Peptides (from the Greek πεπτος, "digestable"), 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 molecule is doubly bonded to an oxygen molecule and the nitrogen has a lone pair of electrons available for bonding.
Peptides (like proteins) occur in nature and are responsible for a wide array of functions, many of which are not yet understood. Antimicrobial peptides generally disrupt the membranes of a target cell, causing lysis of the cell. How this occurs, and what determines the activity and selectivity of these peptides, is currently only known approximately.
It has also been documented that when certain food proteins such as gluten, casein, egg protein and spinach protein are broken down, opioid peptides are formed. These peptides mimick the effects of morphine, and those who are unable to break them down will experience mental illness. These peptides are quite short and are given names such as casomorphine, gluten exorphine and dermorphine.
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. So, in essence, a peptide is a small protein.
There is considerable movement to redefine this arbitrary distinction such 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 which cannot be proteins.