Every peptide has two distinct ends: the N-terminus and the C-terminus. They give the molecule a direction, which is why sequences are always read one specific way. Here’s the simple version.
- N-terminus: the end with a free amine group (–NH₂); the “start.”
- C-terminus: the end with a free acid group (–COOH); the “end.”
- Direction: sequences run N → C.
The two ends
When amino acids link via peptide bonds, the chain that forms still has one free amine group at one end and one free acid group at the other. The amine end is the N-terminus; the acid end is the C-terminus.
| N-terminus | C-terminus | |
|---|---|---|
| Free group | Amine (–NH₂) | Acid (–COOH) |
| Position | Start of the sequence | End of the sequence |
Why it matters
Direction is everything: a sequence read N→C is a different molecule from the same letters read C→N. Many peptides are also chemically modified at one end — for example, an amide cap on the C-terminus — to improve stability. See how sequences are written in Peptide Sequences & One-Letter Codes.
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