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Peptides are the family of molecules formed from the linking, in a defined order, of various amino acids. The link between each amino acid residue is an amide bond, but is sometimes referred to as a peptide bond. A peptide bond is actually somewhat shorter than a typical amide bond and has a partial double-bond character, because the involved carbon molecule is doubly bonded to an oxygen molecule.

Peptides (like proteins) occur in nature and are responsible for a wide array of functions, many of which are not yet understood.

Peptides differ from proteins, which are also long chains of amino acids, by virtue of their size. Traditionally, those peptide chains that are short enough to make synthetically from the constituent amino acids are called peptides rather than proteins. The dividing line is at approximately 50 amino acids in length, since naturally-occurring proteins tend, at their smallest, to be hundreds of residues long.

Notes on terminology:

A polypeptide is a single linear chain of amino acids.
An oligopeptide or (simply) a peptide is a polypeptide less than 30-50 amino acids long.
A protein is one or more polypeptides more than about 50 amino acids long.

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