Educational resource

Peptide bonds

A peptide is a polymer of amino acids joined by amide linkages commonly called peptide bonds. Understanding that condensation chemistry explains why length, formula, and molecular weight are related but not identical concepts on a compound record.

Condensation chemistry

The carboxyl carbon of residue i is covalently linked to the α-nitrogen of residue i+1. Relative to the free amino acids, each peptide bond formed is accompanied by loss of one water molecule.

For an unmodified linear peptide of n residues:

n residues
n − 1 peptide bonds
n − 1 waters lost versus free amino acids

That is why length in this database is the residue count, not the bond count. A 9-residue peptide such as oxytocin has 8 backbone peptide bonds (plus terminal modifications and a disulfide that are separate).

Planarity and geometry

The peptide bond has partial double-bond character from resonance between the carbonyl and the amide nitrogen. The six atoms of the amide unit lie approximately in a plane. Most peptide bonds adopt the trans configuration. Bonds preceding proline can populate the cis isomer more readily because proline’s cyclic side chain alters steric preferences.

Conformational detail (cis/trans populations, φ/ψ angles) is not stored as a searchable field here. It may appear in structure notes when it defines a named analogue.

Directionality

Peptides have direction: N-terminus → C-terminus. Writing a sequence backwards produces a different molecule. Filters and search on this site assume the stored N→C orientation from the cited source.

What is not a peptide bond (but can still close a ring)

Disulfide (cystine) — oxidation of two Cys thiols with loss of 2 H. Does not change residue count. Common in oxytocin, somatostatin-14, insulin, and many toxins.
Head-to-tail lactam — an extra amide between the N- and C-termini (e.g. cyclosporine). Residue count unchanged; one additional condensation relative to the linear peptide.
Side-chain lactam — amide between side chains (for example Lys and Asp/Glu) used to constrain conformation.
Thioether / carba bridge — e.g. carbetocin-type links that replace a disulfide with a carbon–sulfur or carbon–carbon bridge.

Length field conventions

Insulin is length 51 (21 + 30) even though it comprises two chains. A C-terminal ethylamide or threoninol is treated as a modification, not an extra standard residue, unless a specific source counts it otherwise — display notation states the convention used on that record.

Why this matters for mass and formula

Each backbone condensation removes H₂O from the elemental sum of free amino acids. Disulfides remove 2 H each. Terminal amidation replaces OH with NH₂. Getting those corrections right is the difference between a plausible calculated mass and a mismatch with PubChem — see the molecular-weight guide.