Peptides (from the Greek ¦Ð¦Å¦Ð¦Ó¦Ïς, "digestible"), are the family of molecules formed from the linking, in a defined order, of various amino acids. The link between one amino acid residue and the next is an amide bond, and is sometimes referred to as a peptide bond. An amide bond is somewhat shorter than a typical carbon-nitrogen single bond, and has a partial double-bond character, because the participating carbon atom is double-bonded to an oxygen atom and the nitrogen has a lone pair of electrons available for bonding.
Peptides differ from proteins, which are longer chains of amino acids, by virtue of their size. By convention, those peptide chains that are short enough to be made synthetically from the constituent amino acids are called peptides rather than proteins. The informal dividing line is at approximately 50 amino acids in length (some people claim shorter lengths), since naturally-occurring proteins tend, at their smallest, to be hundreds of residues long. So, in essence, a peptide is the component of protein. Because of the arbitrary nature of this definition, there is considerable movement within the scientific community to ascribe the more-specific definition that "a peptide is an amino acid molecule without structure; on gaining defined structure, it is a protein." Thus the same molecule can be either a peptide or a protein depending on its environment, though there are often peptides that cannot be proteins. However, this definition breaks down when describing nonribosomal peptides (which are usually structured). Peptidomimetics (such as peptoids and ¦Â-peptides) are molecules related to peptides, but with different properties.
There are three large classes of peptides, according to how they are produced ‐ ribosomal peptides, nonribosomal peptides, and digested peptides.