms can pair in different ways. Isomers with different pairings have the same mass, so a mass check cannot confirm the pairing. The EMA guideline says characterisation of disulfide bridges and cyclic structures should be performed where relevant. [2]

Questions to send the supplier

Peptides EU says identity is confirmed by mass spectrometry; its FAQ does not discuss terminal modifications. [12] OMNI Peptides keeps its catalog behind a researcher gate, and its public homepage showed no modification details on 30 October 2026. [13] Ask either supplier, or any other, the same questions in writing and keep the replies with the lot record.

• Give the full structure in three-letter notation with both termini shown (H-...-OH, H-...-NH2, Ac-...).
• State any disulfide pairings by residue number, and any cyclisation.
• State the molecular formula and whether the listed weight is for the free peptide or a salt.
• Confirm the certificate mass was calculated for that exact formula.
• If the amide or acid matters to you, ask how the two were distinguished.