Cyclic peptide are polypeptide chains that contain a circular sequence of bonds. This can occur through bonding between the amino and carboxyl ends of the peptide, for example in cyclosporine ; a connection between an amino end and a side chain, for example in bacitracin ; carboxyl end and a side chain, for example in colistin ; or two side chains or more complex arrangements, for example in amanitin, Many cyclic peptides have been discovered in nature and many others have been synthesized in the laboratory. Their length ranges from just two amino acid residues to hundreds. They are often antimicrobial or toxic in nature; They have various applications in medicine, for example as antibiotics and immunosuppressive agents.  Thin-layer chromatography (TLC) is a convenient method for the detection of cyclic peptides in crude extracts from bio-mass.
Cyclic peptides can be classified according to the type of bonds that the ring is involved in.
- Homodetic cyclic peptides, such as cyclosporine A, are those in which the ring is composed exclusively of common peptide bonds (i.e. between the alpha carboxyl of one residue to the alpha amine of another). The smallest such species are 2,5-diketopiperazine ,  being obtained from the cyclization of a dipeptide.
- Cyclic isopeptides have at least one non-alpha amide linkage, such as a linkage between the side chains of one residue to the alpha carboxyl group of another residue, as in microcysteine and bacitracin.
- Cyclic dipeptides such as oriobasidin A and HUN-7293 have at least one lactone (ester) linkage in place of an amide. Some cyclic depsipeptides are cyclized between the side chain of the C-terminal carboxyl and Thr or Ser residues in the chain, such as kahlalide F, thionylpeptolide, and didamine B.
- Bicyclics such as amanitins and phalloidins usually contain a bridging group between the two of the side chains. In amatoxins, it is formed as a sulfoxide bridge between the Trp and Cys residues. Other bicyclic peptides include echinomycin, tryostin A, and cellogentin C.
- There are many bi- and monocyclic peptides that are cycled through a disulfide bond between two cysteines , oxytocin being a notable example.
Cyclic peptides in plants are synthesized through a two-step process; Translation of a linear peptide chain, and its subsequent formation into a cyclic structure through protease-like activities by an enzyme or other means.
Properties and Applications
Cyclic peptides are extremely resistant to the process of digestion, making them of interest to scientists working on novel oral drugs.
- Gramicidin S
- Polymyxin B
- Valinomycin (technically a depsipeptide )
- Yunnanin A