What is a peptide?

A peptide is any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another. This is the definition in the IUPAC-IUB recommendations on the nomenclature of amino acids and peptides, which add that the amide bonds in peptides may be called peptide bonds (IUPAC-IUB 1983, PMID 6743224).

Where the line is drawn

The same recommendations state that peptides with fewer than about 10-20 residues may be called oligopeptides and those with more, polypeptides, and that polypeptides of specific sequence of more than about 50 residues are usually known as proteins, though authors differ greatly on where they start using that term (IUPAC-IUB 1983, PMID 6743224). The boundaries are conventions, not sharp chemical limits.

Sequence notation

When two or more amino acids combine to form a peptide, what remains of each amino acid is called an amino-acid residue. The residue with a free, or at least not acylated, amino group is N-terminal; the residue with a free carboxyl group, or one that does not acylate another residue, is C-terminal (IUPAC-IUB 1983, PMID 6743224). Two peptides with the same residues in a different order are different compounds, so a peptide is identified by its sequence rather than by a name.

Modifications

The N-terminal amino group need not be free: the recommendations note that it may, for example, be acetylated or formylated, and that the C-terminal residue may acylate ammonia, giving a C-terminal amide (IUPAC-IUB 1983, PMID 6743224). A change of this kind is part of the molecule's structure. A review by Werle and Bernkop-Schnürch gives the example of octreotide, a shortened derivative of somatostatin containing D-amino acids, with a plasma half-life of 1.5 hours compared with only a few minutes for somatostatin (Werle and Bernkop-Schnürch 2006, PMID 16622600).

Why this matters for reading research

A publication reports its findings for the material it studied. For synthetic peptides, a review by Lian and colleagues describes structural modifications arising from starting materials, the manufacturing process and storage conditions, and the use of liquid chromatography-mass spectrometry to characterize them (Lian et al. 2021, PMID 34110145). Every entry in this section is tied to the publication it came from.