Each time two amino acids join, the –COOH of the first reacts with the –NH₂ of the second in a condensation reaction. The result is a –CO–NH– bond that repeats along the whole chain and forms its backbone.
That bond has partial double-bond character: it is planar and rigid and does not rotate freely. The flexibility of the chain is concentrated in the angles of the alpha carbons, and those angles give rise to the three-dimensional shape of the peptide: helices, turns or sheets.
By convention, the chain is read from the end with the free amino group (the N-terminus) to the end with the free carboxyl (the C-terminus). When a sequence is written as Gly-His-Lys, glycine is the N-terminal end.
The peptide bond is stable, but the body's proteases cut it at specific sites. A large part of analogue design (replacing an amino acid, using D-amino acids, cyclisation) consists of protecting those cleavage points so the peptide lasts longer.
Related terms
Learn it in depth at the Peptide University
- From amino acid to peptide · Module 1
- Chemical degradation: the weak points of the sequence · Module 11
- Pharmacokinetics: ADME and parameters · Module 2
Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).