The N-terminus and C-terminus are more than the first and last letters of a sequence. They identify chemically different ends of a conventional peptide chain. The identity of those terminal groups helps distinguish molecules that would otherwise share the same residue sequence.
Separate the chain end from the residue name
IUPAC defines the N-terminal residue by its amino group not being acylated by another amino-acid residue, and the C-terminal residue by its carboxyl group not acylating another amino-acid residue. Those definitions permit terminal modifications such as acetylation or amidation; they do not require every terminus to be free.IUPAC Gold Book — Amino-acid residue (opens in a new tab)
For a simple uncapped chain, the amino end and carboxyl end are chemically different. The backbone's internal amide groups are different again. Calling all nitrogen-containing groups amino groups would conceal the distinction between a free terminal amine and an amide within the chain.
| Field | Example of the question |
|---|---|
| Terminal residue | Which residue occupies the end of the sequence? |
| Terminal group | Is that end free, capped or otherwise modified? |
| Side-chain group | Does a residue carry another reactive or ionisable group? |
The same sequence can have different ends
Consider the illustrative sequence Gly–Ala–Ser. A version with a free amino end and free carboxyl end differs chemically from Ac–Gly–Ala–Ser, and from Gly–Ala–Ser–NH2. These examples alter the terminal groups while retaining the same three residue identities in the same order.
The acetyl group is not a fourth amino-acid residue. Likewise, the terminal –NH2 in the amidated form describes an amide at the carboxyl end; it does not reverse the direction of the chain or make that end an N-terminus.
For matching two documents, write the full terminal description once, then use a defined short name. If one document omits the ends, do not assume it describes whichever version is most familiar. The missing field should remain visible until resolved.
Distinguish ionisation from covalent modification
A terminal amino group may be protonated, and a terminal carboxyl group may be ionised; IUPAC's chain-termination notation explicitly accommodates these states. Protonation changes and covalent modifications are different kinds of chemical description.IUPAC — Polypeptide backbone conformations (opens in a new tab)
For example, writing a protonated amino group does not describe acetylation of that group. The first concerns protonation state; the second concerns a new covalent substituent. A salt label elsewhere in a specification cannot substitute for a statement about terminal acetylation.
Nor should a charge drawn for one terminal group be treated as the net charge of the whole peptide. Other ionisable groups may contribute. A complete charge discussion needs the full molecular form and the relevant conditions, not just the first and last residues.
Do not confuse terminal and side-chain chemistry
A residue can contain a side-chain functional group in addition to the groups used in the backbone. A modification named only as amino-group attachment can therefore be ambiguous unless the position is specified. The chain end and a side-chain site are not interchangeable addresses.
In an illustrative peptide containing lysine, an attachment described at the N-terminus differs from one described at a lysine side chain. The attached component might have the same name in both preparations, but the complete connectivity differs. Use the source's positional notation rather than guessing the attachment site.
Keep terminal chemistry in the molecular record
- Record the sequence in its stated N-to-C direction.
- Name the chemical state of each terminus where documented.
- Preserve positions for side-chain modifications and attachments.
- Distinguish protonation, salt form and covalent substitution.
A useful identity record names the sequence, both terminal groups and any other declared modification together. Keeping those fields in one record prevents an abbreviated sequence from silently standing in for several chemically different materials.
Sources and further detail
- IUPAC Gold Book — Amino-acid residue (opens in a new tab)
Includes N- and C-terminal residue definitions without assuming all termini are chemically free.
- IUPAC — Polypeptide backbone conformations (opens in a new tab)
Definitions of backbone torsions and chain terminations, including departures from ideal peptide planes.
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.